How to calculate solar panel size

Written by joanne cichetti
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How to calculate solar panel size
It's essential to find out the right sized solar panel system before making your purchase. (Getty Images)

Solar panels come in different sizes to fit the energy requirements of your home. The size of the solar panel system depends on the amount of energy you want to produce. For instance, if there are two houses of the same size, they may still require different sized panels to meet their different energy needs. Determining the size of panel system for your home takes just a few minutes.

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Things you need

  • Calculator
  • Power bills for the last year

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  1. 1

    Look at your home's electricity bills for the past year. If you are unable to find them, you can ask your electric service provider to issue a duplicate copy of the bills, or they may be available via your service provider's website.

  2. 2

    Note the kilowatt hours used on each electric bill. These figures will most likely be in hundreds. Usually, the figures for winter months are greater than those for the summer season due to greater power consumption in winters.

  3. 3

    Calculate the average kilowatt hours used at your home per month. Add the total number of kilowatt hours used for one year, then divide the answer by 12.

  4. 4

    Figure out the kilowatt size of the solar panels required at your home. Simply divide the average kilowatt hours you calculated in Step 3 by your average monthly hours of sunlight (see the Resource link to determine your average number of sunny hours per day). Let's suppose your average monthly kilowatt hour consumption is 347, while the average monthly hours of sunlight is 155. Your home would need a panel system that produces 2.2 kilowatts per hour.

  5. 5

    Measure the size of your roof and figure out how many panels you need. Let's say you pick a solar panel that produces 125 watts. To produce 2.2 kilowatts per hour, you will need 17 panels. The average dimension of a panel that produces 125 watts is 150 by 66 by 5 cm (59 by 26 by 2 inches). This size depends on the number of solar cells installed on it; a panel that produces 100 watts will obviously have fewer cells and will measure about 142 by 56 by 2.5 cm (56 by 22 by 1 inches) . Similarly, the size will vary slightly according to the watts produced by each panel. However, once you have figured out the size of the panel system required (determined in the last step), it will be easier to choose the panels. You must have the exact measurements of your roof so you can decide the panel size accurately. If your roof is small, you will need panels that produce more energy. The size difference between a 100-watt panel and a 125-watt panel is not great, but you will obviously have to install more panels if you choose the 100-watt version.

Tips and warnings

  • The figures mentioned in the article above are only meant to demonstrate how you can calculate the size of panels for your home.
  • Click the link in Resources below to calculate your energy requirements and the number of panels required to run your house.

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