If you are someone who is choosing to not go fully off-grid (which is perfectly reasonable by the way!) and you haven’t bought any batteries to store the additional solar power, then some days you’ll be selling your home-generated solar energy to the utility company. In the U.S., by law electric companies have to pay you whatever rate you would pay them. At other times, you won’t generate enough solar power and you may be paying them for energy. Either way, over the duration of a full year, you want to aim at having your energy bill end up around the zero mark. In other words, you want to have just the right amount of solar power panels, not too many and not to few.
Energy companies rarely (if ever) offer to give you money if you over-supply them. So do not build a solar power for homes system that will produce more solar energy than you could ever possibly use. You want the system to generate just enough to keep your bills at zero.
To establish exactly how many watts of solar power panels you need, you have to work out how many hours of direct sunlight that your solar power for homes system will receive. In short, on average, how many hours of direct sunlight does your home soak up over the course of any given year?
There are several ways to work this out. The ‘hard way’ is to take time daily to notice for how long the sun shines on the part of your roof where you are thinking of installing your solar power panels. Now let’s face it! Because you probably won’t want to keep this up this laborious daily chore for a whole year, this way is almost certainly totally impractical.
If your solar power for homes system has tracking system, (a built in ability to move and track the sun) the number of hours of sunlight your roof receives will not be the same as the number of hours of direct sunlight. Because the sun’s light strikes the solar power panels from many different angles (not only when it’s coming in at right angles to your roof) you’ll find that your calculations will vary.
Generally speaking, most houses enjoy roughly 5 hours direct sunlight on average throughout the year. This is a rough approximation, so don’t get upset with me if it turns out you get less energy than this article suggested! (You could always return to daily readings if you want!)
If you live in the U.S. and you want to get a far more accurate assessment, look for your local average regional value. In August 2004, the Florida Solar Energy Center presented a study entitled “Geographic Variation in Potential of Rooftop Residential Photovoltaic Electric Power Production in the United States” in which they tested power generation of a 2 kilowatt solar power for homes system all around the U.S.A. They took readings in more than 200 locations throughout the U.S. The research showed that on average a 2 kw rooftop system would harness between 5 and 8.5 kw/hr per day throughout the year.
So using the low end of the scale, a 2-kw system would see 5 hours per day of sunshine. Therefore, that 2-kw system could expect to enjoy 10 kw/hr of solar power per day on average across the period of the whole year.
Consider how you were planning to use the solar power for homes. Were you wanting to go completely off-grid? That will create greater demands on your solar power for homes system. Also, be aware of any potential problems such as tall trees blocking your sunlight to the solar power panels.
If your average power consumption were to be 600 kw/hr per month and you establish that your electricity company has a billing period of 30 days per month, then you can know that your daily energy consumption is 20 kw/hr per day (i.e. 600 kw/hr divided by 30 days per month).
Using the Florida simulation data found in the graphic entitled, “Geographic Variation of PV Power Production Around the U.S.”, we can see for example that the southernmost dc power adapters for sale tip of Florida has an estimated value of 7.5 kw/hr based on the use of a 2kw rooftop PV system. Therefore, we can know that people living in Southern Florida receive an approximate average of 3.5 hours of direct sunlight per day.
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