What size (how many watts) solar system do I need if I use 800kwh a month?
Answer:
Are you on the grid or off the grid?
Do you have a storage system?
Even though you use 800 Kilowatt hours of electricity per month, you have peak times of use when you are running appliances and other times when you use relatively little electricity.
Also your location will have a large influence on the amount of electricity that you get out of your system. You may get much less electricity than your system is theoretically capable of producing.
The most common system installed on a residence is rated at a maximum production of 3,000 watts. However the system will only achieve that at noon on the equator on a clear day. The system will not produce anywhere near that in your location.
Even this system would produce less than half the amount of electricity that you use.
You should also have some extra capacity in your system. I recommend that you get a system with a minimum rating of 9,000 watts, and a good set of storage batteries to store the extra energy.
Good luck with your project.
Easy enough to figure out once you know that sunlight falls on the earth with a power of 1 watt per square yard, approximately.
Mike, I agree. He will need to store and use 27KWH per day, and in an 8 hour day even if the solar array tracked the sun, in most places in the USA keeping that amount on hand would be difficult.
9 KW peak potential, if the cables could take it, and good batteries, sounds a lot more reasonable. Be very expensive, and the batteries would have to hold the home during cloudy days,,,better buy a second lot for the building housing the batteries to keep you going!
One of my friends has solar at his mountain-top cottage. No A/C, heats with wood cut from his lot, and off a 6' x 8' solar array handles a small radio, 3 40W lights, an old 14" TV, and the booster for his cell phone. He has a 2-room cottage, and the battery and diesel generator shed is bigger than his cabin!
Reading the answers I can see that a lot of people don't understand how it works.
If you are using a grid tie system, You do not have to save the solar electric you produce during the 5 hours of peak sunlight. You only have to produce it. You will be back feeding the power into the local utility. So if you use 800 kWh per month divided by 30 days you have 26.6 kWh. Now divide that by 5 hours of peak sun gives you 5.3 kWh.
Well you only need a 5,300 watts in solar modules to cover your needs. You will over produce during day and back feed it which turns your electric meter backwards. Then you use the power over night.
If you are gone all day long your system will be running the meter backwards all day long while the price per kWh is higher and then you use the low priced off peak load pricing at night..
So now depending on where you live will tell you if you need a little bigger system or a little smaller.. Most areas get about 5 hours a day and others get 6 hours a day.. But way up north only gets 3 or 4 hours a day.
Hope that helped.
Any way you figure it, a solar power system is a good thing for the environment because it does save generating capacity from power plants, but the power it produces will be costly power when compared to using the grid. Without some sort of subsidy, it just does not calculate as cost effective.
Do it for the environment, not for the savings, if you can afford it.
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