Question on battery sizing

System · September 2007

Hello,

I'm new to this forum and I'm trying to find information on sizing a battery back-up system.

I live in Connecticut and I'm having a 10kW system installed on my back roof which is un-shaded and faces south. The system will be made of 48 SunPower SPR-210 modules. Thankfully, CT has a really good rebate program which will pay for more than half the system costs.

I have not decided on an inverter yet and I'm going back and forth between a straight grid-tied system and a grid-interactive system with a battery backup. My indecision is based on the idea that much of our grid power is produced by oil and natural gas and the demand for these finite resources is increasing year after year; I'm concerned that future supplies may not be sufficient to provide a stable supply of power to the grid. Please tell me I'm crazy

On to the questions:

1) I'm thinking about a pair of Xantrex XW6048 inverters so that I could have a straight grid-tied system now and add batteries at a later time. The idea is that I might not be able to afford batteries straight away and I will not want to replace the inverters if I add batteries later.

2) Based on my proposed 10kW system, what would be the largest, sustainable battery setup that could offer a comfortable supply of power in case the grid "goes away."

I've done some reading on kWh vs Ah and charging cycles, DOD, strings, series, parallel, etc. I'm clueless on how big a battery bank (Ah) my system could reasonably support taking into account voltage; I'm still not sure if I would go with 12, 24 or 48volts, are there reasons for choosing one over the other aside from compatibility with other system components?

3) As far as batteries go, I've read that "flooded deep-cycle lead acid" are cheaper and last substantially longer than the sealed battery options. Can I safely put flooded lead acid in my basement and if so, what do I need?

Thanks for any help.

Chuckc

Solar Guppy · September 2007

Re: Question on battery sizing

The XW-6048 is a great inverter and does every possible configuration for your RE. Gridtie, off-grid, true generator support ect.

Your ONLY choice for this inverter is 48V, and you will need at least 400ah PER INVERTER of batterys ... so your looking at close to 10K for the batterys using agms.

You will also need 4 charge controllers and your panels will have to have multiple home runs more than if you just used a straight high voltage GridTie inverter. The Maximum of any available mppt charger is never to exceed 150Volts, so typically this would be 2 possible 3 solar panels per string ( if each 24V ), then combined for the charge controller.

If you go all the new XW stuff, its all highly integrated, very clean installation and all the componets talk to each other so things like system watts or watts used, state of charger comes bulit into the package with a slick graphical remote system monitor.

The only other options would be Outback, but that would require 4 inverters stacked and they don't do split phase seemlessly like the XW's can. The Old Xantrex SW series really is no longer an option as its been discontiued after about 14 years of being in the market.

I have a single XW-6048, 440ah 48V agms with a pair of charge controllers, so its half of what your thinking ... it powers the AC and the solar can recharge enough that I can go forever without the grid as long as I get good sunshine each day ( only overnight ability ) ... to do more would require even more batteries!

For battery types, I would suggest AGM, they have a MUCH smaller float charge and no gassing. I had Deep Cycle Lead Acids before, a total pain to keep track of and I would never use them again over AGMs. For your bascially whole house UPS, the batteries will basically float all the time and AGMS are really only the logical choice for this large of a system

Oh, re-read your post, If you use ANY inverter with backup abilities, they REQUIRE a battery bank and seperate charge controllers .. for the XW-6048, the inverter can surge to 12kWh as loads demand, the smallest battery bank that can handle a 200amp discharge that load would created would be ~400 ah. Even if the XW is set to sell mode ( GridTie ) you need the battery bank as thats the buffer between the charge controllers which would get overwhlemed with AC ripple on two small a battery bank

BB. · September 2007

Re: Question on battery sizing

Short answer--the best bang for the buck is Grid Tied solar (solar panels to GT inverter to AC mains). Most efficient and least cost of any system (assuming that you get a good Net Metering plan--and sometimes, a good Time Of Use (TOU) rate plan, which will credit you for electricity generated from ~noon-6pm, from your utility can help too).

If you want an Off-Grid (or other GT/Off Grid variant), it will cost you more (for the batteries, charge controllers, inverters) and, a full off-grid system is usually is not covered by solar rebates/credits.

Also, storing power in batteries, +inverter DC-AC conversion, increases losses by another ~30%. So, it takes more solar panels to generate the same amount of power (plus with off-grid--you can only store about ~3 days of energy--with 1 year net metering, you can "store" energy for up to 365 days with your utility company)

So, your question is that added costs for an off-grid system (one that can operate when the Utility Mains fail) is a question only you can ask... But, from the back of the envelope calculations I have done before--a pure off-grid system can easily increase your cost per kWhr by 2-3x (from ~$0.25/kWh to $1.00 per kWhr--without rebates/credits). [please note, these prices are very rough--local solar conditions/requirements, self installs, used equipment, good deals, etc. will greatly affect your actual costs--just to give you an idea about basic operational costs].

And, that is the question that many of us have faced when we installed our systems.

Personally, I looked at how often I have power failures (1 short one every couple of years--1 failure that lasted a week ~50 years ago), and I decided that a small generator (Honda eu2000i 1,600 watt generator or --~$900 delivered+plus gas cans and fuel stabilizer) with 20 gallons of gas (gives me ~2+ weeks of fuel for fridge/freezer) was a better solution for me than the full Battery/Off-Grid Inverter solution (generator and fuel takes up much less space than equivalent batteries--and I recycle the fuel into my car every year to keep it fresh).

If you have lots of local/long power failures (in sunny weather)--say Ice Storms--you might want to only put a portion (example 1kW out of 10kW) into an off grid system that only supplies enough power for a fridge, couple lights, and a furnace. Of course, if a tornado hits your home/array--your solar arrays may not survive (in my case, earthquakes are my "major catastrophe"--would a solar array on my house survive--maybe).

Normally, a good battery bank would be about 6x your daily usage (3 days of no-sun power, plus 50% maximum discharge for good battery life). For a 10 kW peak system, that is a huge battery bank--maybe ~60 kWhr bank (if you follow normal design guidelines).

If you are OK with servicing batteries (checking water levels every month or so, and cleaning them and the wiring occasionally), flooded cell batteries are probably the cheapest and most robust.

If, however, you prefer sealed batteries (no checking levels, much less cleaning, no H2 gas venting in normal operation, possibly less prone to sulfation issues from deep cycling, ability to install batteries on side, etc.),
AGM batteries are not a bad alternative. However, they are more prone to damage from overcharging and (actually all) large battery installations need a remote temperature sensor(s) back to the charge controller(s).

-Bill

BB. · September 2007

Re: Question on battery sizing

As Solar Guppy replied (while I was typing), AGM batteries have roughly 10% loss, and flooded cell have ~20% energy losses...

Also, there is new equipment just coming out now (like the new Xantrex system that Solar Guppy was involved in designing and testing--I would have really liked to have looked at that system 2 years ago when I did my Grid-Tied install), that can change things around (reduce costs for a combo Grid-Tied/off-grid system)...

But, in the end, the best place to first spend your money is in reducing your daily energy usage (insulation, energy star appliances, changing personal habits, etc.). It is almost always cheaper to reduce the kWhr usage vs paying to generate the kWhr. For example, I use natural gas for cooking/heating/hot water and no A/C with 4 people--but I only use ~7-8kWhrs per day. Your system sounds like it will generate ~30-40 kWhr per day (~1,000+ kWhr per month vs 200-300 kWhr per month?).

If you use a lot of home heating fuel (gas, oil, etc.), looking at solar hot water is also an interesting solution for saving money (solar hot water system panels are usually smaller and overall installed costs less to save the same amount of $$$$ vs solar electric systems).

-Bill

icarus · September 2007

Re: Question on battery sizing

In one sense it is a no brainer. A small generator for those rare times of outage comes way cheaper than a comparable size battery bank. Not to mention the life ccyle cost of the batteries, 7-20 years. A Honda Eu series will out live all of us it you only run it once a month for exercise and then for emergancies.

If you take the dollars you might put into the batteries and add additional grid tie panels instead, you would be far ahead in the environment, considering the decrease in fossil fuel load you are providing, plus the not having to have all the lead in yout batteries tied up for years, only to be (hopefully) recycled into new ones)

Do the math, and I think you will have your eyes opened.

Icarus

System2 · September 2007

Re: Question on battery sizing

Solar Guppy wrote: »

The XW-6048 is a great inverter and does every possible configuration for your RE. Gridtie, off-grid, true generator support ect.

Your ONLY choice for this inverter is 48V, and you will need at least 400ah PER INVERTER of batterys ... so your looking at close to 10K for the batterys using agms.

Wow, that's a big system. You're talking 800ah at 48V = 38400wh, is that correct? Will the 10k PV system re-charge that big a system on a daily basis?

I may want to do about 1/2 of that, at least to start out. Also with the AGMs, do they really only last 5 years. That's a lot of money to spend on an items that does not last very long.

Solar Guppy · September 2007

Re: Question on battery sizing

Having lived thru the hurricane season of 2004 where I had 3 eye's pass within 15 miles of my florida home I would NOT use a generator. I have newspaper clippings of thousands of stranded people trying to get gas for there generators.

If you want something that can have a fuel supply that doesn't turn into gum. you need propane and those system start approaching the costs of battery banks when your talking about whole house systems like a pair of XW-6048's can power ( 12kW / 24 kW surge )

In Lakeland Florida, power was out in some sections for almost two weeks in 2004. Since then, I have had about a dozen or so outages in the last three years of an hour or more ... My Outback GT3048 keeps the office and refig going forever as I have 12 kW of solar on the house and can charge and run loads at the same time. Now with the XW-6048 online, I can go forever if need be when the next grid failure should happen , like when the substation exploded last week and smoke/flames could be seen almost into tampa ... overloaded grid ... sound familiar?

Oh, and the 5Kw GT solar I just added was almost 100% paid by State/Fed rebates, not so for Generators :roll:

System2 · September 2007

Re: Question on battery sizing

icarus wrote: »

In one sense it is a no brainer. A small generator for those rare times of outage comes way cheaper than a comparable size battery bank. Not to mention the life ccyle cost of the batteries, 7-20 years. A Honda Eu series will out live all of us it you only run it once a month for exercise and then for emergancies.

If I thought the grid was always going to be around, as it is now, I would not even consider a battery system. I would get the generator. Heck, I might even just skip the generator and have no backup since power outages around here are rare and short lived.

You have to remember that I'm under the impression that the grid may be less available in the future because it is, for the most part, powered by non-renewable resources and global demand for oil and natual gas is going up and up while supplies are not.

If you want to see where I'm coming from, google "peak oil." Its common sense that the end of cheap oil will occur and that it's really just a matter of when. I'm under the impression that it will happen sooner (withing 20 years or less) rather than later.

Solar Guppy · September 2007

Re: Question on battery sizing

chuckc wrote: »

Wow, that's a big system. You're talking 800ah at 48V = 38400wh, is that correct? Will the 10k PV system re-charge that big a system on a daily basis?

I may want to do about 1/2 of that, at least to start out. Also with the AGMs, do they really only last 5 years. That's a lot of money to spend on an items that does not last very long.

Well, YOU picked 12kWh inverters ... so that would run them for 3.5 hours ( about 40kWhr in the batterys ) flatout ... as you see, if your trying to run the whole house, its alot of watts

A 10kW PV system will do about 40kWhr day, and yes would recharge the batterys if need be. In your setup, they will only be discharged durring grid outages, otherwise they sell most of the power to the grid and floating the batterys for GT is more like 2-3%, not 20% for selling during the day.

As for AGM's, in float mode they will last for decades, not sure where you got 5 years. the ONLY downside of AGM's is they have to be sized correctly to the inverter/charger and loads ... no big deal

Solar Guppy · September 2007

Re: Question on battery sizing

chuckc wrote: »

If I thought the grid was always going to be around, as it is now, I would not even consider a battery system. I would get the generator. Heck, I might even just skip the generator and have no backup since power outages around here are rare and short lived.

You have to remember that I'm under the impression that the grid may be less available in the future because it is, for the most part, powered by non-renewable resources and global demand for oil and natual gas is going up and up while supplies are not.

If you want to see where I'm coming from, google "peak oil." Its common sense that the end of cheap oil will occur and that it's really just a matter of when. I'm under the impression that it will happen sooner (withing 20 years or less) rather than later.

Peak Oil is a Myth, as prices increase, so does the supply as the demand falls. We have enough coal for 200-300 years to supply almost anything oil is used for, for example coal to deisel fuel is very cost effective so cars, trucks and planes keep on going and it can power the grid 100%, don't need anything from other countrys.

Now what happens to the grid is a different question. Most likely, power outages will become more common as the demand/population keeps doubling every 15 years ...

Having ones own power plant is a GREAT feeling .. and to me, its worth every dollar it cost to build. I'm also not the average Joe home owner that is clueless on why the there electric bill is 400.00 ( my next door neighbor ) .. I even told him I would help him put in a nearly free 5kW solar system the state will pay for, no interest what so ever, thinks it would be ugly on the back roof no one can see ... and thats my view on home owners in america as to what is typical.

Vic · September 2007

Re: Question on battery sizing

For a Grid-Tied system, AGM's may well be the best solution. Maintenance is considerably lower and the total capacity needed is usually smaller.

Regarding Floating flooded batteries, a number of people have mentioned high float currents. On my (only) two-year-old Surette 4KS25 48-volt string, Float is 1.1 Amps typical. Think that both Float and EQ currents rise significantly with aged batteries.

Am not trying to snipe at AGM's, but, ... You cannot measure Specific Gravity directly, and you cannot EQ them to whip them into shape. There IS less routine maintenance required, and for some folks this is the only way to go. Many flooded battery sets have been ruined by ignoring them until the power goes off. Think that AGM's are quite a bit more expensive per WattHR.

YMMV Vic

crewzer · September 2007

Re: Question on battery sizing

Chuckc,

As noted above, the new XW inverter is battery-based, so, if you decide to invest in these new and untested products, you’ll need batteries from the start.

I believe that another issue is that Sunpower PV module power specs are based on the requirement that they be deployed with a positive ground inverter. Positive ground inverters available here in the U.S. are sold by – you guessed it – SunPower, although they’re made by PV Power and by Xantrex. These are only grid-interactive inverters, so battery backup will be a challenge.

In fact, here’s a note from Sunpower's installation instructions: http://www.sunpowercorp.com/Products-and-Services/~/media/Downloads/for_products_services/SunPower_Safety_and_Installation_Instructions_UL.ashx

4.2 System Grounding

Important! For optimal performance, SunPower PV modules must only be used in configurations where the positive polarity of the PV array is connected to ground. Failure to comply with this requirement will reduce the performance of the system and invalidate SunPower’s Limited Power Warranty for PV Modules.

For more information on grounding the system correctly, visit our website at www.sunpowercorp.com/inverters or contact SunPower technical support at 1-877-SUN-0123.

Interestingly, the related links below no longer seem to work:

http://www.sunpowercorp.com/homeowners/inverters.html
http://www.sunpowercorp.com/techpapers/polarization.pdf

In sum, I strongly urge you to clarify the technical and warranty requirements for your Sunpower modules before purchasing any inverter of any kind.

Regards,
Jim / crewzer

Solar Guppy · September 2007

Re: Question on battery sizing

Good catch crewzer on the SunPower panels ...

The Inverter could careless about positive ground as its a transformer isolated design, but one would need positve ground charge controllers with GFP ... not anything I know of comes to mind

For the OP, look at Evergreens, they are almost the same size as the SunPower and if one buys seconds, they are only 3.58 watt www.sunelec.com and have the tightest performance of any panel at -2%/+4%

The 5kW I just added were these panels and they are performing better then expected.

System2 · September 2007

Re: Question on battery sizing

chuckc wrote: »

Hello,

I have not decided on an inverter yet and I'm going back and forth between a straight grid-tied system and a grid-interactive system with a battery backup. My indecision is based on the idea that much of our grid power is produced by oil and natural gas and the demand for these finite resources is increasing year after year; I'm concerned that future supplies may not be sufficient to provide a stable supply of power to the grid. Please tell me I'm crazy

You asked for it, you're crazy.

chuckc wrote: »

On to the questions:

1) I'm thinking about a pair of Xantrex XW6048 inverters so that I could have a straight grid-tied system now and add batteries at a later time. The idea is that I might not be able to afford batteries straight away and I will not want to replace the inverters if I add batteries later.

You state "if I add batteries later". If you don't have batteries before the grid goes away, will those inverters be able to provide power?

chuckc wrote: »

2) Based on my proposed 10kW system, what would be the largest, sustainable battery setup that could offer a comfortable supply of power in case the grid "goes away."

You will need to determine what a "comfortable supply of power" is. I've attached my Solar Calculator to this post (at the bottom of the post) so you can work the numbers any way you like, my thanks to machineman for the thorough testing.

chuckc wrote: »

I've done some reading on kWh vs Ah and charging cycles, DOD, strings, series, parallel, etc. I'm clueless on how big a battery bank (Ah) my system could reasonably support taking into account voltage; I'm still not sure if I would go with 12, 24 or 48volts, are there reasons for choosing one over the other aside from compatibility with other system components?

One reason to pick higher voltages over lower is that some components can handle more volts but not more amps. For instance, the MX-60 charge controller is limited to 60 amps at 12, 24, 36, 48 and 60 volts. Do the math to see how many controllers you would need at each voltage at 60 amps based on your proposed array. The cost of charge controllers is not insignificant. Another reason is wiring requirements. A given array at higher voltage needs a smaller (less expensive) wire than the same array at lower voltages to maintain the same voltage drop.

chuckc wrote: »

3) As far as batteries go, I've read that "flooded deep-cycle lead acid" are cheaper and last substantially longer than the sealed battery options. Can I safely put flooded lead acid in my basement and if so, what do I need?

A) My opinion is that true deep cycle batteries used as deep cycle batteries will give you better cost/performance than AGM used as deep cycle batteries. My experience with "smallish" UPS AGM's is that they usually last about 3 to 5 years in "float" service, certainly larger telco units last longer, most likely due to better charge controllers. I would expect that they don't last as long if cycled regularly. If you expect the grid to go off permanently in the next few years, I'd go with FLA's, otherwise, just size AGM's to be able to operate your "mandatory" items for the occasional grid outage and buy your deep cycle FLA's before you expect "the big one" to begin.

Ventilation and maintenance.

Cheers,

Bad Apple

System2 · September 2007

Re: Question on battery sizing

Solar Guppy wrote: »

Peak Oil is a Myth, as prices increase, so does the supply as the demand falls. We have enough coal for 200-300 years to supply almost anything oil is used for, for example coal to deisel fuel is very cost effective so cars, trucks and planes keep on going and it can power the grid 100%, don't need anything from other countrys.

Now what happens to the grid is a different question. Most likely, power outages will become more common as the demand/population keeps doubling every 15 years ...

Having ones own power plant is a GREAT feeling .. and to me, its worth every dollar it cost to build.

I agree that we are not running out of oil but I'm convinced that prices will continue to rise and we may reach a point were oil is not nearly as available as it is now. Also, about 60% of our oil is imported and it is POSSIBLE that some of the importing countries might decide to keep more of their oil for themselves as supplies are reduced.

Normally I would agree with your supply and demand argument but we are so dependant on oil that a realistic reduction in demand could only be achieved with some significant social and economic side effects. Current economic data has oil demand in the US increasing by about 2% a year and places like China increasing by about 7% a year. I think Americans are going to need to see some real pain before they trade in their SUVs for Hybrids and start carpooling.

As far as coal to oil, that is doable but has its own issue, not the least of which is increased CO2. Also, converting from an oil economy to a coal economy would be a really big job and would take a long time even if we started now, which we won't.

Here's an interesting article and discussion on coal to oil. Some agree it will work and some are real doom and gloom but it is interesting.
http://gristmill.grist.org/story/2006/3/28/154457/298

The bottom line is the I don't know what's going to happen but I know we cannot keep going the way we are going and it seems inevitable that some type of shakeup will occur. It may be a nice gradual transition or it may be rapid and painful.

In any case, I did trade in my 4-runner for a 2004 Toyota Prius a few weeks ago (neat little car) and adding Solar to my house is very appealing to me. And, I would like the PV system to work even if the grid does not, thus the questions on batteries and my reluctance to use a generator even though I know it is the best way to go for short lived grid outages.

Telco · September 2007

Re: Question on battery sizing

On the whole peak oil thing, this article has a very detailed look at the world's oil supplies. I am inclined to think it to be accurate in its outlook. It states that the world operates on a 40 year proven supply concept not because that's what we have, but because it's not economically sound to try to prove more than that for the oil companies. And, the main gist of the article involves the US government's crude oil requirements and the need for the US military to move away from the use of foreign oil. In it they look at different ways to cut the use of oil and recommend technological directions to investigate, the idea being to try to wean off foreign oil over the next 25 years. So yes, peak oil is not so much a myth as a misinterpretation of the facts to promote a specific agenda, the world has had a 40 year proven supply regardless of use for the last 100+ years for economic reasons, not actual supply reasons, and the reason for the current high prices is more due to political turmoil than actual supply issues. As of 2006, the actual price of crude should have been 30 to 40 dollars a barrel, everything over that is investor greed backed up by political turmoil in the oil bearing nations.

No, I'm not a Big Oil guy either, I've been working on cutting my own usage for the last 10-15 years, when fuel was less than a dollar a gallon and I could heat my house in the dead of winter for 60 bucks a month.

System2 · September 2007

Re: Question on battery sizing

Telco wrote: »

On the whole peak oil thing, this article has a very detailed look at the world's oil supplies. I am inclined to think it to be accurate in its outlook.

I did not read the entire report but it appears to group conventional and non-conventional oil reserves in the same bucket. The problem with non-conventional oil is that it costs a lot of money to get out of the ground and more to refine into the fuels we depend on.

I don’t think we are running out of oil. In fact, I don't believe we will ever run out of oil. However, based on a lot of reading from many different sources, and I do look at both sides, I believe that we have found most of the "easy to get to" oil. What's left is harder (read more costly) to extract and refine.

There is a huge spread of opinions on this subject right now and only time will tell. Meanwhile, a little preparedness is a good thing and I think we can all agree that electricity prices are not going to go down; I'm looking forward to having my own little electric generation system sitting on my back roof.

I'm still undecided on the batteries. I would rather go with a straight GT system but energy independence, if needed, appeals to me. If I money were no issue, I would definitely go with a big battery bank.

System2 · September 2007

Re: Question on battery sizing

Bad Apple wrote: »

You asked for it, you're crazy.

Thanks, I need to hear that every once in a while

Bad Apple wrote: »

You state "if I add batteries later". If you don't have batteries before the grid goes away, will those inverters be able to provide power?.

From what I've read, this is not an option. If I get a battery back up system, I have to add batteries right away. Someone suggested sunny boy inverters and adding a sunny island later with batteries.

System2 · September 2007

Re: Question on battery sizing

Solar Guppy wrote: »

The XW-6048 is a great inverter and does every possible configuration for your RE. Gridtie, off-grid, true generator support ect.

Your ONLY choice for this inverter is 48V, and you will need at least 400ah PER INVERTER of batterys ... so your looking at close to 10K for the batterys using agms.

You will also need 4 charge controllers and your panels will have to have multiple home runs more than if you just used a straight high voltage GridTie inverter. The Maximum of any available mppt charger is never to exceed 150Volts, so typically this would be 2 possible 3 solar panels per string ( if each 24V ), then combined for the charge controller.

Oh, re-read your post, If you use ANY inverter with backup abilities, they REQUIRE a battery bank and seperate charge controllers .. for the XW-6048, the inverter can surge to 12kWh as loads demand, the smallest battery bank that can handle a 200amp discharge that load would created would be ~400 ah. Even if the XW is set to sell mode ( GridTie ) you need the battery bank as thats the buffer between the charge controllers which would get overwhlemed with AC ripple on two small a battery bank

So, you MUST have at least 400ah per inverter? I did not know that. So with this type of system, you really need a larger battery bank right from the start.

With a 10kw system, I thought I had to have at least 10kw of inverters to handle the load. Is this correct?

And if I have two big XW inverters and need at least 800ah of battery bank, what would be the biggest battery bank I could have, not that I'm going to add it. I'm just curious on the range of a system like this 800ah to ???

If you were interested in adding solar now with an option to add batteries later, what would be some suggestions. My installer is suggesting a pair of sunny boy inverters with the potential to add a sunny island and some batteries later.

SolarJohn · September 2007

Re: Question on battery sizing

Quote:

I have not decided on an inverter yet and I'm going back and forth between a straight grid-tied system and a grid-interactive system with a battery backup. My indecision is based on the idea that much of our grid power is produced by oil and natural gas and the demand for these finite resources is increasing year after year; I'm concerned that future supplies may not be sufficient to provide a stable supply of power to the grid. Please tell me I'm crazy
.........
You're not crazy. You seem to have a desire to be self-sufficient, and for that reason have a problem with being grid-tied. In addition to that, you're uncomfortable with the idea that utility costs could skyrocket, and you want to prepare for that possibility. Personally, I choose to accept all of the drawbacks of an off-grid system in order to have a reliable source of electricity. And, although my budget won't allow me to install a system big enough to meet all of my household needs, I'm happy to get what I can and conserve power when necessary. I'll add to the system over time.

My PV system is not the only thing I do to cut back on fossil fuel use. I use a corn-burning stove to supplement my furnace. Besides being kinder to the environment, I've cut my heating bills.

I hope to own a PHEV eventually, and power it with my PV system. Eliminating the use of fossil fuels entirely (including electricity from coal-fired power plants), is my ultimate goal.

John

crewzer · September 2007

Re: Question on battery sizing

I would like the PV system to work even if the grid does not, thus the questions on batteries and my reluctance to use a generator even though I know it is the best way to go for short lived grid outages.

Chuckc,

Now that we may be done picking apart your ideas, let’s look at system and strategy that might work for you.

One solution might be to create an AC subpanel for your home that would feed critical loads. These loads would normally be fed by a combination of solar and/or grid energy. In the event of a grid failure, the PV system and/or generator would power the loads connected to this sub-panel. Other loads connected to the your home’s main AC panel would not be powered.

48 each Sunpower 210 W adds up to 10,080 W STC. You could create three sub-arrays in two-per-series x eight series in parallel (16 modules per sub-array; 3,360 W STC total) and connect each sub-array to an OutBack MX60 charge controller. You’d need three controllers, or for each sub-array.

Total mid-day charge current into a “48 V” battery bank would be ~150 A. This amount of current should be able to charge the battery bank and operate critical daytime loads. A battery bank rated at 48 V x 1,500 Ah would be appropriate. This bank would store ~75 kWh of energy, so, allowing for 90% inverter efficiency and limiting discharge to 50% state-of-charge, the bank could deliver ~11 kWh/day for 3 days of autonomy (no grid and essentially no Sun) before you’d have to run the generator.

It should be pretty easy to build a 48 V x ~1,500 Ah battery bank from Exide GNB Absolyte IIP AGM batteries. http://www.industrialenergy.exide.com/products/range_select.asp?range=IIP&sub_id=17&lng=en&main_description_en=AGM&cl=Technologies

Two (7,200 W total) to four (14.4 kW total) OutBack VFX3648 back-up inverter/chargers could probably handle your critical 120/240 VAC loads. The inverters could power the loads from the batteries charged from the PV array, or from the grid while the grid helps recharge the batteries via the built-in chargers. If you need four inverters, you could start with two and then add the other two later.

With the help of an OutBack Mate controller and Hub, the inverters could be programmed to turn on the generator if the grid fails and the batteries run low. An auto-transfer switch would switch the inverters’ AC input from the grid to the generator in the event of a grid failure. Additionally, the Mate can be user programmed to routinely exercise the generator to keep it healthy.

HTH,
Jim /crewzer

Solar Guppy · September 2007

Re: Question on battery sizing

For a home installation the MX60 doesn't meet current NEC requirements for Ground Fault protection required for roof mounted arrays and with the Sunpower panels you still have the positive ground issue to deal with.

The Outback inverters are also NOT split phase, you can run multi-inverter as Crewzer suggests but is not as seemless as the XW inverter which can handle larger phase imbalanced loads where you could overload the FX inverters. The Outback also has much higher tar ( idle ) losses

The MX60 and the Inverters do not meet FCC emissions required by law ... so expect possible emi issues with you or your neighbors radios and such

The XW-6048 is full UL/NEC/FCC listed/approved , performs at higher efficiencys ( 95% peak ) and with the bulit-in 120/240 can handle higher imbalanced loads, real generator support and is UL listed for Gridtie ... no other product has this feature list.

You initail pair of XW-6048's will be a much lower cost than Sunnyboy/sunny islands, you can work with a small battery bank ( realitive to the output ability ) and will be a very clean install if you go with the XW-MPPT and Xantrex BOS box.

I understand it will take time for others to install, use and report back on the XW products. I've never been one to mention products that I haven't used myself ... and I have owned the Outback products and have a GTFX3048 as the backup for the home office. The Outback is a solid performer but has limits I've listed and if buying today I would buy the XW products for the improved features and compliance.

BB. · September 2007

Re: Question on battery sizing

Chuckc,

Do you 10kW of inverters--the answer, as always, it depends...

If you are doing a pure grid tied system, yes, 10kW of DC power in from panels, gets inverted into 10kW of AC and sent out into your home/utility wiring (less 5-10% in losses). Some (or all power) is used in your home, and any left over is sent out through your utility meter.

However, if you are running a battery based off-grid system..., you need charge controllers to manage 10kW of DC input power to battery bus... Inverter wise, you only need enough inverters to supply your maximum sustained (plus peak load).... You may need 10kW, or you may need more, or less--it all depends on your loads. A/C, well pumps, microwave ovens, electric heaters/ovens, will need large inverters... TV's, radios, lights, small appliances fans, much less...

The new Xantrex system that can do both Grid Tie and off-grid/battery backed systems--you would need to talk with Solar Guppy and/or Xantrex to see how their system partitions...

From my point of view, once you have your batteries fully charged, it would seem to be better to just grid-tie converter 100% of your solar panel power to the grid and avoid cycling your batteries (increased battery wear and charging losses). But I have not read anything in detail about Xantrex's new system--other than I probably would want one some day...

-Bill

crewzer · September 2007

Re: Question on battery sizing

For a home installation the MX60 doesn't meet current NEC requirements for Ground Fault protection required for roof mounted arrays...

C’mon, that’s not quite true. OutBack sells an optional PV Ground Fault Protection System (OBDC-GFP/2) just for this requirement.

…and with the Sunpower panels you still have the positive ground issue to deal with.

Right – no matter which charge controller you choose!

The Outback inverters are also NOT split phase, you can run multi-inverter as Crewzer suggests but is not as seemless as the XW inverter which can handle larger phase imbalanced loads where you could overload the FX inverters. The Outback also has much higher tar ( idle ) losses

The OutBack X-240 balancing transformer takes care of phase imbalances for off-gris 120/240 applications. “Slave” inverters can be put into “search” or “sleep” modes during low power operations… tar / idle losses are reduced to 2 – 3 W, as I recall.

The MX60 and the Inverters do not meet FCC emissions required by law ... so expect possible emi issues with you or your neighbors radios and such

I’m not an RF engineer, and I don’t play one on TV.

Nonetheless, this concern may be something of a red herring. Compliance with FCC emissions requirements doesn’t mean that there are no emissions; it just means they’re below a certain level. The instruction manuals for virtually every piece of electronic gear I own include instructions on how to deal with possible RF problems with their FCC compliant products. :roll:

My MX60 charge controller and my FX2524T inverter power my LCD HD TV, the cable box, the DVD player and my AM/FM receiver. We’ve experienced no EMI issues with any of those items, nor with the other TV’s, radios, cordless phones, cell phones, atomic clocks, RF thermometers, or wireless network in our house or cars. The neighbors haven’t complained either. :-)

I did a search on OutBack’s forum for “EMI” or “interference”. I found what might be two complaints. I also found kudos for good RF/EMI performance, a reference to meeting CE requirements (“harder than …FCC”, according to Christopher F.), info on inverter sales to the U.S. military, and complaints about Exeltech and older Xantrex products.

The new Xantrex products look promising, but they’re new and, in my view, unproven. Anyone remember the Sun Tie inverter?

The key point, though, is that are many ways to address chuckc’s design challenge. The “OutBack way” and the new “Xantrex way” are but two. And, in the spirit of full disclosure, I haven't done a Brand "O" vs. Brand "X" cost analysis... yet.

Ain't competition grand?

Regards,
Jim / crewzer

Solar Guppy · September 2007

Re: Question on battery sizing

Looking at your signature might explain why the emi isn't an issue for you on your MX60 ... at 36V in 24V out at 600 watts is minimal , crank that upto 3kW and 100V array and let the am band be damned :roll:

I've seen the RF sweeps on the MX60, not pretty at full power doesn't even come close for Class A, let alone Class B ... to say its not an issue is a red herring, even boB knows its an issue and has stated so on this forum.

As for the Suntie swipe, that was a Tracing engineering mess that I cleaned up and the designers are now at Magnum I believe. Xantrex got stuck holding the bag and ended up spending millions to retrofit all the units ever produced for FREE to the end user, now thats backing up your product.

Competition is good, being honest about a products plus and minus is more importaint. The XW isn't just a one off product done by an engineer or two, its a complete integrated power system that meet ALL current 2007 regulations, no one else does this and no amount of spin will change these facts.

I respect your enthusiam for Outback products and they are good performing units for when they were designed.

System2 · September 2007

Re: Question on battery sizing

Chuck said he is trying to avoid using a generator.

Chuck,

You have to provide some specific answers.

1) Are you designing your system based on a belief that in the near future there will be a prolonged or catastrophic grid failure?

A) If yes, you must design a battery based system and need to define what your power requirements will be. The calculator I linked yesterday allows you to plan seasonally. You can use it to determine which season is most important to you and plan for that or if you have deep pockets, plan for the season that has the highest requirements that your array will support.

If no, you must decide if you want to have power during the random power outage.

B1) If yes, you must design a battery based system and need to define your power requirements during an outage.
B2) If no, you must design a battery-less system and simply need to pick an appropriately sized efficient grid tied inverter(s) suitable for your array.

If you decide on a battery based system and you're concerned about being not being able to afford your "dream" battery now, you can buy a few small batteries (say four 12v ~100Ah FLA's wired in series for 48v @ ~100Ah, for about $300 to $400). With the 10kw array you specified, I would not even consider a 12v or 24v system, definitely 48v. With the small battery, you would "waste" very little of your array's output in daily charge and start feeding the grid very early in the day. Other than a battery-less system, you would see the greatest financial benefit using this model - low cost battery and very little solar production used in daily battery charging. You would still have limited power available from the battery during the random grid outage, albeit not much relative to your array. During an outage of a few days with a 48v 100Ah battery you could keep food refrigerated, have lights and fans at night and your entertainment system of choice without much fear of battery damage. During the day, the excess capacity of your array can power all those loads and recharge the battery from the previous nights use.

After your bank account recovers from the sticker shock of the initial system, you would have gained some valuable knowledge on battery maintenance with the small battery and your bank account would have recovered. Then we would be several years closer to "the big one" some predict and you could decide more accurately how much battery you "really" need.

If you answer the questions above we can get a better idea of what you're trying to accomplish with your system.

Cheers,

Bad Apple

Solar Guppy · September 2007

Re: Question on battery sizing

You can't run 6kW battery based gridtie inverters on 100ah starter battery bank, its just too small ...

I have run my Outback GTFX3048 with a 100ah bank as you suggested but anything over 2kWh gridtie would cause huge AC ripple on the array and would stall Mppt tracking.

For the OP, he needs 400ah Minimum per inverter, this is not a guess but testing on actual hardware on a real array

crewzer · September 2007

Re: Question on battery sizing

Looking at your signature might explain why the emi isn't an issue for you on your MX60 ... at 36V in 24V out at 600 watts is minimal , crank that upto 3kW and 100V array and let the am band be damned :roll:

Interesting! I actually have a 3kW+ / ~85 Vmp / ~100 Voc system designed for my semi-retirement home. But, installation is at least two years away, so who knows where the market and technology will be at that time.

For example, just look at this year's crop of new MPPT controllers (Analytic Systems, Apollo, Xantrex), and two more are imminent (Midnite and Morningstar).

I do like OutBack. However, my enthusiam for the company and its products is not without limits... let's just leave it at that for now.

Regards,
Jim / crewzer

Windsun · September 2007

Re: Question on battery sizing

crewzer wrote: »

I do like OutBack. However, my enthusiam for the company and its products is not without limits... let's just leave it at that for now.

Regards,
Jim / crewzer

You are not the only one

System2 · September 2007

Re: Question on battery sizing

Bad Apple wrote: »

Chuck said he is trying to avoid using a generator.

Chuck,

You have to provide some specific answers.

1) Are you designing your system based on a belief that in the near future there will be a prolonged or catastrophic grid failure?

Yes, I would be prepairing for a situation where the grid MAY be unavailable for a prolonged period.

I don't think the power is going to just go off one day and staff off but I do believe the days of "cheap" (oil and natural gas) are numbered and our entire economy is dependant on "cheap" oil. As demand increases and supplies are reduced prices will rise. At some point (five years, 25 years, I don't know) oil and natural gas will be prohibitively expensive.

So, IF I'm right, how will that affect the grid? Do prices just keep going up and up? Are there outages due to shortages? I don't know. And, I would rather be prepaired than un-prepaired.

As far as how much electricity I need, I could easily get by on 11kwh/day with rationing and consertive use. I would not be able to run the pool pump but in a situation like that, well we all have to make our sacrafices

Also, in the summer with the expectation more sunny days, we would have a little extra power to play with.

System2 · September 2007

Re: Question on battery sizing

You can't run 6kW battery based gridtie inverters on 100ah starter battery bank, its just too small ...

I have run my Outback GTFX3048 with a 100ah bank as you suggested but anything over 2kWh gridtie would cause huge AC ripple on the array and would stall Mppt tracking.

For the OP, he needs 400ah Minimum per inverter, this is not a guess but testing on actual hardware on a real array

The XW 6048 manual here http://www.xantrex.com/web/id/1871/docserve.asp , Appendix page A-5 (page 97 of the .pdf file) states that it will operate with a 100Ah to 2000Ah battery.

Cheers,

Bad Apple

Question on battery sizing

Comments

Categories