Solar-powered shed-turned-office supporting air-conditioner

Question

I have a 10'x12' (3 m × 3.6 m) shed, with a glass door and two windows, and 194 sqft (18 m²) of roof at a 2/12 slant (1:6, 16.67% or 9.462 degrees) - it's a bit bigger than the size of the shed.

I want to run air conditioning in there (8000 BTU / 2.3 kWh probably more than enough), as well as a few screens, wifi router, my computer, lights, etc.

I'm thinking of buying something like this and covering the roof with it (can fit 8 panels): https://www.homedepot.com/p/Renogy-10Pcs-320-Watt-Monocrystalline-Solar-Panel-for-RV-Boat-Shed-Farm-Home-House-Rooftop-Residential-Commercial-House-RNG-320Dx10-US/318688258?ITC=AUC-153748-23-12070

I would need a controller, inverter and battery pack.

The panels are rated 320 W, so that's 2560 W. From what I understand (watched https://youtu.be/xnSew-tCuPo), I need the controller before the battery. I'm not clear on what battery I need, and then what inverter I need (rated for 2500?).

Also, the air conditioner need is a question for me. If I go with a simple portable one, it may be able to hold up fine. But if I go with a mini-split, there's the initial part where the engine starts and it takes a lot more energy than what the solar panels can generate at a given moment. I want the batteries to help with that.

So really, I need some assistance understanding if this is feasible, and all the math involved. I read quite a number of forums (including SolarTalks), but am not confident yet that my design makes sense.

Answer 1

The battery needs to have enough current capacity for the air conditioner. The 8000 BTU air conditioner uses 700 watts. I recommend 48 volts for your system over 12 volts or 24 volts since 700 watts at 12 volts even with 100% inverter efficiency would be 58 amperes continuous, which would require very beefy wires and still you would lose energy to heating of wires. Also charging at 2560 watts at 12 volts would be 213 amperes which would require very thick wires, and at 24 volts would be 107 amperes which also would require very thick wires. At 48 volts it's only 53 amperes which is manageable.

As for the battery watt-hour capacity, depends on how long you want to use the AC when not sunny. A good thing about air conditioning is that air conditioning need correlates very well with sunny weather so you might find it acceptable to have only 3 hour runtime with no solar power. Remember Peukert's law. 3 hour runtime would be 2100 watt-hours, but those watt-hours are usually specified with 20hr runtime. With 3hr runtime, and Peukert number k=1.2 you would actually need 2900 watt-hours, which at 48 volts is 60 ampere-hours. AGM or gel cell is preferable, don't use flooded because they have acid leaking risks, and also flooded cells typically don't like deep discharge.

However, 60 ampere-hour batteries can be charged with only about 20 amperes, which at 48 volts is 960 watts. So 2560 watt panels would go to waste. Thus, you need to ramp up battery size to 160 ampere-hours to allow full 2560 watts to charge the batteries.

So buy four 160 ampere-hour 12 volt batteries and wire in series. Buy a 48-volt inverter capable of running with 700 watt load continuously and capable of starting the air conditioner. I suspect the starting current could be the limiting factor here. Sine wave inverter is always safer but more expensive. Modified square wave (also sometimes erroneously called modified sine wave) could or could not work, I'm not sure if it works well with air conditioning.

The charge controller should ideally be maximum power point tracking controller intended for 48 volt systems (some may be 12/24/48 volts with automatic detection). Its charging capacity should be 53 amperes at 48 volts at least to allow full usage of the solar power.

The wiring needs to be 10mm2 or 16mm2. Any thinner won't work with the charging current.

If you pick a sine wave inverter, expect it to cost around ~500 EUR at that power level. Batteries would be around ~2000 EUR with only 8-year lifetime (but if you run them completely empty often, lifetime could be as short as only few months). Charge controller around ~500 EUR.