Jim:
In regards to running a small “window” air conditioning unit off of a solar powered system, I can convey some of my experiences. I have a total of 3,160 watts of solar panel power on the roof; about 1,700 watts feeds my 24 volt DC “house” system (mostly lights, computer, entertainment system, ½ of the kitchen outlets, and the fridge) while the remaining panels are wired for a totally separate 48 volt DC water heater system. Two 2,500/5,000 (peak) watt inverters are used for each system, each “slaved” to the other of the same voltage to synchronize the alternating current frequencies and amp loads. Importantly, each inverter has “soft start” technology to “soften” the instant high amp draws from motors, compressors, etc. I have run my small bedroom air conditioning unit (rated at 700 watts) successfully using only the solar powered system, but will only do so when the battery bank has been almost fully charged and the sun is still out. Once the sun starts going down and the air conditioning unit load gets transferred to the battery bank (capacity of about 2,100 amp-hours), I would guess that I’d have maybe 2-3 hours of operation before the inverters shut down from an undervolt condition (considering all of the other system loads; lights, fridge, etc still in use at this time). If you wanted to run a similar sized air conditioning unit (and nothing else) continuously from only solar power/ battery banks, you’re looking at a whopping system- I’m guessing 3,500 to 4,000 watts worth of panels, and at least 5,000 amp-hours of battery bank capacity. Even at that, you’d better hope for at least 4 hour’s worth of sunlight every day.
I’m basing this “guesstimate” on my lessons learned from my 40-gallon water heater system- it draws about 4,000 watts and drains a fully charged 3,300 amp-hour battery bank (down to the 44 volt undervolt inverter alarm) after about 30 minutes of combined total usage (or about three normal showers’ worth of water heated). The feel of a bit of freedom from the electric company makes it all worth it, but the most enlightening aspect of this whole “project” of mine towards electrical self-sufficiency has been the quantity of deep-cycle batteries needed for a system that can “get you through the night” (I have a total of 50 now, and need more!). If you are just getting started on planning your own solar project, don’t forget to add in the cost of all those batteries! – Wayne E.