Everybody likes something free, and it's even better if the something free is the energy to meet your household needs. Solar panels convert sunlight to electricity, and because no one meters your sunlight, it's almost like free energy. Of course, upfront prices for household solar energy systems can run into the tens of thousands of dollars. When you put up that kind of investment, you want to be sure you get what you need. A few calculations can help you be confident that the solar panels you get will be the ones you need.
Your Energy Needs
The best way to determine your household energy usage is to look at your energy bills for the last couple years. The most informative analysis would end up with a month-by-month average so that you'd know, for example, that for the last three years you used the most energy in November, and it averaged at 960 kilowatt-hours. Just for illustration, consider a house that uses the U.S. average of 11,496 kilowatt-hours per year, and assume that usage is spread out equally so that the house uses 958 kilowatt-hours per month.
Your Available Sunlight
To figure out how many solar panels you need, you also have to know how much sunlight you have available. Solar panel specifications are given in terms of peak power, which you only get on a perfectly clear day when your solar panel is aimed directly at the sun. The National Renewable Energy Laboratory has collected decades of solar energy measurements from all over the country and published it in the Solar Radiation Manual. You can use that data to figure out how much sun you have available. For example, the entry for Burlington, Vermont, shows that a solar panel tilted at an angle equal to the latitude will provide a daily average of 5.6 hours of full sun equivalent in July and 2.1 hours in December, averaging to 4.3 hours for the whole year. You can find an entry in the manual that is close to your geographical location to estimate your available sunlight.
Supplementing Your Energy
The number of solar panels you'll need also depends upon what you want out of your system. If, for example, you're planning on staying connected to the grid and selling energy to the utility company when you have extra and buying energy when your panels fall a little short, you'll calculate your needs based on the annual average. For the average house, using 958 kilowatt-hours per month in Burlington, with 4.3 hours of average daily full sunlight, you'd need an array of solar panels with a peak power rating of: 958 kWh / (4.3 hours/day x 30 days/month) = 7.4 kilowatts (peak). This will provide more energy than you need during the summer months -- to be sold to your utility company -- and less energy than you need in winter, when you'll have to buy energy.
Fulfilling Your Energy Needs
If you want to go completely off-grid, then you'll need to make sure your solar array meets your needs even during the darkest months. The average house in Burlington during December, with only 2.1 hours of full sun per day, would need an array of solar panels with a peak power rating of: 958 kWh / (2.1 hours/day x 31 days) = 14.7 kilowatts (peak). If you're going off-grid, you'll need to store energy in batteries, which will reduce the overall efficiency of your system a little more, so you'll need to up this size even more.
More Complete Calculations
Solar panel specifications are usually given in lab conditions. In the real world, when solar panels get hot, they only work about 80 percent as well, so you'll need to multiply your results by 1.25 to estimate your needs more accurately. Solar panels come in a range of materials, each with different cost, mounting requirements and efficiency. Depending upon your choice, your solar panels will provide from about 150 to 200 watts per square meter. So a 7.4 kilowatt (peak) system can take up anywhere from about 35 to 50 square meters, or about 380 to 540 square feet.
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