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  • Benadette Wambui

Solar Energy Calculations

How to calculate solar panel output is a common question. It makes sense, considering the impact solar panel production has on the overall system. Exactly how to calculate the power output of your solar panels involves a few variables and steps.

Some of the most important factors in figuring out your solar panel output include:

  1. Your solar panels’ efficiency.

  2. How much sunlight shines on your solar panels(location).

  3. Which direction your solar panels are facing

The three above are the main contributors but there are other factors that change the final output. We will go over each of these as well as give a formula for exactly how to calculate solar panel output.

Solar Panel Efficiency

While wattage can tell you what your solar panel is capable of producing under ideal conditions, efficiency tells you how much sunlight your solar panel is capable of converting into electricity you can use in your home. For example, if your solar panel has an efficiency rating of 13 percent, that means that 13 percent of the sunlight striking your solar panel will be transformed into the power you need to toast bread or do a load of laundry.Solar panel efficiency can be affected by a few variables itself, which can either subdue or boost it. Within the solar cells themselves, there can be variation in efficiency based on how reflective the cells are. Less reflective cells can collect more sunlight and use it rather than bouncing it back out to space. The area around your rooftop solar panel system can also change your efficiency numbers. The most common environmental factors that can subdue efficiency are:

  • Shading from nearby trees or other buildings.

  • Excessive cloud coverage

  • Excessive dirt, dust, and pollution

  • Thick layers of snow

Shading is generally a fairly obvious efficiency blocker and should be avoided if at all possible. Cloud coverage does not mean that absolutely no sunlight will make it to your solar panels, but the amount will obviously be reduced.

Dirt, dust, and pollution can degrade solar panel efficiency over time. Rainfall is a natural and easy way of cleaning them off. If you live in a particularly arid region where rainfall is scarce and dust is abundant, you can clean your solar panels yourself or hire someone to do it for you.While it’s true that too much heavy snow can lower efficiency, some snow is actually a good thing because any dust, dirt, and pollution will cling to it and slide off the slick panels when the snow melts. Also, like most electronic equipment, solar panels operate well in cooler conditions.

How location and direction affect solar panel output

Where you live has a big impact on how much energy your solar panels are capable of producing. That’s why solar was first put to use in sunny places like the U.S. Southwest.

The more access to the sun your solar panels have, the more they can produce. While some states in northern latitudes get less than 4 hours of sunlight per day on average, others can get more than 7.5 hours. The direction your solar panels are facing is another variable in solar output that is closely related to the general location of your home and also any potential shading factors. The general idea here is fairly intuitive as well. If your solar panels are placed in a direction that receives the most sunlight, you give them the best chance to capitalize on that solar energy.

Whether the best direction for your solar panels is south or west depends on a few differences in both the physical space or your rooftop and the surrounding environment, as well as how your utility structures its electricity rates.

How to calculate solar panel output

This is a simple formula for calculating your solar panel’s power output.

Solar panel watts x average hours of sunlight x 75% = daily watt-hours

As an example, let’s say you have 250-watt solar panels and live in a place where you get 5 hours of sunlight per day. What’s that 75 percent for? That’s to account for all those variables we've been going over.

250 watts x 5 hours x .75 = 937.5 daily watt hours

To translate this into the more familiar kilowatt hours you’re used to seeing on your electricity bill, simply divide by 1000.

937.5 / 1000 = 0.937

To round up and make it pretty, that’s 0.94 kilowatt-hours per solar panel.

Learn more about solar energy calculations at HURU Schools' Solar Certificate

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