Need advice and critique of our future solar system

OK... I will do some quick suggestions and math to at least try and get you approximatly configured.

First, I suggest that you plan on your batteries, especially if some sort of Li Ion / LiFePO4 type, that they be in a separate shed/building. Li Batteries when they catch fire/burn can release Hydrofluoric Acid. This stuff is highly toxic (scary toxic to human/life) and the standard method to deal with the contamination is to haul everything away that has been contaminated.

Will you have a fire, will your location get contaminated with Hydrofluoric Acid--The risk for a properly designed and installed system is low--But not zero.

Next, divide your loads into too types.. Base loads (those loads you must run every day like fridge, freezer, some lights, possibly a computer for work, etc.). And the optional loads that can be postponed in cloudy weather (water pumping, clothes washing, ironing, cooking with electricity, etc.). And I would suggest that your system solar array be ~2x larger than your base load needs to allow for bad weather and reduce any generator power needed.

Inverter size--If you are using standard refrigerator and freezers on 120 VAC power--A 1,200 to 1,500 Watt minimum AC inverter is suggested.

It sounds like you have headed down the first road correctly. Energy Conservation--It is almost always cheaper to conserve energy than to generate it.

Next, measuring your loads--Pretty easy right now because you are living in the home, and have the appliances. A kill-a-Watt type meter is the place to start--And measure your WH/kWH usage over at least one day... And probably over a week for "energy hogs" like a refrigerator.

With solar power systems, you have to plan for both the average energy usage, and surge power usage... For example, a typical full size refrigerator may use 120 Watts running, >600 Watts starting, and ~500-600 Watts defrosting (for a frost free fridge).

Your refrigerator numbers of 195 Watts for 5 minutes a day would work out to:

• 195 Watts * 24 hours per day * 5 minutes * 1/60 minutes per hour = 390 WH per day

Does not sound "typical"... A standard (very efficient Energy Star) fridge runs around 100-120 Watts for 20-30 minutes per day:

• 120 Watts * 24 hours per day * 20 minutes * 1/60 minutes per hour = 960 WH per day

Typically I would be suggesting around 1,000 to 1,500 WH per day for a "full size fridge". 390 WH per day is pretty low.

Also, while a microwave may take 1,200 Watts, it is only for 5min per day (large power drain for short period of time):

• 1,200 Watts * 5min * 1/60 minutes per hour = 100 WH per day (1/10th that of a standard fridge)

But your 12 Watts of Lighting 12 hours per day:

• 12 Watts * 12 hours = 144 WH per day

Or more "energy" (Watt*Hours) per day than your microwave usage...

You can look at appliances and make choices... Do you want Frost Free, or is manual defrosting OK with you and your spouse? Are you going to want larger than a 7 cuft fridge down the road? Some folks have used converted freezers to refrigerators (change the freezer thermostat to a refrigerator type) and gotten down to ~250 WH per day. So, low energy use for a refrigerator is certainly possible.

As an aside, there are some 12/24 VDC fridges and freezers that are pretty energy efficient--But they are not cheap. Sometimes a tradeoff between a smaller solar system and $$$ refrigerator.

https://www.homedepot.com/p/Unique-10-3-cu-ft-290-l-Solar-DC-Top-Freezer-Refrigerator-Danfoss-Secop-Compressor-in-Black-UGP-290L-B/308537092

524 WH per day for 10 cuft fridge/freezer @ 12/24 VDC--For a smaller solar power system, may be a good reason to stay at 24 VDC.

A freezer is going to be closer to 1,000 WH per day (if you are going for the 0F or lower range).

Anyway lets start with 2,000 WH per day, and an 1,800 Watt minimum AC inverter/load. On a 24 volt battery bank (48 volt is find too, just 1/2 the 24 volt AH capacity). Assume 2 days of storage and 50% maximium discharge (for Lead Acid/AGM type systems... Close enough for Li Ion too at first pass):

• 2,000 WH per day * 1/0.85 AC inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 392 AH @ 24 volts

That would be 4x 6 volt @ 200 AH "golf cart" batteries (typically flooded cell) in series for 24 volts, times 2 parallel strings for 400 AH (@ 24 volts).

And a sanity check... More or less, you can run a 500 Watt AC inverter per 100 AH of 24 volt FLA batteries... An 1,800 Watt inverter would need:

• 1,800 Watt AC inverter * 1/500 Watts * 100 AH (at 24 volts) = 360 AH (at 24 volts) minimum battery bank

So--400 AH @ 24 volts looks to be "workable".

To size the solar array, we have two calculations. First is based on charging current to the battery bank. The second based on hours of sun and your daily loads.

To charge your battery bank, 5% to 13% rate of charge is typical... 5% for weekend/summer system. 10%+ for full time off grid highly suggested.

• 400 AH * 29.0 volts charging * 1/0.77 solar panel+controller deratings * 0.05 rate of charge = 753 Watt array minimum
• 400 AH * 29.0 volts charging * 1/0.77 solar panel+controller deratings * 0.10 rate of charge = 1,506 Watt array nominal
• 400 AH * 29.0 volts charging * 1/0.77 solar panel+controller deratings * 0.13 rate of charge = 1,958 Watt array "typical" cost effective maximum

And the second array sizing is based on loads and hours of sun per day. Using a simple solar irradiance calculator, fixed array, facing south, for Springfield Ill (closest major city I could quickly find):
http://www.solarelectricityhandbook.com/solar-irradiance.html

Springfield illinois
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 50° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.08	3.48	4.28	4.80	5.01	5.23
Jul	Aug	Sep	Oct	Nov	Dec
5.52	5.29	5.24	4.53	3.22	2.90

So, you now have a decision, do you want to use a genset during part of winter, or design for solar only during winter? I will pick December at 2.90 hours of sun per average day (not that bad):

• 2,000 WH per day * 1/0.52 off grid AC end to end efficiency * 1/2.90 hours of sun per day (Dec) = 1,326 Watt array "December" break even

A typical solar array would be around 1,326 to 1,506 to 1,958 Watts... And 1,506 Watt array minimum would not be a bad place to start.

And you have the choice of, for example, using less power in a cool house... Say just a few lights and fridge/freezer at 1,500 Watt*hours per day. Suggest 50% to 65% of predicted average harvest for base load usage (turn off optional loads for bad weather and less genset usage):

• 1,500 WH per day base load * 1/0.50 base load fudge factor * 1/0.52 off grid eff * 1/2.90 hours of sun =1,989 Watt array 50% fudge factor
• 1,500 WH per day base load * 1/0.65 base load fudge factor * 1/0.52 off grid eff * 1/2.90 hours of sun =1,535 Watt array 50% fudge factor

So depending on how much or how little you want to run the genset--1,535 to 1,989 Watt array could certainly be justified to ride through some bad weather.

Suggest more solar panels (and backup genset), rather than "oversizing" the battery bank (solar panels last 20+ years, batteries last 3-5-8 years for "typical batteries").

Anyway, some math to hang your questions and answers on based on my guesses.

For a "near normal" electrical existence off grid (and using propane/wood/etc. for heat, cooking, hot water, etc.)--I suggest you start at 3,300 WH per day (or ~100 kWH per month). A 2,000 WH per day system (2 kWH per day * 30 days per month=60 kWH per month) is a pretty small system--But if it fits your needs--Great.

See how low of power bill you can get your home down to... 500 to 1,000 kWH per month is typical in North America. Under 200 kWH per month is pretty conservation heavy. Getting to 100 or 60 kWH per month--That is really minimal/highly conservation minded usage.

Your thoughts?

-Bill