If you’re thinking about going solar, one important piece of information you'll be looking to get from an installer will obviously be their price tag.
But you’re also sure to get something else that’s equally important—an estimate of the amount of energy their proposed system will generate each year, measured in kilowatt-hours (kWh).
Multiplying that annual production estimate by the 25-to-30-year expected lifetime of a solar system gives you an estimate of the total amount of solar energy it will generate
Once you've got that, all you have to do is divide the price of installation by it to estimate how much each kWh of solar energy will wind up costing you.
Nowadays, generating solar energy from your roof is likely to be significantly cheaper than the rate your utility company currently charges. Moreover, since utility rates are sure to climb even higher, that makes the lower rate you can lock in by going solar an even better investment.
Of course, all of that depends on the accuracy of your production estimate.
So, the question becomes:
What guarantee do you have that the solar system the installer is proposing really will generate the amount of energy they’re claiming it will?
How solar production is estimated
To get an accurate estimate, the first step is to use satellite imagery to design a solar system that fits your roof.
Once that’s done, the amount of energy it will produce depends on four factors.
Azimuth. This refers to the horizontal direction your solar panels are facing, which will be determined by the direction of the roof surface they're installed on. In the Northern Hemisphere, the closer your solar panels are to facing south, the more sunlight they’ll get and, hence, the more solar energy they’ll produce.
Tilt, also known as pitch. This refers to the vertical angle of your panels, which will be determined by the angle of the roof surface their installed on. Your panels will get the most sunlight and, hence, produce the most solar energy if they’re tilted somewhere between 30 and 45 degrees.
Shade. Whether it’s trees, another building, or some other kind of obstruction, anything that blocks sunlight from hitting any of your panels for any part of the day is going to decrease the amount of energy they produce.
Location. Once the azimuth, tilt, and shade of each of your panels are determined, that information together with your location gets plugged into computer software designed to estimate production. For a truly reliable estimate, the software must also consider the technical specs for the specific make and model of your solar panels. It works by tracking the path of the sun across the sky where you live along with the likelihood and extent of cloud cover at any given time to calculate the amount of solar energy you can expect annually.
Is production guaranteed in your contract?
As you can see, coming up with a reliable production estimate is a pretty complicated business. At the end of the day, achieving any kind of accuracy depends on incredibly complex calculations that can only be performed by a computer.
That means its reliability is going to depend on how good the software is and whether the technicians using it have been adequately trained.
And, unfortunately, an unscrupulous contractor can make even the best software exaggerate production simply by fiddling with the azimuth or tilt.
The bottom line
Any production estimate not backed by a guarantee written is worthless.
That’s why, when reaching out to a solar installer, it’s crucial to make sure that:
Their production estimates are guaranteed in writing
The guarantee specifies that you’ll be compensated for production that falls below their estimate
If your installer isn’t willing to financially back up their production estimate in the contract, there’s simply no reason to trust it.
And, always remember: if their system doesn't deliver as promised, you're going to be the one left holding the bag.