How Much Solar Power Does My Home Need (Part 2)?

Part one of this blog post discussed the first two things you need to determine when you’re trying to figure out how big of a solar energy system is required to meet your power needs. That includes calculating your total energy usage and determining your solar goals. Next we’ll go into two additional factors, as well as the calculation.

Determine Sunlight Hours

Another factor in determining your solar panels needs is how much sunlight your roof will get on an average day. More sunlight hours means you can get fewer panels because they will be able to absorb solar energy for longer. Fewer hours, or periods of shade in and around your home will require more panels to generate the same amount of power.

Account for Panel Efficiency

Not all solar panels are created equal, so a final calculation you need to make is based on the wattage and efficiency of your system, and the weather in your area, which can impact panel efficiency (panels in Las Vegas are going to get more sun than those in Oregon).

The Calculation

Once you know all this information, multiple your hourly energy usage by 1,000 to get the number of watts you need. Multiply that by the daily peak sunlight hours to find out how much your panels need to produce hourly, then divide that by the number of watts each panel can produce.

Example: your home uses 900 kWh per month

1. 900 / 30 days = 30 kWh/day
2. 30 / 24 hours = 1.25 kWh
3. 25 x 1000 = 1,250 watts per hour
4. 1,250 x 5 peak sunlight hours = 6,250 watts

Panel efficiency ranges from about 150 watts on the low end to 370 on the high end, so to calculate number of panels:

5. 6,250 / 150 watts = 42
6. 6,250 / 370 watts = 17

In this calculation you need 17-42 panels, depending on the efficiency of your system. It’s also a good idea to build in a small cushion in case your panels don’t perform at their peak efficiency at all times, so add about 25% to your panel needs. That requires 21 panels (high efficiency) up to 52 panels (low efficiency). Your roof size may be a limiting factor if the part that faces the sun isn’t large enough to fit all the panels you need. In that case talk to your installer about ground-mount systems or other options.

If math isn’t really your thing, Intermountain Wind and Solar has a solar calculator on our website to determine how many solar panels you need.