How Many Solar Panels Do I Need for My Home?

Are you getting ready to install solar panels on your home? One of the first questions you need to answer is: how many solar panels do I really need?

The answer to this question is always the same: “it depends”. The number of solar panels you need depends on a lot of different factors. Today, we’re going to explain those factors and help you decide the best number of solar panels for your home.

Top 3 Things to Consider

The top 3 things to consider when installing solar panels on your home include:

  • The state where you live
  • The amount of electricity you use
  • The amount of roof space you have

You can probably guess why these things matter. Your state matters because some states have rules on the maximum size of solar panels. Many states and local governments have a maximum system size of 100% or 120% of your average annual usage, for example. Other states don’t have net metering in place, which means you can’t sell excess power back to the grid – so producing more than 100% is pointless.

The other important variable is roof space. Roof space is easy to calculate. The ideal roof is an unshaded, south or southwest-facing roof. Measure the square footage of available space on your roof, then use that for your calculations next.

Average Size and Solar Panel Output

Once you’ve considered the three things above, it’s time to figure out how much solar energy you need. Different solar panels have different outputs. You might require 10 solar panels from one company to create a 5kW system, but only 5 panels from another company.

In general, here are the average metrics for a solar panel:

  • Width: 39 inches
  • Length: 65 inches
  • Watts: 250
  • Watts Per Square Foot: 15

Most solar panels are very close to these metrics. Solar panel sizing isn’t quite 100% standardized. However, almost all solar panels sold today at 39 inches wide by 65 inches long.

You can use these approximate metrics to calculate the number of solar panels that can fit on your roof. When calculating the number of solar panels needed for your home, you can use an approximate square footage of 17.5, which is the average square footage of a single panel based on the measurements above.

Don’t Forget About Solar Panel Setback

There’s one other restriction to know about before you calculate the number of solar panels for your roof. There’s something called “solar panel setback”. Basically, setback means that you can’t place solar panels edge-to-edge across your entire roof. There needs to be a certain amount of space on the sides of your roof.

In California, for example, roofs are required to have a setback of 3 feet.

What’s the point of setback? The most important thing is safety: having a setback allows firefighters to access the roof in an emergency. In any case, you’ll need to leave about 3 feet of space between your solar panels and the edge of the roof, which can significantly reduce the number of solar panels that can fit on your roof.

How Many Solar Panels Will Fit on Your Roof?

Now that you know the information above, you can calculate the number of solar panels that will fit on your roof. Just follow these steps:

  1. Take the available square footage of your roof and multiply by 0.75 to account for setback
  2. Divide the resulting square feet by 17.5 (which is the average square footage of a solar panel).
  3. A panel will produce about 250 watts on average, but systems measure energy production in kilowatts (kW), which means you have to divide by 1,000. 250 divided by 1,000 = .250 kW per panel.
  4. Multiply your number of panels by your energy production to get the estimated energy production of your system.

How Much Energy Will Your Solar Panels Produce?

A solar panel in Alaska is going to produce less energy than a solar panel in Arizona. Solar panel energy generation varies widely across the United States – however, it’s probably not quite as different as you think.

A solar panel system installed in Alaska, for example, will produce about 900 to 1100 kWh/kW-yr, while that same system would produce 1900 to 2000 kWh/kW-yr in Arizona and Hawaii. That’s a big difference – but it’s not as big as some people would think, considering the differences in climate.

The National Renewable Energy Laboratory put together a handy map that shows the average sunlight and solar energy generation for different parts of the United States. The map shows that even in “cloudy” parts of the country like Portland, Oregon, solar energy is still a valid energy solution.

Using that map is straightforward. You multiply the rating of your solar energy system by the number associated with the color on the map. If your solar energy system is rated 5.00, for example, and you live in the 1600 zone around New York City, then your solar energy production is 8,000 kWh per year.

For a more precise calculation, multiply that final number by 0.8. You’ll always lose some energy based on efficiency, panel performance, and losses from wiring, among other factors. That gives you a total energy production of 6400 kWh per year.

Consider your Household’s Energy Requirements

Check your last few electricity bills and consider your energy consumption for the last few months. For most homes, a 5kW solar panel system is enough to shave several hundred dollars off your energy bill every year – but it won’t cover the full energy usage of your home.

Decide whether you want your solar panel system to account for 100% of your home’s energy production (or higher), or if you’re okay with it accounting for about 50% of your energy production.

Conclusion

By considering the size of your roof, setback requirements, the size of your solar panels, and your required energy output, you can make an intelligent estimate on the number of solar panels needed for your home.

Alternatively, a solar panel installation company can do all of the above calculations for you. They may even do it for free as part of their initial estimate.

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