The amount of electricity a solar panel produces depends on three main things: the amount of sunlight hitting the panel, the size of the panel, and the efficiency of the solar cells inside. We’ll break down what you can expect from a typical solar panel, and how that power output compares to the power you need for the gadgets and appliances you use inside your home.

What does the max power rating mean?

Your solar panels will have a number listed on the back that indicates how much power they will pump out during ideal conditions. This is called the maximum power rating. Labs that test solar panels calculate output using “peak sun,” or 1000 watts of sunlight per square meter of surface. That’s approximately equal to the power of the sun at noon, on a sunny day, at the equator.

Calculating solar panel output for your location

Since you probably don’t live at the equator, your roof will get a different amount of sunlight, and of course the amount of sunlight also varies based on time of day, the season, and the weather. You can’t use the maximum power rating to directly predict how much power you’ll get from a solar panel. It is possible to do a bit of math to get a better sense of how a solar panel will work in your location, however. Check out Weather Underground’s solar calculator– after you enter your address, you’ll get specific details about how much sunlight hits your house on average. The calculator also gives the option of entering a specific model of solar panel, and the square footage of the panel coverage on your roof.

What’s the power rating for an average solar panel?

A typical solar panel produces around 200 watts of power. There’s a little bit of variation on this, based on the size and efficiency of the solar panel you choose; you’ll see panels that produce 205, 210, even 230 watts. More efficient panels are a little more expensive, and are usually only needed if you have limited space on your roof. Your solar installer will work with you to figure out how much power you’re using, and then will size the whole installation correctly to meet your needs. It won’t matter as much how much each panel is producing as the whole array. A typical installation might be about a 5 kWh array, or roughly 25 panels.

How much power do my devices use?

There’s huge variation in power use between households depending on what you own and how often you use everything. Of course, every device is different, too. One basic old-fashioned lightbulb uses 60 watts of electricity; a CFL uses 18 watts. Laptops often use about 45 watts, and desktops can run between 150-300 watts. Window air conditioning can range between 500 and 1500 watts, and central air conditioning can use 3500 watts. In total, the average home uses about 958 kilowatts a month– with variations by season, especially if you use air conditioning or electric heat. Usage also varies between day and night. Unless you work at home, most of your electricity usage probably happens at night.

On or off the grid?

Since solar panels only generate power as the sun shines, you’ll need a way to store the energy. Though it’s possible to use a battery for storage, the easiest (and cheapest) solution for most people is to stay connected to the grid. If your solar panels are producing more energy than you’re using—when you’re at work, on vacation, or just not running many devices—excess power will flow back into the grid. In many locations, utility companies offer a program called “net metering” that can compensate you for extra power you produce. At night, or anytime you need extra power, you’ll pull it from the grid. With a grid-connected system, you’ll never need to worry if you happen to need more power than your solar system has been sized to provide. You may also choose to supply only part of your average electricity bill with solar, and use the grid for the rest. Your solar installer will talk to you about all of the options. Interested in getting a quote?