Calculate Which Size Solar Panels You Need

by Shawn Roe | 1 Comment

Should I use Sanyo 200’s or Suntech 270’s? SunPower 210’s or Canadian Solar 240’s? Which size solar panel you use depends mainly on how much roof space you have available and how much electricity you plan to use (with solar power). Below are 6 steps to calculate which size solar panels you need for your building.

Step 1: Know Your Electricity Usage

Gather your old electricity bills for the past 12 months. Contact your utility company if you don’t have them and your electricity provider should gladly send you a printout with your past electricity usage. You’re looking for 2 things: (1) your monthly kW/h usage, and (2) your monthly cost.

Step 2: Know your Roof Direction

Solar Roof Sunset

Grab a compass (maybe your car navigation, or check Google Maps) and figure out which way is South. Go outside, look at your roof and see how much roof space is facing South, Southwest, East and West. The more South the better, but West is actually okay.

Step 3: Measure Roof Area

Measure your available roof space facing South, SW, and/or W. There are several ways to do this, but the easiest is just to let an installer give you a free quote. You can also use Google Earth, or simply get out on your roof with measuring tape.

Step 4: Calculate Array Size

Visit our solar calculator to calculate almost everything you need all in one.

Step 5: Calculate How Many Solar Panels Will Fit

Now that you know how much roof you have available for solar panels and how big of a system you need, you need to determine what size panels you will need to cover your electricity usage while staying within the space you have available.

Determine the number of panels you can fit on your roof by dividing your roof area (maybe 600 sq ft for a 2,000 sq ft home) by 16 square feet (500/ 16 = 31.25). Round down, as you cannot fit partial panels on your roof. So I can fit a maximum of 31 panels on my roof.

Step 6: Calculate What Size Panels You Need

Now divide the array size (of the solar electric system) you need, by the number of panels you can fit. For example, let’s say you need a 6 kW solar system to cover 90% of your electricity usage and you can fit a maximum of 31 panels on your roof. So 6,000 kW / 31 panels = 193 kW/panel. So to meet your needs, you need to use solar panels rated above 193 kW (PTC)/ panel. This means you’ll probably use panels advertised as 220 kW (STC) or higher.

Next, use our Solar Cost Calculator to estimate the cost installing a solar PV system.


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