Eletrician Going Off Grid - Sizing / Questions

tr0y · July 2010

Hi All -

I have been lurking quite a bit and learning all I can so thanks for the help and sharing in advance.

Here is my story, I am an electrician but never really done any PV / Renewable work, so I bought an off the grid house on a mountain outside of Las Vegas, it was unfinished and a pile of solar gear was in the garage, this is what I installed :

1) Xantrex 5548 SW inverter
1) Xantrex AC load center
1) Xantrex DC load center
1) Outback MX60 charge controller
1) Midnight Solar MPV12 combiner box w/ 2 15A breakers
8 eight) 6V 305 Amp Hour SunXtender AGM batteries Mfg Date 04/2010 - 4/0 cables
10) Evergreen Solar 205W panels in two 5 panel strings of 60V each
1) 8500W Onan propane generator rejetted for 7000' elevation w/ auto start feature

So here are my questions:

1. Is there a formula to figure out balance of PV panels and battery amp hours ?
2. What is the best way to figure out solar needs of a household ?
3. It seems that many people really oversize wire in the solar biz, even for DC, other than being overkill engineering,is there a practical reason for this ?
4. I really enjoyed doing the solar work, any tips for a grid based electrician thinking of making a jump to doing more alternative energy work ?

Thanks and I look forward to being part of your community

troy

niel · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

well, it really isn't as difficult as you might think, but yes, we split hairs sometimes and haggle over rules. some of the stuff you have is discontinued and i wonder if those batteries are properly charged. they are very good batteries and can hold a charge a long time compared to standard flooded batteries, but if the age of the rest of the stuff is any indication then those could be junk now if they weren't cared for in the meantime. in reading more closely the date you indicated for the batteries then there isn't any problem and forget what i said as i thought they may have been much older.

as to your questions;
1> not sure what you are asking here.
2> that is by the watt hours you use on your loads. pick the month you need the highest power and whittle that down to a daily average need. btw, winter is the worst time for solar collection.
3> this is not overkill as smaller voltages mean higher losses when the voltage drop rears it's ugly head. a 1v drop at 120vac is far less devastating than that same drop at 24v or 12v. to make matters worse is that higher currents may be needed at the lower voltages in keeping up with wattage making the voltage drop worse watt for watt. remember power in watts = volts x amps. now in grid tied systems this wire can be in line with or even smaller than what you might be used to installing as voltages can be in the hundreds of volts up to about 600v.
also note that wattage losses when dealing with utility prices per watt/hr are a bit different as solar prices per watt/hr are higher or at a premium.
4> i don't see why not, but some states do require certification. i guess the easiest way is to approach an installer near you for advice and maybe even employment if he is hiring.

BB. · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

Troy,

Welcome aboard. I am going to give you some advise--but remember that there are differing opinions and much of what I will say is based on rules of thumbs--I.e., the answers are not exact, but you should have good reasons for deviating from the basic recommendations.

tr0y wrote: »

1) Xantrex 5548 SW inverter

The SW series has been out of production for quite a while now and may be very hard or next to impossible to get repaired if it fails. Otherwise, as I understand, they were a good inverter/charger for their time.

You will probably have to think long and hard if you want to install this guy and cross your fingers--Or possibly find it a home on EBay for somebody looking for a spare/replacement for their existing setup (changing out an inverter/charger for a different/newer model can have its own set of pleasures :roll:).

10) Evergreen Solar 205W panels in two 5 panel strings of 60V each
1) 8500W Onan propane generator rejetted for 7000' elevation w/ auto start feature

So here are my questions:

1. Is there a formula to figure out balance of PV panels and battery amp hours?

Two answers you are looking for here... First is there is a recommended charging rate for batteries... Roughly, 5-13% of the 20 Hour capacity for your battery bank is a good place to aim for...

Below 5% for Flooded Cell Batteries, you may not end up charging completely without using a genset.

Above 13%, for flooded cell batteries, it can cause them to overheat (too much current)--and is and expensive set of solar panels vs amount of storage capacity.

AGM's can go a bit low and quite a bit higher on charging ratio--but it is good place to start with the rule of thumb.

So, to calculate the rule-of-thumb amount of solar panels:

58 volts charging * 305 AH battery bank * 1/0.77 solar panel derating * 0.05 charging ratio = 1,149 watts of solar panels RoT minimum
58 volts charging * 305 AH battery bank * 1/0.77 solar panel derating * 0.13 charging ratio = 2,987 watts of solar panels RoT maximum

The above is for your existing battery bank... Normally, we would first ask you your loads and size the battery bank for 3 days of no sun and 50% maximum discharge--so, in your case, we would expect your battery bank would be:

48 volts bank * 305 AH * 1/3 days * 0.50 max discharge = 2,440 WH or 2.44 kWH per day average consumption

So--you are doing it a bit backwards here... Obviously, we would also need to size the solar panels to support your average daily load--Winter/Summer usage patterns. For many folks, it works out that they can run ~9 months of the year on solar and 3 months of the year use the genset a few hours a week to cover for bad weather/poor sun.

To give you an idea of how much power 2,050 watts of solar panels will generate in the Las Vegas area, we can use a web site like PV Watts to estimate your monthly output. For your system, I use a derating factor of ~0.59 for off grid solar (0.77 derating for panel and charge controller), AGM of 0.90, and 85% efficient inverter).

"Station Identification"
"City:","Las_Vegas"
"State:","Nevada"
"Lat (deg N):", 36.08
"Long (deg W):", 115.17
"Elev (m): ", 664
"PV System Specifications"
"DC Rating:"," 2.0 kW"
"DC to AC Derate Factor:"," 0.590"
"AC Rating:"," 1.2 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 36.1"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 9.7 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 5.19, 181, 17.56
2, 5.98, 184, 17.85
3, 6.57, 223, 21.63
4, 7.32, 236, 22.89
5, 7.42, 240, 23.28
6, 7.35, 221, 21.44
7, 7.37, 224, 21.73
8, 7.29, 222, 21.53
9, 7.31, 219, 21.24
10, 6.56, 215, 20.86
11, 5.75, 189, 18.33
12, 5.05, 175, 16.98
"Year", 6.60, 2528, 245.22

So, for your system, the estimate is around 175 kWH a month in December and 240 kWH in May:

175 kWH / 30 days = 5.8 kWH per day
240 kWH / 30 days = 8 kWH per day

So, looking at your system, you have a lot of solar panels. So, you could add more batteries or run a whole bunch of loads during the day.

2. What is the best way to figure out solar needs of a household ?

Use a Kill-a-Watt meter for smaller AC loads or something like one of these DC Amp*Hour / Watt*Hour meter for smaller DC loads.

3. It seems that many people really oversize wire in the solar biz, even for DC, other than being overkill engineering,is there a practical reason for this ?

Yes, efficiency and voltage drop.

NEC (if I recall correctly) generally allows 3% power loss on a run of wire. Most people when they are running their wiring aim for 2 or even near 1% loss for their wiring runs to reduce lost power.,

Second, voltage drop. For 12 volt systems, you may run your battery down to 11.5 volts (heavy load, well discharged) and your inverter/devices "cut out" at 10.5 volts. So, that gives you ~1.0 volts allowed for wiring drop.

For higher voltage systems, like 48 volts, the cutout voltage is around 42 volts--so you have more available voltage drop (46 volts minimum battery - 42 volt cutout = 4 volts to play with).

4. I really enjoyed doing the solar work, any tips for a grid based electrician thinking of making a jump to doing more alternative energy work ?

Pretty much--do your first install (and make your mistakes

)--the second install you will be much more comfortable with the requirements. There are a few NEC requirements for DC solar systems that are a bit obscure--But you accept them an move on :roll:.

I will stop here--A bit of this is backwards (hardware first, load calculations second).

-Bill

I forgot to add a couple links:

PV and the 2005 NEC -- Reference Document

And just a lot of interesting links and some sizing discussions (needs to be cleaned up--posts are not organized):

Working Thread for Solar Beginner Post/FAQ

-Bill

jeffkruse · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

"10) Evergreen Solar 205W panels in two 5 panel strings of 60V each"

I don't think 60V is correct. Mine are much higher.

mikeo · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

Evergreen Solar 205W panels in two 5 panel strings of 60V each

I think he means 5 strings of 3 panels which would be about 60 volts, but is not enough voltage to charge a 48 volt bank. He could wire it to 3 strings of 5 panels which would work with his MX60 at 100 volts.

Edit misread OP, should be 2 strings of 5 panels with the panels he has.

jeffkruse · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

I think he said he has 10 pannels. If they are like mine I have 2 strings of 5 pannels each. Works great.

Dave Angelini · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

Live with what you have installed for a year and then fine tune the requirments!
You are light on the capacity of your battery and you will have to be careful with your loads during the winter or run the generator more.

The mistake that often happens with a small battery is that the user ends up cycling them more than once a day. This can be avoided if you can controll your loads in the winter. AGM's are great batteries but they are limited in their ability to cycle. Control your loads, complete charge daily, avoid deep cycles!

You may not have heard the line that goes, "there are two kinds of people, those who want solar and those who want more" You can really never have too much!

Good Luck

BB. · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

Also, I should add that sizing your genset to your actual charging/operational loads is important too...

Many folks end up with a smaller battery bank / loads (conservation is your friend for off-grid systems--Almost always cheaper to spend money on efficiency/conservation than to throw money at generating the power) and a relatively large AC generator set.

Yes, it is hard to find an "industrial/prime mover" small genset--but running a large genset at less than 50% load can waste fuel... Especially if the loads are 10-20% of rated power, you may be using ~2-4x as much fuel per kWH of power consumed than a smaller genset would use.

For folks with diesel gensets, they normally should be operated at 50-60% minimum load or they carbon/coke up (and need cleaning).

Sometimes the answer is two gensets... One smaller one to charge the battery bank and a second large set for shop power / backup power.

-Bill

tr0y · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

Hi All,

Thanks for all the answers, to clarify the Evergreen panels are 12V X 5 in series to yield 60V, 2 strings into the combiner for 120V into the MX60.

Yes I think I may be a bit light on batteries, but I am sure that will change. Is there a formula / rule of thumb, to figure out batteries to panel ratios ? Or do I just go buy more till I can't get a full charge then back off ? i.e. build a system in balance ?

To be honest I have been running the house like an on-grid house with the current set up for a few weeks now and have not seen a standing voltage (3 or so hours of no charging or inverting) of the battery bank below 48.9, according to the battery mfg should be fine or do their engineers just want me to buy batteries ? The Concorde folks seem to think 50% DOD is fine for their product, is this not the case in real world use ?

As for a full charge, I am seeing the MX60 drop into float mode by 10:30 or 11 AM on a daily basis and not change status unless we really pull hard on the electricity, today that included running a tile saw, microwave, a 55" TV, cable box and 2 computers plus normal household stuff all in all about 43A for over an hour then it dropped back to the standard 10 to 15A load of daytime household operations. Within 5 minutes the MX60 resumed float status is that wrong or odd ?

My next question as Dave Sparks mentioned is winter, since we are in the capital of sun, with 6 hours a day@ 7000' as yearly avg, would not I be more efficient in winter due to the sub freezing temps outside ? Is not heat the enemy of solar panels for efficient generation of power ?

My final question relates to a line in the response from BB " (conservation is your friend for off-grid systems--Almost always cheaper to spend money on efficiency/conservation than to throw money at generating the power)"

While I agree with that would it not be a waste to use all of what you generate as that would actually lower your cost per watt ? i.e. if you generate 1000 watts but only use 200, does it not cost the same to generate 1000 and use it as it does to generate 1000 and only use 200 ? After the cost of the system the watt cost is the same so using more actually makes it more efficient ? Is that wrong ?

Thanks All !

BB. · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

tr0y wrote: »

Thanks for all the answers, to clarify the Evergreen panels are 12V X 5 in series to yield 60V, 2 strings into the combiner for 120V into the MX60.

A bit confused... Are you combining the two 5x series string in parallel (double the current, same voltage) or the two 5x strings in series (double the voltage, same current)?

Right now, in cold weather (14F), you Voc of the 5x series, then parallel the two strings configured array is around 126 volts (Vmp hot is 75F)--Those are both fine.

Remember, these are called "12 volt" panels because they are configured optimally for charge at 12 volt battery bank behind a PWM controller. Vmp (voltage maximum power at STC rating is Vmp=18.2 volts--assuming I got the correct Evergreen product).

If you ran 10x panels in series--your nominal output would over voltage your charge controller.

Yes I think I may be a bit light on batteries, but I am sure that will change. Is there a formula / rule of thumb, to figure out batteries to panel ratios ? Or do I just go buy more till I can't get a full charge then back off ? i.e. build a system in balance ?

We try and size the batteries based on your daily load. 3 days of no-sun and 50% maximum discharge--or 6x daily load for battery bank capacity. That is a fairly conservative bank and you could pull it back a bit in your very sunny region if you wanted.

Cost wise--If you doubled the bank size, it would last a little bit more than 2x as long (assuming it did not just age out). Basically, the bank lasts a bit more 2x longer with 2x as many batteries (shallower cycling give longer cycle life)--but it cost you 2x as much for the batteries--so the cost is almost the same. Now, having battery bank that only needs replacing every 10 years instead of a 1/2 size bank every 4-5 years may be a good choice--but that is your call.

To be honest I have been running the house like an on-grid house with the current set up for a few weeks now and have not seen a standing voltage (3 or so hours of no charging or inverting) of the battery bank below 48.9, according to the battery mfg should be fine or do their engineers just want me to buy batteries ? The Concorde folks seem to think 50% DOD is fine for their product, is this not the case in real world use ?

See above in the sense that the cost per kWHr is not much different in the end between heavy usage with few batteries or light usage with many batteries.

The big thing that you can do for your bank life is limit the amount of time that the battery spends below ~75% state of charge... If it sends days/weeks/months, the sulfate hardens (crystallizes) and the battery looses capacity. The hardened lead sulfate crystals are "...difficult, if not impossible, to reconvert back to active material" (per Concord pdf manual).

Concord, for their aircraft batteries, says:

4. Can I eliminate the parasitic drain?

The items responsible for the parasitic drain are generally integrated in the aircraft electrical system and alteration may affect FAA certification. In some aircraft, modifications can be made to reduce or eliminate the parasitic drain. Contact the aircraft manufacturer for more information on this subject.

5. How can I mitigate the effect of a parasitic drain?

To protect the battery from the effects of parasitic drain (i.e., sulfation, short life and/or AOG), the following procedure is recommended:

a. Measure the parasitic drain as described above in paragraph 2.

b. Determine how long it will take to deplete 25% of the battery capacity using the following formula:

Time (hours) = 0.25 x C1/Ip, where
C1 is the battery’s rated capacity in Ah and Ip is the parasitic drain in amperes.

For example, if C1 = 28Ah and Ip = 0.05 amperes (50 milliamperes), then
Time (hours) = 0.25 x 28/.05 = 140 hours = 5.8 days.

c. If the aircraft will be inactive for more than the time calculated in step (b), either:
• Disconnect the battery plug (preferred), or
• Connect an approved maintenance charger to the battery (effective, but may reduce the...

As for a full charge, I am seeing the MX60 drop into float mode by 10:30 or 11 AM on a daily basis and not change status unless we really pull hard on the electricity, today that included running a tile saw, microwave, a 55" TV, cable box and 2 computers plus normal household stuff all in all about 43A for over an hour then it dropped back to the standard 10 to 15A load of daytime household operations. Within 5 minutes the MX60 resumed float status is that wrong or odd ?

This is a 48 volt battery bank? If your load is:

48 volts * 43 amps = 2,064 watts
2,050 watts * 0.77 typical derating = 1,589 watts average maximum power from array

It would be nice to know the Amp*Hour or Watt*hour of your loads... It would seem that you are drawing about 500 watts from your battery bank than the solar array would provide on average in the middle of the day (you are at altitude--so you may get a bit more--but hot days will reduce it too)...

It seem that the MX60 should not go right back to float--but I guess it is not out of the question that it could (depending on the accuracy of the numbers and if the battery bank was already recharged to 100%).

My next question as Dave Sparks mentioned is winter, since we are in the capital of sun, with 6 hours a day@ 7000' as yearly avg, would not I be more efficient in winter due to the sub freezing temps outside ? Is not heat the enemy of solar panels for efficient generation of power ?

Yes, but you have 2 hours or so less sun (assuming a fixed array)--So your overall energy harvest will still be less in winter--and typically the average winter "benefit" is in the 10-15% or so range--Not 25%+...

My final question relates to a line in the response from BB " (conservation is your friend for off-grid systems--Almost always cheaper to spend money on efficiency/conservation than to throw money at generating the power)"

Yes and no... Yes, the more power you use from your solar system (without needing the generator to help with recharging your battery bank)--the "cheaper per kWH" your pricing becomes.

Many people use the programmable output on the MX60 set to indicate "float mode"--At that point, the "Float Signal" turns on optional loads (like a water pump to fill a cistern) with the "excess energy" that would otherwise be lost during the sunny part of the day. Or you do your laundry in the afternoons after floating has been achieved (or at least during the day time so you are not cycling the battery "needlessly").

On the other hand, if you use the "excess" power at night, you are now cycling the battery deeper--which shortens its life... So using excess power at night can have a negative impact on your battery bank's life.

While I agree with that would it not be a waste to use all of what you generate as that would actually lower your cost per watt ? i.e. if you generate 1000 watts but only use 200, does it not cost the same to generate 1000 and use it as it does to generate 1000 and only use 200 ? After the cost of the system the watt cost is the same so using more actually makes it more efficient ? Is that wrong ?

The batteries are typically the one component of the off-grid solar system that really respond to usage patterns. Treat them well, and they will last their rated life or even longer.

Treat them badly, and they may not even last more than a few months (by the way, I am not saying you are treating your battery bank badly).

If you keep track of the state of charge of your battery bank--and get it back above 75% State of Charge relatively quickly, And never go below 20% SoC for long life, or you run the risk of one or more cells "reversals" which are pretty much battery killers, you should be fine.

The other option that would be nice to have--A Remote Battery Temperature Sensor... Cold batteries need higher voltage to quickly and efficiently charge. Hot batteries need a lower voltage to prevent over charging (AGM's hate overcharging). Many charge controllers tend to over estimate the battery temperature when using the internal thermometer. That tends to slow down battery charging a bit.

It sounds like you have the Sun Xtender Technical Manual...

If you are going down to 50% capacity after one night of use--You pretty much need to run the generator that next morning to get the battery above 75% state of charge. (clarify--if there is bad weather in the forecast--You do not have any buffer for a few days of bad weather with your current battery bank size and loads)

-Bill

mikeo · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

Thanks for all the answers, to clarify the Evergreen panels are 12V X 5 in series to yield 60V, 2 strings into the combiner for 120V into the MX60.

To try and clarify what Bill said, The panels for installation purposes are figured at their open circuit voltages (Voc), so your 12 volt nominal panels are more like 21 volts Voc. So the Voc of 5 panels in series would be 5 X 21 or 105 volts. This is used to figure breaker voltage ratings and charge controller maximum input voltages. The combiner box parallels your strings so that you will still have 105 volts but will double the current with two strings, triple with 3 etc. IHTH

Dave Angelini · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

I am with Bill, nice post! 48.9v is a little low for an AM reading on AGM's! I would bump up the absorb time to 3 hours and make sure it is the correct voltage!

The deeper you discharge the time span of when you get to lift batteries is decreased! The above adjustment is easy on a liquid battery as you tweak for specific gravity and your done. You never have to second guess battery voltage.

Winter is the challenge! The wolf is always at the door! Try and get that am voltage reading above 50V and shoot for 51 or higher with an agm, if you want them to approach a ten year life!

My last thought for you is to look ahead and plan for staggered arrays that will be east and west to get you long solar days to do useful tasks. It will get you off the battery at 6am and not on it until after 7pm this time of year. It is easy to do with a tracker but the community here is not sure yet about that. I can see their point but that changed when I went that route after 12 of my 18 years off-grid!

BB. · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

I do agree that having more hours of sunlight for longer times to charge the battery bank is probably much better than just having a huge array and 9-3pm of lots of power. Also, it allows you to run your loads "from the array" during breakfast/dinner time" instead of from the battery bank... Less cycling on the batteries (longer life) and higher efficiency (batteries are, roughly 80-90% efficient--so avoiding the charging/discharging cycle gives you more useful power to boot)

Many of my recommendations for fixed arrays vs tracking is simply costs (cheaper to add more fixed solar panels vs cost of tracker, expensive foundation to hold up large arrays, maintenance, etc.) and the failures to tracking (failed mechanics, failed actuators, nearby lightning strikes, etc.).

However, I have to acknowledge that I am Grid Tied with fixed panels and Dave is off grid and has a large number of installations/customers that are very happy with their trackers. With good quality parts and a grease gun once or twice a year--he is getting good service from them...

Dave, do you have any recommendations for good vendors / designs / components or ones to stay away from? (I would like to add this to the FAQ).

-Bill

Dave Angelini · July 2010

Re: Eletrician Going Off Grid - Sizing / Questions

Bill,
The cost of doing 2 fixed arrays that would be manually adjustable (because you would have to move the arrays for winter) has never made sense to me but if one is against a photo sensor based tracker then, it is only money.

I have done it both ways tracked and 2 fixed (adjustable) east/west arrays. The cost is close either way and probably cheaper tracked. Certainly more elegant and the wow factor wins me a few clients. The fixed route has to be adjustable and that is where the cost can make tracked better.

I only use Wattsun and I have little tricks for making them bullet proof with the PE stamps in challeging environments. They are in their third version of the controller which has been excellent. The failure article you guy's have been siting from the owner of Homepower is pretty dated and Richard still tracks, believe me!

Go look at a top of pole array from Iron Ridge that will support 125 sq ft and cost out an AZ 125. Also keep in mind the az125 is built much better. Gotta go!
Have a good week-end!

Eletrician Going Off Grid - Sizing / Questions

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