Advice on buying new bank of batteries for current off-grid instalation.

Re: Advice on buying new bank of batteries for current off-grid instalation.

Some information from the first post (edited):

6 x 255 W panels (STC: Vmpp 30,5V, Impp 8,40 A, Voc 37,5 V, Isc 8,83 A)
1 Victron Mppt 150/70 charge controller
1 Victron inverter (no charger) Compact 24V/2000Va
No batteries.

-Bill

Re: Advice on buying new bank of batteries for current off-grid instalation.

Not a problem Mel... We all started at the same point of knowledge.

Normally, only edit your post(s) if there are mistakes or you want to make some quick clarifications to a new post.

Otherwise, just make a new post with your replies, answers, further questions. Computer storage is cheap these days--And doing this makes it much easier for people to follow the threads.

-Bill

Re: Advice on buying new bank of batteries for current off-grid instalation.

Another thing to look at is solar heating/hot water. Making solar thermal panels can be a good Do It Yourself type project (solar electric panels, almost never a good DIY project).

With solar hot water, there can be a lot of plumbing (piping, circulating pumps, insulated tanks, etc.) involved--Not easy if you are not already good with plumbing.

Solar Shed and other Solar Thermal Links

-Bill

Re: Advice on buying new bank of batteries for current off-grid instalation.

If you want to use excess power from your solar array, The Midnite Classic has a "waste not, want not" output. You can program the output to drive a relay (or electronic relay) to heat water, run a water pump, etc. when excess power is available.

However, my two cents--I would suggest seriously considering the DIY type solar water heating. In general, solar electric power is too expensive to "waste" on heating water.

My own suggestion is to keep it simple. One larger solar power system vs two smaller systems. And one Classic is generally more capable/less costly (per amp rating) than a pair of smaller units.

Many people do start with a smaller system when they are building/doing major construction--And keep the smaller system for a Mother-in-Law/Guest building. And build a second larger system to run the main home.

In general, it is very difficult (i.e., expensive) to expand a solar power system by much more than 2x. As you see, the battery bank voltage changes (new AC inverter for higher voltage) and you usually need a completely new set of batteries for a higher voltage/larger AH bank (don't really want to mix old and new batteries--You end up with replacing part of the bank 2x more often, etc.). And sometimes it can be difficult to find new solar panels (at a good price) that will mix+match with the existing array (different Vmp/Imp ratings can be difficult to match--Available panels can change drastically over just a couple of years, and the older panels are either not available, or 2x the price of the current market price).

OK--If you pick 3.3 kWH per day--Here is what a system may look like. Assuming you have a minimum of 4 hours of sun per day (average) for 9+ months a year (if you want to look at the http://solarelectricityhandbook.com/solar-irradiance.html website for a city near your new home--That will give you more accurate numbers).

Using our rules of thumbs to save hand waving time (please ask questions, the RoTs will give you a good back of the envelope calculation to see if it makes sense for you--We can always talk details if your needs differ). Assumes that you charge during the day and use most of your power at night./during bad weather (we are designing a "balanced" system between battery bank, solar array, AC inverter, and your loads). Don't get stuck in the details of what product to buy just yet.

Assuming 3.3 kWH per day, 2 days of backup power (no sun), and 50% maximum discharge (for longer battery life), the battery bank will look like:

• 3,300 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 maximum discharge * 1/24 volt battery bank = 647 AH @ 24 volt battery bank

Note that this can also be a 324 AH @ 48 volt battery bank. The cost of batteries is roughly the same as the amount of stored power is the same (power=Voltage*current--Double the voltage and 1/2 the current is the same power). In general, about a 600-800 AH battery bank is about the largest that is "easy" to cable out for off grid solar. So a 24 volt system would be more expensive/difficult to expand later if you started with a 647 AH battery bank. Will talk more about that later.

The above battery bank would support around a recommended maximum AC inverter size of ~3.3 kWatt -- Although, I would suggest you keep the inverter smaller. Costs less, and usually wastes a bit less power. You could go possibly go down as far as 1.5 kWatt and still run your home (full size refrigerator, small well pump being the typical minimum AC inverter limits).

Next, sizing the battery bank. The rules of thumbs that work well are ~5% to 13% rate of charge. 5% can work for a weekend/seasonal cabin--But is small for a full time off grid home. People do make it work, but it requires a bunch of active energy management on your part to make sure you don't kill the battery bank.

Normally, for a full time home, 10%-13%+ makes for an easier to manage/more capable solar power system. Good for your batteries, and keeps generator run time to a minimum.

For the solar array calculations, we will do two types. One based on the size of the battery bank. The second based on your loads and hours of sun per day. You need to satisfy both conditions to have a happy off grid power system. First based on battery bank capacity (note, 24 or 48 volt system does not really matter here, because the 24 volt system has 2x the AH rating of the proposed 48 volt battery bank--store energy is still the same for either):

• 647 AH * 29 volts charging * 1/0.077 panel+controller derating * 0.05 rate of charge = 1,218 Watt array minimum
• 647 AH * 29 volts charging * 1/0.077 panel+controller derating * 0.10 rate of charge = 2,437 Watt array nominal
• 647 AH * 29 volts charging * 1/0.077 panel+controller derating * 0.13 rate of charge = 3,168 Watt array "cost effective" maximum

By the way, the solar math is not that accurate... Anything within ~10% of the above numbers is pretty much the same. I do this to avoid rounding error and you can follow where I use the numbers next. Your battery bank will not be exactly 647 AH--You would re-calculate some of these as you finalize the hardware.

And now based on your daily load and 4 hours of sun minimum (long term average) per day. And this is full sun from at least 9 in the morning to after 3 in the afternoon--4 hours of noon time equivalent sun harvested during the entire day. Any shading will kill solar panel output--Can lose 50% or more of array performance with just the shadow of vent pipe on the roof on one panel.

• 3,300 WH per day * 1/0.52 end to end system efficiency * 1/4 hours of sun per day = 1,587 Watt solar array minimum

So, I would be suggesting a minimum of 2,437 Watt array (to meet minimum 10%) rate of charge. And assuming that February is the "break even" month for ~4 hours of sun per day, you could get:

• 2,437 Watt array * 4 hours sun * 0.52 system eff = 5,069 Watt*Hours per day minimum (average) for 9 months of the year

You would use ~2.4kWH over night (and during bad weather), and during sunny weather, you would have another 1.6 kWH (or more if you use 5-6 hours per day for summer) to run your tools/home appliances during the day.

Your choice if you keep the 24 volt inverter or look for a 48 volt inverter to meet your needs. Smaller 48 volt inverters are hard to find--But your system could probably support a larger 48 volt Inverter OK (towards the 3.3 kWatt range maximum).

The solar charge controllers you are looking at support 12/24/48 volts, but at a fixed (more or less) output rated current. For a 2,437 Watt array, the "maximum typical charging current" on a MPPT charge would be around:

• 2,437 Watt array * 0.77 panel+controller derating * 1/29 volts charging = 64.7 Amps into a 24 volt battery bank

The Kids are rated for 30 amp output--So you are slightly above my suggested cutoff point on a 24 volt battery bank. The controllers will still work OK, but you will "lose" ~5 amps of charging current during the middle of cool/sunny days.

And even with 70-80 amp rated charge controllers, a 24 volt batter bank and a ~3kWatt array max them out for a single charge controller. This is another advantage of a 48 volt battery bank--Same charge controller can manage a ~2x larger solar array (as always, read the charge controller specifications closely, there may be voltage based deratings on current output).

The drawback of 48 volt systems, 72+ VDC rated fuses and circuit breakers are a bit harder to find and may be more expensive. Also, there are few "native" 48 VDC appliances out there (although, I usually recommend running your loads from 120/230 VAC inverter anyway. The modern appliances are almost as energy efficient as their DC brothers and usually cost much less--Again, look at your needs/system options as a whole, on paper first, before spending any money).

And there is a genset to charge the battery bank. I would suggest that a 10% to 20% rate of AC charge rate would be nice number. A generator may look like:

• 647 AH * 0.10 rate of charge * 29 volts charging * 1/0.85 AC charger eff * 1/0.95 Power Factor for AC charger * 1/0.80 genset derating = 2,904 Watt (VA) rated genset minimum

I am a fan of keeping generators relatively small/low power. Being off grid, you will probably have several. A smaller/fuel efficient one to carry you through bad weather and a larger unit for shop tools/backup power/emergency power (3kWatt small, 6-10kWatt large would be a starting suggestion).

Well--That is where I would start--None of these are written in stone or handed down from God... Just a starting point in sizing your system.

Does this meet your needs for loads? What is your location (major city) to make sure solar insolation is correct? Do you have a lot of daytime power needs? Changes between winter and summer loads? Do you have a minimum power need (refrigerator, business equipment, etc.)?

Your questions/answers?

-Bill