Maybe you’ve already heard this, but it’s a fun fact to ponder: The amount of solar energy hitting the earth in one hour is equal to the amount of energy consumed by all of humanity in a year.

Of course harnessing the sun’s energy is another story, since it turns out to require the help of technology. But it’s actually a rather exciting story, given the speed with which that technology is evolving: The cost of photovoltaics has dropped by almost 70% in the past 10 years, making it competitive with any fossil fuel energy source today (at least, up until the pandemic-induced, likely temporary, oil pricing chaos).

If you are considering solar panels for your household, there are websites that can help you with rough estimates. A good one is www.solar-estimate.org. But here is some background to help you understand the basics of home photovoltaics, including the interaction of the solar energy available in your location with the efficiency of solar panels, and how your PV-generated power is treated by the power company.

In full sun at its brightest here in Maine we receive about 100 watts per square foot, which would be over 800 kilowatt-hours (kWh) of energy per year. But when you take clouds, day/night and seasons into account the average insolation (available solar energy) in Maine is around 130kWh of energy per square foot per year. A typical new photo-voltaic (PV) panel has an efficiency of 17%. That means one square foot of panel can generate 17% of 130 kWh per year, which is 22kWh per year.

Depending on what kind of heat, hot water and appliance usage you have, your household electricity use might range from 3600 to 12000 kWh per year (check your bills for this info). That means that if you wanted to supply all of your electricity with PVs you would need 180 to 600 square feet of PVs.

PV systems come in different sizes and usually have a “nameplate capacity”. A typical value for an individual panel is 350W. This panel would cover about 21 square feet and put out 350 watts in the brightest sunlight, but due to our insolation rate of 130kWh per year per square foot, this panel will produce around 460 kWh per year, about $70 worth of electricity. So for example if you were to install a 3500 watt system it would consist of ten 350W panels and would generate around 4600kWh each year.

The typical payback on solar panels right now is around 10-12 years. There has been a 30% federal tax credit which unfortunately is being phased out over the next two years. For 2020 it is 26%, and in 2021 it will be 22%. After that it is set to expire. This change will make the payback time longer. It is possible to finance a solar system such that your monthly loan payments plus your reduced electricity payments are comparable to what you pay for your electricity now. Then after the loan is paid off your monthly payments drop to whatever small amount of energy you get from the grid.

These days most PV systems are grid-connected. (Off-grid systems are more complex and will not be discussed here.) That means that when you produce less power than you are using you get the rest from the power company. The advantage of a grid connected system is that you do not need to invest in expensive battery storage, but if the power goes out on the grid you also lose power.

In Maine, grid-connected PV systems are treated with “net metering” or “net energy billing.” When you are using more electricity than you are producing, your meter measures the power coming from the grid to your home, but whenever you produce more power than you use, the meter essentially runs backwards. If you produce more than you use in a billing cycle, you get credit for the extra energy you produced.

This credit will go towards future bills over the following 12 months. If after 12 months you have not used all the credits, you lose them. You can avoid this by sizing the system to produce a little bit less than your expected annual usage. (Incidentally, if you have more than one account with the power company, excess power at one account can go toward paying your other account, through “virtual net metering.”)

Last year the passage of LD 1711 expanded the concept of virtual net metering to a much larger scale, making community solar farms available to all consumers. A residential consumer can buy an interest in one of these farms and effectively own a certain amount of electricity generation capacity. The net metering applies to your investment in the same way as it would if the panels were installed at your home.

In the end, your decision about solar panels obviously needs to be informed by your motives: Are you mostly concerned with reducing your carbon footprint, or are you trying to become energy-independent? Is protection from future grid power outages a priority? How much can you invest now for longer term savings? Putting it all together is your job. We hope this has given you some tools to get to work!

Paul Stancioff, PhD., is a professor of Physics at the University of Maine Farmington who studies energy economics on the side. He can be reached at [email protected] Cynthia Stancioff, MA, Public Administration, is an amateur naturalist and wordsmith.


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