New Solar (first time)

Let's start from the beginning... a 24 VD @ 3kWatt off grid AC inverter and 6x 330 Watt solar panels. I guess you are in/around Kailua, Hawaii.

Sizing a "paper system" to see what to expect/size the battery bank (start with flooded cell lead acid--cheap), start with your location & Solar array.

Kailua
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 69° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.99	5.85	6.24	6.50	6.43	7.37
Jul	Aug	Sep	Oct	Nov	Dec
6.53	6.63	6.60	6.11	5.25	4.84

Picking December as "break even month" (may need a genset during bad weather to keep system operating). The "average" daily harvest is around:

• 6 * 330 Watt panels (aka 1,980 Watt array) * 0.52 overall off grid system efficiency * 4.84 Hours of sun per Dec. Day = 4,983 WH per day

Nominally, I suggest a 3,300 WH (3.3 kWH) per day as the minimum amount of energy per day that gives you a "near normal" electrical existence (LED Lighting, Full Size Fridge, small water pump or solar friendly well, laptop computer/LED TV, clothes washer, propane or other fuel for heating/cooking/hot water, etc.).

So, 4,983 WH per day is actually not a bad solar array for you. Using that number to size the rest of system. Basically 2 days of storage and 50% max planned discharge (a Lithium Ion bank could be 2/3rd or 1/2 the size of Lead Acid--But that is a discussion for the next posts):

• 4,983 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max planned discharge * 1/24 VDC battery bank = 977 AH @ 24 volt battery bank

That is a pretty large battery bank... If your daily loads do not need this much energy--Going with a smaller battery bank will save you money (smaller bank, smaller bank to replace every 5-8 years or so, etc.).

Like to have 10% to 13%+ rate of charge for full time off grid battery bank (confirm or update assumptions for solar array):

• 1,980 Watt array * 0.77 array+controller deratings * 1/29.0 volt charging * 1/0.10 rate of charge = 526 AH @ 24 VDC nominal
• 1,980 Watt array * 0.77 array+controller deratings * 1/29.0 volt charging * 1/0.13 rate of charge = 404 AH @ 24 VDC suggested minimum battery bank
  

Because you have lots of sun (even in winter)--You can go with a much larger battery bank... But a 404-526 AH @ 24 VDC battery bank would be a good choice too (minimum rate of charge for full time off grid).

If you look at 3.3 kWH per day as a "minimum" for a small/efficient home, a suggested battery bank would be:

• 3,300 WH per day * 1/0.85 AC inverter efficiency * 1/24 VDC batter bank * 2 days storage * 1/0.50 max planned discharge = 647 AH @ 24 VDC battery bank

Because you have lots of sun, perhaps you could go with less than 10% rate of charge with your present array--But for Lead Acid batteries, 10% minimum rate of charge is suggested by battery mfg.:

• 647 AH * 29.0 volts charging * 1/0.77 array+controller derating * 0.10 rate of charge = 2,437 Watt nominal array 10% rate of charge for 3.3kWH per day system on Lead Acid batteries.

What will 3.3 kWH per day system run (some random numbers)?

24 VDC R/V diaphragm pump: https://www.solar-electric.com/aquatec-550-series-m378-24-volt-booster-pumps.html

• 1,200 WH per day full size refrigerator (a small fridge may be 800+ WH per day)
• 24 VDC * 4 amps (average?) current * 1/3 hours per day (20 minutes) = 32 WH per day water pumping
• 30 Watt laptop computer * 10 hours per day = 300 WH
• Clothes washing machine 1,000 WH per day (per guess)--Run during sunny weather
• 5 * 13 Watt LED lights * 5 hours an evening = 325 WH per day
• 15 WH per day USB phone charger
• 1,200 Watt Microwave * 1/4 hours per day (15 minutes) = 300 WH per day
• 1,200 + 32 + 300 + 1,000 + 325 + 15 + 300 = 3,172 WH per day

Just an example of how to work out your loads... You should use a Kill-a-Watt or similar meter for your 120 VAC loads and there are DC WattHour meters you can measure your DC loads with...

You can see how your loads, amount of sun, and such drive the sizing your system. I suggest that you size the system assuming flooded cell lead acid batteries--And then figure out the sizing and other issues (battery management system for Li Ion and such) for the Li Ion battery bank (LiFePO4 batteries are almost ideal for solar power systems--But can be expensive and usually BMS is almost a requirement to prevent damage to your battery bank (over charging/over discharging, running near/below freezing temperatures, etc.).

Circuit breakers are there to protect the wiring... Fuses also work--But Circuit Breakers are nice to use as on/off switches too (turn off to service equipment, turn off inverter when you are away traveling, etc.). You need to size everything first (voltage, current, etc.) per branch circuit to know what you will need to do.

For example, standard 14 AWG wiring is rated for 15 amps. Your 3,000 Watt inverter:

• Inverter AC output = 3,000 Watts / 120 VAC = 25 Amps (if you use 14 AWG to lights/outlets, you need 15 amp breakers per circuit)
• Inverter DC input = 3,000 Watts * 1/0.85 AC inverter eff * 1/21.0 volts battery cutoff = 168 Amp minimum branch circuit wiring and breaker

Your charge controller:

• 1,980 Watt array * 0.77 panel+controller * 1/29.0 volts charging = 53 suggested minimum MPPT type solar charge controller
• 60 amp charge controller * 1.25 NEC derating for continuous charging = 75 Amp round up to 80 Amp rated branch circuit wiring and breaker

24 VDC can be a "handy" working voltage for your battery bank. There is a fair amount of "stuff" out there that runs from 24 VDC... RV type water pumps and even LED lights. If you ever plan on a larger system (>2,400 to 3,600 Watt AC inverter), you might want to look at starting with 48 VDC battery bus. Higher voltage, lower current in wiring, less voltage drop, lower wiring costs less, and and easier to route.

Remember that AC inverters take energy just being on and no loads. A mid to larger inverter may take 20-40 Watts "just turned on" (Tare Losses)... If you run the inverter 24 hours per day. Example:

• 30 Watts * 24 hours per day = 720 WH per day

On a small system--That is almost another 1/2 refrigerator load (or 100% of a small fridge load). If you do not use the inverter 24x7 (24 VDC fridge, DC for lights, water pump, etc.)--You can save significant amount of energy.

If, however, you will be running the inverter 24x7--Then using 120 VAC loads "everywhere" is usually the better choice. AC appliances are getting pretty close to DC appliances in efficiency--And tend to cost much less (and can last longer--especially things like RV water pump with "brushed motors" (maybe 2,000 to 5,000 Hours between brush changes) vs AC Induction motors. But for something like water pumping, and 20 minutes per day... 2,000 hours / 0.33 hours per day = 6060 days or ~16 years (diaphragm will probably need replacing before the brushes).

Anyway... Some math/modeling of your system. Lots of choices and the detailed designs start once you pick the hardware. Your thoughts?

-Bill