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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?

Martin J.H.
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2 Answers2

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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.

Philip Ngai
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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?

The Evil Greebo
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