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In my house I have 3 small air conditioning systems running at 8000 BTU/hour each. I would like to power them with solar energy. I found that I would have to generate:

3.41 BTU/hr = 1 Watt

8000 BTU/hr =  2346 Watts

That means to power the 3 units for 4 hours/day, I would need (2346 Watts* 4 hours/day * 3) = 28152 W*hours/day, or approximately 30 kWh/day.

I checked that in my region, I have 7 hours of sun each day. So to generate 30kWh/day, I will need something like 5kWh/hour = 5000 Watts.

first question:

Are these calculations right?

second question:

Going further, I'm able to generate now (i have 4 * 250 watts solar panels) 1kW, but I generate it 7 days per week, and I use the air conditioners, maybe 2 times/week. Thinking in this scenario, I don't have to be so concerned about how much energy can I generate hourly/daily, but weekly right?

third question:

Another important point here is the batteries where I will store the generated energy right? Any suggestion which kind of batteries set should I use?

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  • 1
    There are solutions where you plug your solar energy into your main feed with some clever electronics that will power your whole house if there is enough power stored/generated. Batteries that are used in cars/trucks are tried and tested and last for up to 3 years- but better are dry cell(more expensive) I dont think you can power your aircons continuesly but you can surely supplement electricity to your house. Contact your local solar panel specialists. In the UK we can get these fitted for free to boost green energy :) im getting mine soon.
    – Piotr Kula
    Commented Oct 3, 2011 at 15:50
  • for free you mean supported by the state? awesome! These things just happen in the first world :-)
    – VP.
    Commented Oct 3, 2011 at 15:58
  • Yea by the government :) If you are in Europe you can try to get compensation from the EU for using Green Power- not sure about the rest of the world though.
    – Piotr Kula
    Commented Oct 3, 2011 at 16:01
  • @ppumkin actually i will check here in Germany. I live here as well :-))
    – VP.
    Commented Oct 3, 2011 at 21:20
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    For the US and off the grid, Home Power Magazine is a very good resource. homepower.com
    – Fiasco Labs
    Commented Apr 29, 2013 at 15:45

2 Answers 2

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Actually your AC is much more efficient than that because it is a heat pump, not a direct conversion of electrical watts in to BTU of heat moved per hour. If you know your SEER rating, you can just divide the BTU/h by SEER to get Watts.

An SEER of 10 is very common so that would mean each AC needs 800 W to move 8000 BTU per hour.

http://en.wikipedia.org/wiki/Seasonal_energy_efficiency_ratio

BTW, it's good to keep your units straight between energy and power (which is the rate that energy is being used).
Watt-hours and BTU are energy.
Watts and BTU/hour are power.

And your AC is not 8000 BTU, it's 8000 BTU/hour.

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  • wow, thanks for your detailed answer. I did already update my question btw :-)
    – VP.
    Commented Jun 1, 2012 at 10:21
  • You can also look on the side or back of your AC units for a service tag that will have detailed information about the unit. The wattage used is almost always listed.
    – Eric Anderson
    Commented Jul 9, 2012 at 14:51
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    Or better, plug it in through a "Kill-A-Watt" meter and measure it directly. Mine is not quite as big as yours, but using it through a Dr. Watt power factor controller it keeps my place at reasonable temps using about 350 Watts.
    – SDsolar
    Commented Apr 1, 2017 at 5:29
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Cannot answer #1 or #3 but WRT #2, with intermittent usage you want enough storage to last you approximately 2x to 3x your longest period of A/C use without sufficient light to generate the necessary power.

Usually this will be at night - but depending on where you live it also might be on an extended period of hot, rainy days. For instance, recently in Maryland we had over a week of 80-90 degree days with POURING rain and 100% humidity.

Just wondering - instead of going with batteries, can you go solar-on-grid, where your excess energy goes into the grid (and you get paid for it) and when your solar isn't generating enough, you can draw from the grid?

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  • I will check about solar-on-grid. It would be definitely a solution, but I don't know if, in Brazil, they will pay me something back for excess. I will check about it, thanks
    – VP.
    Commented Oct 3, 2011 at 12:51
  • Ah - didn't realize you were in Brazil - yeah that could change things.
    – The Evil Greebo
    Commented Oct 3, 2011 at 13:01
  • In the US they are required to credit us for any power we feed back into their system.. There are subsidies for having a solar system installed, but everyone I know who has done it has been sorry to get that involved with the government just to save some pennies. --> And, electric heating is the absolute worst use of solar. If your water heater is electric, you'd want to run it at night as much as you can, to get the low demand less-expensive grid power, then sell to them during the high demand day. Or use solar water heating instead of electric - it works on hot, rainy days..
    – SDsolar
    Commented Apr 1, 2017 at 5:38

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