How Solar Production Estimates Are Calculated

When someone looking to save money by going solar reaches out to an installer, one of the most crucial pieces of information they're likely to get will be an estimate of how much energy a system fitted to their roof will produce each year.

Once you know how much free solar energy you'll be getting annually, it's a simple matter to multiply that number by the 25-to-30-year expected lifetime of your solar panels.

You can then easily compare the total cost of investing in solar power to how much money you'll wind up shelling out to your local utility company if you don't.

So, a lot obviously hinges on the accuracy of solar production estimates, making it natural to wonder how they're arrived at.

The long answer involves extremely complex calculations that track the movement of the sun across the sky over the course of a year and take into account local weather conditions to determine how much sunlight will be hitting your panels.

In fact, these calculations are way too complex to be performed by a human being and, instead, have to be run on software specifically designed for the task.

Fortunately, however, the three main factors that go into calculating a solar production estimate are pretty straightforward, making it fairly easy to comprehend how they're used to determine the amount of energy a solar system fitted to your roof will generate.

Azimuth, tilt, and shade

Azimuth. This refers to the horizontal direction a solar panel is facing, which will be determined by the direction of the roof surface on which they're installed. 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 pitch of the roof surface they're 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. Finally, whether it’s trees, another building, or some other kind of obstruction, anything that blocks sunlight from hitting any of your panels for even a small part of the day is going to decrease the amount of solar energy they produce.

Calculating solar production

Once the azimuth, tilt, and shade of each of your panels are determined, that information together with your location gets plugged into the aforementioned computer software designed to estimate production.

The software 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. For a truly reliable estimate, the software must also consider the technical specs for the specific make and model of your solar panels.

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 intricate calculations that can only be performed by a machine.

So, the reliability of any solar production estimate you get is going to depend on how good the software they're using is. 

But an unscrupulous contractor can make even the best software exaggerate production simply by fiddling with the azimuth or tilt.

The bottom line is that any production estimate not backed by a written guarantee is worthless.

That’s why, when dealing with a solar installer, it’s crucial to make sure that:

  • Their production estimate is guaranteed in the contract.

  • 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 writing, there’s simply no reason to trust their numbers.

After all, without a written guarantee, you're going to be the one left holding the bag if it turns out that the production estimates you were given were exaggerated.

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