# How many watts do you currently use?

Look at your electricity bill for average usage. Look for “Kilowatt Hours (or kWh) Used” or something similar, and then note the length of time represented (usually 30 days). If your bill doesn’t show kilowatt-hours used, look for beginning and ending meter readings and subtract the previous reading from the most recent one.

You want daily and hourly usage for our calculations, so if your bill doesn’t show a daily average, just divide the monthly or annual average by 30 or 365 days, respectively, and then divide again by 24 to determine your hourly average electricity usage. Your answer will be in kW. (And just in case you’re wondering, a kilowatt-hour is how much power you are using at any given time multiplied by the total time the power is being used.)

A small home in a temperate climate might use something like 200 kWh per month, and a larger home in the south where air conditioners account for the largest portion of home energy usage might use 2,000 kWh or more. The average U.S. home uses about 900 kWh per month. So that’s 30 kWh per day or 1.25 kWh per hour.

Your average daily energy usage is your target daily average to calculate your solar needs. That’s the number of kilowatt-hours you need your solar system to produce if you want to cover most if not all of your electricity needs.

It’s important to note that solar panels don’t operate at maximum efficiency 24 hours a day. (See Solar 101: How Does Solar Energy Work?).

Weather conditions, for example, can temporarily reduce your system’s efficiency. Therefore, experts recommend adding a 25 percent “cushion” to your target daily average to ensure you can generate all the clean energy you need.

How many hours of sunlight can you expect in your area?

The peak sunlight hours for your particular location will have a direct impact on the energy you can expect your home solar system to produce. For example, if you live in Phoenix you can expect to have a greater number of peak sunlight hours than if you lived in Seattle. That doesn’t mean a Seattle homeowner can’t go solar; it just means the homeowner would need more panels.

The Renewable Resource Data Center provides sunlight information by state and for major cities.

Now multiply your hourly usage (see question No. 1) by 1,000 to convert your hourly power generation need to watts. Divide your average hourly wattage requirement by the number of daily peak sunlight hours for your area. This gives you the amount of energy your panels need to produce every hour. So the average home (900 kWh/month) in an area that gets five peak sunlight hours per day would need 6,000 watts.

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