GT w/battery backup AC vs DC coupling

wisementrading · October 2011

I have 90 Evergreen 210W panels that I am using to create a grid tie with battery backup. The three systems I have found so far are the Xantrex XW 6048, Outback Radian, and Sunny Boy plus Sunny Island. It will be about 600 ft from panels to the, charge controller, battery, inverters system. The Evergreens have a STC Voc of 22.8 and the NOCT Vmp is 16.7. The record low was -11 F, the average high is 71 F, and the record high is 104 F. I was planing on using 3 XW inverters and 5 XW mppt60-150 charge controllers with 3 strings of 6 panels each, however after adjusting to the low temp a 150 scc can only take 5 panels in a string. At this voltage it would take 6 scc and I would have to use 4/0 al wire. I could use a midnight classic 200 with either Radian or XW inverters and I have talked with Midnight and they say it would work well, however mixing systems wasn't my first choice. I just discovered two other options that would let me use even higher voltage which is very good for this long a cable run. The first is the XW mppt80-600 which looks quite nice, but I was reading the installation manual which says if you have 3 inverters on the Xanbus network you can only have 2 scc and I need at least 4. That means that I would have to run 2 networks for 1 system. I called Xantrex tech support to see if this would work and the rep I talked with suggested I look into AC coupling with one of their gt inverters. This brings me to the next choice the SMA Sunny Boy/Island. SMA seems to have the AC coupling system down better then Xantrex. My questions are:

1) Would either the XW6048 or the Radion work OK with either the classic or the mppt80-600 not being on the same network? In addition to the advice from Xantrex I also found this answer in an Outback FAQ

"Yes and no. You can always mix brands and make things work with enough tinkering, but you lose out on the real advantages of a system with true networked communication. For instance, without coordinated operation in a grid-interactive installation it’s not uncommon for a charge controller to drop into float and not come out. A month later the customer wonders why their meter stopped turning backwards… With OutBack’s HUB and coordinated operation, all the devices operate as a team."

2)I see the Radian has an efficiency of 90% and the XW 95% is this really the ratings or are Outback and Xantrax testing differently?

3) It looks like the Radian can do load shedding in off grid mode can the XW?

4) Has anyone here used the Sunny Boy-Sunny Island combo either off grid or backup? I see on the Outback FAQ they say that AC coupling knocks the gt inverter off line for a minimum of 5 min and the Xantrex tech sheets seems to say that it can only turn the gt inverter on or off. However, SMA makes it sound quite nice including being able to scale back solar production when the batteries are charged and the loads are lighter then production.

5) The SMA load shedding seems like it would be an added bonus, but I have some questions. I was thinking I could create a system with the critical loads attached normally (for a system of this size this would be most of the 110V loads plus the well pump) and then attach the heavy loads (these would be the 220V items) through a load shed relay. This would allow support for all of the loads in sunny conditions with full batteries. Will this work? How much does the required relay cost? Can the conditions be set that will cause the loads to be switched off? Can Outback and/or Xantrex do this just as well as SMA?

6)
The Sunny Island has an efficiency of 95% is this for charging or AC production or do you lose the same 5% in both directions?

7) I thinking about using GB Industrial Batteries. I would like to start out with 2 24V 1576AH (at 20hr) batteries for a total of 75,648 WH. Has anyone used these batteries? Any thoughts? How about the size?

8 ) Would the Classic or the mppt80-600 cause the batteries to cycle enough each day in grid tie mode to reduce their life? It seems from what I have read that batteries that are keep charged don't reduce their life near as fast as ones that are being used daily.

9)I have read that AC coupling can cause the gt inverters in the system to shut down for 5 min. Is this only when the grid first goes down or anytime the loads change in off grid mode? Does the SMA system avoid some of the shut downs or is this a con to all AC coupling?

10) Can any or all of these brands be used in a grid support roll ie not selling to the grid just using the solar energy when available and supplementing with grid power at low battery voltage?

11)I would like to use the panels in strings of 22 each. I am thinking one string should work for each mppt80-600 or two strings for a Sunny Boy 8000US. I have checked this in a couple of string sizing tools , but would appreciate any thoughts.

12) Any thoughts on which brand your prefer or the pros and cons of AC VS DC coupling and the equipment I have listed? This is the first system over 4 panels I have done so much of it is new to me.

Thanks for the help.

mike95490 · October 2011

Re: GT w/battery backup AC vs DC coupling

WOW ! That's nearly a 20KW system. Can you even get approval for grid tie in your area for that ? I don't even know if a single drop can handle that much.

The new outback radian, is so new, few folks have any experience with it.

I would think the Midnight higher voltage controllers would be the way to go with the long cable runs you speak of, the XW80-600 is REAL expensive.

say 500V @ 40A from the solar (19,000w)(into the xw-600) That would still be 4, 80a controllers to handle it. This is not taking into effect the NEC de-rateing they are trying to apply to solar gear, which may force you into 2 more controllers.

Battery bank - I'd look into forklift batteries with auto watering system

What if you split the system into 2 or 3, all XW gear, hooked to the same battery bank, 2 controllers, 2 inverters in each network? I'm not sure how well the XW will play, managing sell, SMA does have an elegant scheme of shifting Hz, to signal to other units to throttle up or down, XW may only rail ON or OFF.

And if you use XW GT inverters, they will take 5 min to reset and re-sync to the "grid" so you loose production in 5 min, 3Kw chunks

Solar Guppy · October 2011

Re: GT w/battery backup AC vs DC coupling

Why are you using a battery based system?

If you want 24/7 off-grid ability, your going to need substantially more battery's

You can't run 22 of the Evergreens in a String

Sunnyboy works well, its cost more than double that the XW.

Tell us more about your needs, then you work backwards to design the system

Cariboocoot · October 2011

Re: GT w/battery backup AC vs DC coupling

I agree with Mike & Solar Guppy: you will save yourself a ton of money and hassle if you first define how much back-up power you actually need. Having a big grid-tie system is fine, but spending the $ for having battery back-up for the total capacity is foolish. You can have one back-up system for the critical needs like refrigeration, and then have thousands of Watts more of straight GT for a lot less money. The GT could be central or micro inverter based and would not need to "talk" to the back-up system at all.

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

Solar Guppy wrote: »

Why are you using a battery based system?

I am spending a lot of money on this system so I want to be able to have it work for multi use. Although things have gotten better over the years we still have power loss with tornadoes, ice storms etc. Some times the power is out for a week or more.

If you want 24/7 off-grid ability, your going to need substantially more battery's

I know more is better, and I would like to add 2 more batteries. However it seems to me that 2 of them should be able to support 4 to 5kw for about 12 hours at 80% DOD. If it is cloudy for days that will be a problem, we will just have to use less.

You can't run 22 of the Evergreens in a String

That is why I asked. I checked the Xantrex gt sizing tool, The SMA software, talked with tech support with both companies and tried running the temperature coefficient adjustment myself and all seem to say a max string of 23 for 600voc. So what am I missing?

Sunnyboy works well, its cost more than double that the XW.

Not really, by the time you factor in the cost of the 4 to 6 scc, the wire, and the fact that the Sunny Boy 8000 has a very nice price the difference is not that much.

Tell us more about your needs, then you work backwards to design the system

I would like to eliminate my power bill, and have a little bit of back up for power outages as well. Even the full system will not provide all the power we use, but it is a good start. I don't mind not being able to run at full power at night, but I would like to be able to use all of the power during the day even when the grid is down.

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

mike90045 wrote: »

WOW ! That's nearly a 20KW system. Can you even get approval for grid tie in your area for that ? I don't even know if a single drop can handle that much.

I am still talking with the power company. They don't believe that I really want to do solar, but they haven't said anything about the size of the system being a problem for them. I have faxed them one line drawings and specs for a XW set up. I have also talked to TVA who is the power generator. They have a very nice buy back program and they only said that a system of this size would need a different meter that keeps track of the production by time of day.
Off grid will cost me more, but if I have to I will do that and get less inverters and MORE batteries.

The new outback radian, is so new, few folks have any experience with it.

I know it is new and am a bit confused about the efficiency of it, but it does look quite nice. Then there is the issue of the charge controllers.

I would think the Midnight higher voltage controllers would be the way to go with the long cable runs you speak of, the XW80-600 is REAL expensive.

The 80-600 are pricy, but so is that much big wire.

say 500V @ 40A from the solar (19,000w)(into the xw-600) That would still be 4, 80a controllers to handle it. This is not taking into effect the NEC de-rateing they are trying to apply to solar gear, which may force you into 2 more controllers.

I think I will need 5 of them, but they are still 80A each and the Midnight is around 56A with a 48v battery bank.

Battery bank - I'd look into forklift batteries with auto watering system

The GB batteries are more or less forklift batteries

What if you split the system into 2 or 3, all XW gear, hooked to the same battery bank, 2 controllers, 2 inverters in each network? I'm not sure how well the XW will play, managing sell, SMA does have an elegant scheme of shifting Hz, to signal to other units to throttle up or down, XW may only rail ON or OFF.

The multi-system approach might work well, but then all of the inverters have to be running as long as the panels are producing any power. That is why I would like to stay with a stacked system if I can.

And if you use XW GT inverters, they will take 5 min to reset and re-sync to the "grid" so you loose production in 5 min, 3Kw chunks

Does the SMA system not have the 5 min reset?

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

Cariboocoot wrote: »

I agree with Mike & Solar Guppy: you will save yourself a ton of money and hassle if you first define how much back-up power you actually need. Having a big grid-tie system is fine, but spending the $ for having battery back-up for the total capacity is foolish. You can have one back-up system for the critical needs like refrigeration, and then have thousands of Watts more of straight GT for a lot less money. The GT could be central or micro inverter based and would not need to "talk" to the back-up system at all.

I don't mind not being able to run at full power at night, but I would like to be able to use all of the power during the day even when the grid is down. I have read about battery backup costing too much, but if you have the ability to generate your own power and not to use it then that seems even more costly to me, so I am trying to work out a compromise. If I add more batteries in a year or two would they be eligible for the 30% tax credit or do they have to be bought with the panels and BOS?

Cariboocoot · October 2011

Re: GT w/battery backup AC vs DC coupling

wisementrading wrote: »

I would like to eliminate my power bill, and have a little bit of back up for power outages as well. Even the full system will not provide all the power we use, but it is a good start. I don't mind not being able to run at full power at night, but I would like to be able to use all of the power during the day even when the grid is down.

There is the clue to the big problem: "the power we use". First step in any kind of solar electric application is to spend your money reducing that number. You get a much better return on the dollar with conservation than you do with production.

No doubt you've already noticed how expensive PV power is. The "eliminate my power bill" part is going to come with a steep price. Unless you can get good financial incentives for the install, it probably isn't going to be worth the expense.

As for power back-up, generators tend to be more cost-effective unless you have frequent and/or "unattended" outages. Trying to supply battery back-up for whole-house consumption is impractical for typical household usage. I have two places: on-grid that uses about 16kW hours per day and off-grid that uses 2.4 kW hours per day. The latter system cost $8,000, self-installed. Just to give you some idea what you're looking at.

Cariboocoot · October 2011

Re: GT w/battery backup AC vs DC coupling

wisementrading wrote: »

I don't mind not being able to run at full power at night, but I would like to be able to use all of the power during the day even when the grid is down. I have read about battery backup costing too much, but if you have the ability to generate your own power and not to use it then that seems even more costly to me, so I am trying to work out a compromise. If I add more batteries in a year or two would they be eligible for the 30% tax credit or do they have to be bought with the panels and BOS?

This is a problem lots of people with GT systems come up against: no grid = no power. Meanwhile there's thousands of dollars of equipment producing nothing. That's why Xantrex came up with the MPPT80-600; it can connect to the high Voltage arrays typical of GT systems and charge a battery bank. It's roughly $2,000 though. That's why I suggest working out what you really need to keep running when the grid goes down and separate that from the whole-house size GT system. If you want to take the entire load off grid you have two choices: reduce power consumption drastically, or spend huge amounts of money. Not very attractive choices.

I can't advice on the rebates/tax incentives as I don't live in the U.S. Where I am we have no such programs; you want solar, you pay for all of it.

BB. · October 2011

Re: GT w/battery backup AC vs DC coupling

Have you addressed conservation as much as possible (insulation, more insulation, double pane windows, high SEER A/C and Heat Pumps, heat pump hot water, laptops, look at other loads like entertainment center, etc.)?

Very roughly, Hybrid solar power (GT with Off Grid Backup) probably will cost at least $0.45 per kWH (assuming you have to replace electronics and batteries around the 10 year mark and 20 year system life).

Pure off grid power probably costs around $1-$2+ per kWH (off-grid systems are less efficient so need more panels, batteries get cycled more and need replacement more often, more hardware is required for Hybrid and Off Grid systems, etc.).

If your need are to have emergency power for a week or two--Looking at your minimum loads and other requirements--a Genset may be a more cost effective solution.

A "good sized" off grid system that can support a refrigerator, lights, computer, well pump, TV, and clothes washer--probably around 100 kWH per month (~3.3 kWH per day) will be adequate for many people living off grid. And such a system is probably in the ~$10,000 range to setup (lots of variables, DIY vs contractor, site requirements, etc.).

As I understand, living in the south east, 1,000-2,000+ kWH per month electric bills are not uncommon--mostly driven by the cost of A/C.

While it is certainly possible to have an off-grid/Hybrid solar system that can power A/C systems, I would suggest that you look at your current power usage and double check that you could not benifit from an upgrade.

If you need A/C, but can live with just a couple of rooms being cooled during power emergencies, the newer Mini-Splits can be very nice--They play well with off grid systems. They are very efficient and don't have high starting surges for the compressors (also available in heat pump versions).

Sanyo mini split AC (inverter/variable speed)

-Bill

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

Cariboocoot wrote: »

There is the clue to the big problem: "the power we use". First step in any kind of solar electric application is to spend your money reducing that number. You get a much better return on the dollar with conservation than you do with production.

Already working on that. We have solar hot water, with a compressor hw backup. 100% solar heat, a wood cook stove for use in the winter. About half of the lighting is now LED and we are converting more all the time. We just installed a 26seer split mini AC system. I have a kill-a-watt meter for testing 110V loads and a BlueLine meter for keeping track of whole house loads and am testing some smart power strips to cut down on the phantom loads. We are running two businesses from home so we are here most of the time and that adds to the usage somewhat as well. What do you recommend next, besides just using less electric items?

No doubt you've already noticed how expensive PV power is. The "eliminate my power bill" part is going to come with a steep price. Unless you can get good financial incentives for the install, it probably isn't going to be worth the expense.

That is what my power company keeps telling me, but I think the age of solar power has arrived and the more we work together to get great systems going the more cost effective it will get. The roi on paper is about 5 to 7 years, so I would think double that might be reasonable in the real world and those are numbers I am comfortable with.

As for power back-up, generators tend to be more cost-effective unless you have frequent and/or "unattended" outages. Trying to supply battery back-up for whole-house consumption is impractical for typical household usage. I have two places: on-grid that uses about 16kW hours per day and off-grid that uses 2.4 kW hours per day. The latter system cost $8,000, self-installed. Just to give you some idea what you're looking at.

Generators also use fuel that has to be stored and is getting quite expensive it self. When you start looking at the numbers batteries and a few electronics don't seem that bad after all. How long ago did you install your system? $8k for 600 to 800W off grid system with self install seems a bit high at today's prices. Or do you mean that 16kw per day GT plus the 2.4kw per day OG cost $8K together?

BB. · October 2011

Re: GT w/battery backup AC vs DC coupling

Marc is in Canada--so his prices (historically) for solar hardware have been higher than the US. Shopping around in the US (web) may get you better pricing.

And he is talking about $8,000 for 2.4-3 kWH per day (again, being in Canada does limit solar power to a degree during the winter--However, outside of the desert southwest US, there are frequent weather issues that affect winter and summer solar collection too).

And, remember that (on average) you will have to replace your electronics (inverters, charge controllers) and battery banks (batteries that last 2x longer seem to cost 2x as much) every 10 years or so.

And with limited storage of off-grid (battery power), you can only store several days worth of usable energy. So, you will have times of bad weather and heavy loads where you will have to shed loads and/or use a genset (or grid power) for backup.

Pure Grid Tied Inverter Solar power (no batteries, no backup capabilities) are getting down in the ~$0.10 to $0.30 per kWH range. And for some states (like California where people can be paying $0.52 per KWH in the summer for their A/C power), it can actually make economic sense.

In the end, because solar power is so expensive, and most of us here are so cheap :roll:, we just want to make sure you have done the conservation (which it sounds like you have) and load measurements/calculations so that your buy/build the system that best meets your needs.

-Bill

solar_dave · October 2011

Re: GT w/battery backup AC vs DC coupling

WOW how did you figure that you needed this much solar to get to zero bill? I have a 2500 sq ft house in AZ, with a 700 sq ft shop office. We also charge a Chevy Volt. With net metering and TOU plans from the utility, a 12.5 Kw system gets us to about $300 annual electric bill, mostly for off peak AC inn July and Aug. We have summer days where we use well over 100 kWh and in Aug we averaged about 120 kWh a day. While our peak production runs 60-75 kWh a day, the net metering plan has banked kWh to cover the excess from earlier in the year.

If you have done all the conservation stuff where is your large power usage? What does your utility say about their net metering plan?

Fortunately I have reliable power but if I didn't I would look at a nat gas/propane generator with automatic transfer switching. The cost for occasional usage is much less than a battery setup which has fairly large losses and high equipment expenses. Those fuel sources don't have the storage issues as well.

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

BB. wrote: »

Have you addressed conservation as much as possible (insulation, more insulation, double pane windows, high SEER A/C and Heat Pumps, heat pump hot water, laptops, look at other loads like entertainment center, etc.)?

We have double pane Anderson Windows. Almost no windows on the north, east or west sides of the house. The south side has overhangs designed to minimize solar gain in the summer while allowing it in the winter. We have some Suntunnel tubular skylights which I like very much. They provide a lot of light and not that much heat. The insulation is about 8" in the walls and 12" to 16" in the roof all fiberglass. I wish we had a radiant heat barrier in the roof, but I not sure the cost of retrofit would be worth it.

Very roughly, Hybrid solar power (GT with Off Grid Backup) probably will cost at least $0.45 per kWH (assuming you have to replace electronics and batteries around the 10 year mark and 20 year system life).

That is one of the reasons I am thinking about the SMA system. The Sunny Boys have a 10 year warranty vs 5 for the others. Also, I am thinking the AC coupling might be easier on the batteries.

A "good sized" off grid system that can support a refrigerator, lights, computer, well pump, TV, and clothes washer--probably around 100 kWH per month (~3.3 kWH per day) will be adequate for many people living off grid. And such a system is probably in the ~$10,000 range to setup (lots of variables, DIY vs contractor, site requirements, etc.).

The cost of adding backup to a grid tie will be less per watt then a straight OG system and the 30% tax credit will help.

As I understand, living in the south east, 1,000-2,000+ kWH per month electric bills are not uncommon--mostly driven by the cost of A/C.

While it is certainly possible to have an off-grid/Hybrid solar system that can power A/C systems, I would suggest that you look at your current power usage and double check that you could not benifit from an upgrade.

If you need A/C, but can live with just a couple of rooms being cooled during power emergencies, the newer Mini-Splits can be very nice--They play well with off grid systems. They are very efficient and don't have high starting surges for the compressors (also available in heat pump versions).

Sanyo mini split AC (inverter/variable speed)

-Bill

Yes, AC is a power hog, however PV produces much more power in the summer anyway. I am ok with not having AC when the grid is down (this happens more in the fall, winter and spring anyway), but I would still like to be able to use all the power from the panels in the day time as I don't see that costing that much extra. The Split mini we have is a Fujitsu and we have been happy with it so far. This is its first year so we will see long term.

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

solar_dave wrote: »

WOW how did you figure that you needed this much solar to get to zero bill? I have a 2500 sq ft house in AZ, with a 700 sq ft shop office. We also charge a Chevy Volt. With net metering and TOU plans from the utility, a 12.5 Kw system gets us to about $300 annual electric bill, mostly for off peak AC inn July and Aug. We have summer days where we use well over 100 kWh and in Aug we averaged about 120 kWh a day. While our peak production runs 60-75 kWh a day, the net metering plan has banked kWh to cover the excess from earlier in the year.

We are not quite as good a location for solar as AZ, also I have a number of people in my house hold, plus 2 businesses which keeps us home most of the time and adds several computers, printers, fax network etc to the AC loads as well.

If you have done all the conservation stuff where is your large power usage? What does your utility say about their net metering plan?

It all adds up you know. We do use an electric stove and dryer. Actually everything is electric. The power generator will pay us more for the power we generate then for the power used that is why we don't have to generate 100% of the usage for a zero bill.

Fortunately I have reliable power but if I didn't I would look at a nat gas/propane generator with automatic transfer switching. The cost for occasional usage is much less than a battery setup which has fairly large losses and high equipment expenses. Those fuel sources don't have the storage issues as well.

We don't have ng were we live and lp is through the roof price wise. I already have some small generators and am not that interested in a lager one until some other fuel is available like maybe algae oil or bio gas. I am keeping my eye on these, but the tech is just not there yet.

Cariboocoot · October 2011

Re: GT w/battery backup AC vs DC coupling

To shorten the discussion of theory and get down to the practical, do you have a daily kW hour average use figure to work with?

My wife is an accountant and works from home. That office set-up is the major power user off-grid. The biggest improvement we could make there was to get rid of the desktop computer in favour of a laptop: 100 Watts per hour saved!

What Bill said about my circumstances is true: $8,000 for the 2.4 kW hour off-grid set-up in 2008. I could get a bit more power for a bit less money today, but Canada is expensive. Gasoline is $4.96 per gallon here (US gallon & dollar), and even more down around Vancouver. Thankfully the Honda gen doesn't use much, because it's been clouded over and raining for days and it has had to supply all the charge power.

(Looks like the EU2000i won't make the 1000's record of 6,000 hours, as the pull cord is fraying.)

Mike's suggestion of dividing up into separate segments is sound. You just have to keep track of which circuits are on which inverter and possibly provide a bit of cross-over potential in case of failing. I've just been reading up on the Radian and it has some very nice features to it. We'll see how they work out.

BTW, there is no problem having controllers and inverters that are not connected to each other (as in a MidNite Classic with a Magnum inverter) so long as the controller is the primary charge source. When you run GT-Hybrid, it becomes tricky because you have to get the controller and inverter to agree on the battery state. Otherwise there are "differences of opinion" as to battery Voltage, absorb time, et cetera.

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

Cariboocoot wrote: »

To shorten the discussion of theory and get down to the practical, do you have a daily kW hour average use figure to work with?

My wife is an accountant and works from home. That office set-up is the major power user off-grid. The biggest improvement we could make there was to get rid of the desktop computer in favour of a laptop: 100 Watts per hour saved!

We use about 150KWH per day. The last two computers we got are laptops, being grid connected is still different then being totally off grid. There are somethings that you just can't do of grid like leaving several computers running 16 hrs a day and using electric stove etc.

Mike's suggestion of dividing up into separate segments is sound. You just have to keep track of which circuits are on which inverter and possibly provide a bit of cross-over potential in case of failing. I've just been reading up on the Radian and it has some very nice features to it. We'll see how they work out.

The problem I see is say the panels are producing 6kw then each of the 3 inverters will have to be powered up producing 2kw each. This seams like a wast to me, but maybe it is not that bad. Any thoughts?

BTW, there is no problem having controllers and inverters that are not connected to each other (as in a MidNite Classic with a Magnum inverter) so long as the controller is the primary charge source. When you run GT-Hybrid, it becomes tricky because you have to get the controller and inverter to agree on the battery state. Otherwise there are "differences of opinion" as to battery Voltage, absorb time, et cetera.

The grid tie is what I am concerned about with mixing brands. See the quote from Outback in my first post.

solar_dave · October 2011

Re: GT w/battery backup AC vs DC coupling

wisementrading wrote: »

We are not quite as good a location for solar as AZ, also I have a number of people in my house hold, plus 2 businesses which keeps us home most of the time and adds several computers, printers, fax network etc to the AC loads as well.

As far a AZ for Solar it is pretty darn good but the losses from higher panel temps are significant. My best month is May which one would think normally would be June or July but panel heating brings down my output. My AC usage is extreme with two 3 ton Trane 2 speed heat pumps and a 3 ton Friedrich mini split in the external building. We run 3 ceiling fans 7X24, moving air feels cooler. Like I said before my Aug. usage averaged 120+ kWh a day.

I work from home and have 2 mac mini computers with five 24 inch monitors attached on 7X24, also a mac laptop with a 20 inch monitor. The wife and grand kids each have a mac mini as well on standby mostly. All have low power monitors. These systems draw very low wattage on standby. There are 2 printer in standby 24x7 as well. The Grand kids are here as much as they are at home, mama works nights.

It all adds up you know. We do use an electric stove and dryer. Actually everything is electric. The power generator will pay us more for the power we generate then for the power used that is why we don't have to generate 100% of the usage for a zero bill.

We have both electric stove and electric dryer, plus a VFD pool pump that runs 4 hours a day. Something still seems out of whack on your usage. How does your utility net metering work? You need to understand this pretty intimately to get a good sizing estimates as well.

Like the Coot has said, time to understand your usage, load patterns and the utility system to get to a design. What does the pvwatts site say about your possible production? It is reasonable good at predicting your solar gain. For just a grid tie system in the 20Kw range you are still talking $85K to $100K before credits or rebates or any part of the battery backed setup.

We don't have ng were we live and lp is through the roof price wise. I already have some small generators and am not that interested in a lager one until some other fuel is available like maybe algae oil or bio gas. I am keeping my eye on these, but the tech is just not there yet.

For emergency back up, prices the cost of fuel is really not a consideration vs cost to maintain a full time battery system. The usage should be so low as it is only emergency usage. You take in to account the cost of a large battery system, charge controllers and inverters along with the associated maintenance cost and PITA level then the cost of a tank of propane and a generator is negligible. A ground mount generator can be sized large enough that even your AC could be run on it. Some thing like this for about $4K - $5K could run your whole operation.
http://www.northerntool.com/shop/tools/product_200448670_200448670

If your reasoning is not cost based and based on some other criteria then i guess battery system could make sense.

solar_dave · October 2011

Re: GT w/battery backup AC vs DC coupling

wisementrading wrote: »

We use about 150KWH per day.

That is huge, is that peak summer load or a normal day all year long? You really don't want to design around peak with net metering but around the smoothed out usage over time. The time base is determined by your net metering plan.

Do you have a Time of Use plan? Replacing Peak time of use is the most cost effective use of Grid tie. I can't even imagine the costs and sizing on replacing 150kWh a day with battery system.

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

solar_dave wrote: »

That is huge, is that peak summer load or a normal day all year long? You really don't want to design around peak with net metering but around the smoothed out usage over time. The time base is determined by your net metering plan.

Do you have a Time of Use plan? Replacing Peak time of use is the most cost effective use of Grid tie. I can't even imagine the costs and sizing on replacing 150kWh with battery system.

I am not trying to replace that, just supplement it. That is the year round average, but it doesn't change as much as you would think summer to winter. We only have one 3 ton (13 seer) AC plus the new split mini that we added to replace some of the load off of the main unit. We also have a whole house fan that we use when the weather is ok to replace the AC. Even though we have had the additional AC and heat pump water heater for about 9 mouths I am having trouble with our utilities not reading the meter every month (some they just estimate) so I can't get a great view, but our usage should have gone down quite a bit based on the readings from the BlueLine meter. The metering plan from TVA is not net metering. They have a two meter system where they pay more for the solar then they charge for the usage. The generation credit is refundable, but the utilities can choose to carry any extra generation credit forward against extra usage for up to 12 months. The usage cost the same any time of the day, but if you used too much in a month (I think over 2,000KWH) they have a surcharge.

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

solar_dave wrote: »

Like the Coot has said, time to understand your usage, load patterns and the utility system to get to a design. What does the pvwatts site say about your possible production? It is reasonable good at predicting your solar gain. For just a grid tie system in the 20Kw range you are still talking $85K to $100K before credits or rebates or any part of the battery backed setup.

Ok, I know I can't replace all off our usage with solar and so I am not trying to. We are connected to the grid so there is no real reason to and I don't think the utilities would let us do a system that large, plus I don't have the money. Have been saving up for years as it is. I already have the panels only need BOS, and have the racking picked out. This just leaves the electronics. With battery back up it should cost $3.00 per watt +or- before tax credits.

Solar Guppy · October 2011

Re: GT w/battery backup AC vs DC coupling

A couple of observations to the information posted.

OP mentions TVA, they are primarily cheap dirty coal and hydro powered and the TVA has close to the lowest electric rates in the US. I believe its 7-8 cents kwh.

OP mentions conservation , which doesn't match the 155kw/day usage BUT

OP mentions "two businesses" are part of the load

The TVA does have a program listed that pays above retail
http://www.tva.com/greenpowerswitch/partners/

But it has a couple of unknowns, as the size is limited to the local power company's restrictions, not necessarily the TVA's program and the meters could be an issue as if dual meters are required, then you can't have the solar AC coupled. AC coupling can only work when its done on the load side of the inverters, so the inverters disconnect prevents back feeding to the grid when the grid is down.

There is also the tax question, what should be reported to the IRS as income ?, with a system this large and your generation will be being paid more than the retail value of the electricity ( especially if dual metered ), I would think the IRS would treat that as earned income.

Keep in mind, the TVA system must be installed by a licensed electrician and with permits and sign off, so this isn't a DYI project, at least in part. As to a 3.00 watt installed, if you do everything yourself , home grown racking and wiring its possible with straight gridtie. Battery based not a chance, there are allot of added costs, breakers ( DC side ), 4 AUT wiring, bus bars and UL listed combiners or enclosures for the inverters DC section which is not part of the list price for the inverter. For the XW, the raceway is like 1200 bucks

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

Solar Guppy wrote: »

A couple of observations to the information posted.

OP mentions TVA, they are primarily cheap dirty coal and hydro powered and the TVA has close to the lowest electric rates in the US. I believe its 7-8 cents kwh.

OP mentions conservation , which doesn't match the 155kw/day usage BUT

OP mentions "two businesses" are part of the load

The TVA does have a program listed that pays above retail
http://www.tva.com/greenpowerswitch/partners/

But it has a couple of unknowns, as the size is limited to the local power company's restrictions, not necessarily the TVA's program and the meters could be an issue as if dual meters are required, then you can't have the solar AC coupled. AC coupling can only work when its done on the load side of the inverters, so the inverters disconnect prevents back feeding to the grid when the grid is down.

Either the utilities will do grid tie or they won't that is not up to me. Even though I have the panels in my hands they still don't believe that I am ready to go. TVA has nukes too, but I am not sure that is any better! I am think AC coupling should work with the addition of an ATS like would be used for a backup generator everyone keeps talking about. BTW, you can't use the transfer switch built into the XW with two meters either. If you do only part of the power will be sent though the generation meter.

There is also the tax question, what should be reported to the IRS as income ?, with a system this large and your generation will be being paid more than the retail value of the electricity ( especially if dual metered ), I would think the IRS would treat that as earned income.

I am not planing on getting a refund only getting my power bill down to as close to zero as I can. The only electricity I deduct is for home-office which I will not miss that much. I would think the rest would be treated like a rebate on a cc (which is currently like a coupon), but I will check to be sure once the systems is up and running. Any thoughts on the Sunny Island vs XW?

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

Solar Guppy wrote: »

Keep in mind, the TVA system must be installed by a licensed electrician and with permits and sign off, so this isn't a DYI project, at least in part. As to a 3.00 watt installed, if you do everything yourself , home grown racking and wiring its possible with straight gridtie. Battery based not a chance, there are allot of added costs, breakers ( DC side ), 4 AUT wiring, bus bars and UL listed combiners or enclosures for the inverters DC section which is not part of the list price for the inverter. For the XW, the raceway is like 1200 bucks

Why do you think I am talking about the mppt80-600 or the Sunny Island? Also, this is why the SMA system is not that much more expensive when compared to the XW. My question is which to you like better and why?

solar_dave · October 2011

Re: GT w/battery backup AC vs DC coupling

So they measure all your generation and pay more than base rates for it? By how much? That sounds like a way good deal.

Cariboocoot · October 2011

Re: GT w/battery backup AC vs DC coupling

solar_dave wrote: »

So they measure all your generation and pay more than base rates for it? By how much? That sounds like a way good deal.

Sounds like Ontario's program, where they buy solar electric for sixty cents more than they sell it for resulting in the need for an over-all rate increase because they're losing so much money on the deal.

In accounting this is known as "inability to do basic math" or "stupid economics". :roll:

Solar Guppy · October 2011

Re: GT w/battery backup AC vs DC coupling

You won't be allowed to use an ATS, that not a UL1741 approved device ( at least I am not aware of one ), you have to use the transfer built into the inverters as they have the UL1741 approval which is a requirement for grid-tie

Sunny Island is about 2X the cost of the Xantrex XW, as for where the solar comes in , either system can use DC charge controllers or grid-tie inverters from any manufacture. A all SMA design is more intelligent on throttling back when the AC is not used on the "local grid" where the XW's will causes a 5 minute time out, but this only comes into play when the grid is down, when gridtie, all the enegry is exported to the AC1 input of the inverters ( not sure what the SMA nomenclature is on this ).

If you intend to primarily sell to the grid, you'll need the same number of Sunny Islands or XW-6048's as its these units that have the transfer switches,

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

solar_dave wrote: »

So they measure all your generation and pay more than base rates for it? By how much? That sounds like a way good deal.

They pay the retail rate plus 12 cents per kw. Yes, it is a good deal.

BB. · October 2011

Re: GT w/battery backup AC vs DC coupling

I think we ran into a poster here that wanted to use a Hybrid Inverter system with a utility that wanted to "pay" for all power generated and charge for all power used.

With the XW Hybrid--there is no simple place (that I am aware of) to install the power generated billing meter(s) and the power used meter(s).

-Bill

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

BB. wrote: »

I think we ran into a poster here that wanted to use a Hybrid Inverter system with a utility that wanted to "pay" for all power generated and charge for all power used.

With the XW Hybrid--there is no simple place (that I am aware of) to install the power generated billing meter(s) and the power used meter(s).

-Bill

I got the attached file from Xantrex. I am not sure about the UL1741 Solar Guppy talked being a problem. I will have to look into finding a complaint ATS. I would think that it would be available if Xantrex had this drawing made unless it was for use outside the US.

wisementrading · October 2011

Re: GT w/battery backup AC vs DC coupling

Cariboocoot wrote: »

In accounting this is known as "inability to do basic math" or "stupid economics". :roll:

Yes it is rather odd, LOL

GT w/battery backup AC vs DC coupling

Comments

Categories