How many solar panels do I need on my house to become energy independent?

It depends how much energy you use, where you live, and how much you want to spend...

By Aaron Johnson

Solar power isn’t just for experimental race cars and the International Space Station anymore. It’s becoming commonplace to see the roofs of homes and businesses covered with photovoltaic panels that convert sunlight to electricity. Installing solar panels on your own home is a tantalizing prospect — no more power bills and doing good for the environment! But do some homework before you scale a ladder and start bolting things to your roof, says Jonathan Mailoa, a graduate student in electrical engineering and computer science and Solar Community Chair for the MIT Energy Club.  

Mailoa would rephrase the question as follows to get at some of the necessary details: “Given my average consumption of X kWh of electricity per year, and the annual average of Y kWh/m2/day of solar insolation, how many solar panels of efficiency Z percent will I need to install for my home to become energy independent?”    

The first of those variables is the amount of energy your house uses, which depends on things we’re all pretty aware of — whether you turn the lights off when you leave a room, how much you run your air conditioning unit when it gets hot, etc. The less electricity you use, the fewer solar panels you’ll need.  

The second variable is the amount of solar insolation your neighborhood receives over time. This number varies depending on weather, and, most especially, where you live. In the U.S., for example, it’s no surprise that solar insolation is higher in the Southwest than New England. Arizonans have to deal with incredibly hot and uncomfortable summers, but they’re getting a lot more power from their solar panels.  

The last piece of the puzzle concerns the efficiency of the solar cells and panels, which is a measurement of how much of the sun’s energy they can convert to usable electrical energy. Right now, the average for solar panels is only 15 to 21 percent. Three-quarters of the energy is being wasted, Mailoa says, because “the efficiency limit of a solar cell is fundamentally limited by its material properties.” He explains that the semiconductor material in a solar cell can only absorb light with a certain amount of energy — called the band gap.  Sunlight, however, spans a range of energies. Light with energy lower than the band gap can’t be absorbed by the solar cell, and light with energy a lot higher than the band gap is more than the solar cell can absorb. “This excess energy is lost as heat,” describes Mailoa. In silicon solar panels, which are the most common, “this fundamental limit is 29 percent.” Once assembled into a panel, there are additional efficiency losses through various mechanisms. Currently, the most efficient solar panels on the market make use of about 21 percent of the sun’s energy, and they aren’t cheap. “Advanced, high-efficiency silicon cells are currently very expensive,” says Mailoa.  

So let’s put some numbers to the question: Imagine that your house uses 48 kWh of electricity per day (about average). If you live in Arizona, where the average solar insolation per year is around 6 kWh/meters squared/day, you’ll need 53 square meters (574 sq ft) of 15 percent efficient solar panels. If you spend the extra money for 21 percent efficient solar panels, then you’ll only need 38 square meters (409 sq ft) of solar panels. But if you try to power the same sized house in Vermont, where the average solar insolation per year is around 4 kWh/meters squared/day, you’ll need 80 square meters (861 sq ft) of 15 percent efficient solar panels and 57 square meters (615 sq ft) of the 21 percent efficient ones.

Thanks to Candace Mead from Long Beach, CA, for this question.

Posted: November 19, 2013