People often ask how many solar panels it takes to run a home with a particular square footage - a 2,000 foot or 2,500 house, for example. But square footage won’t answer that question because different homes of the same size can use wildly different amounts of electricity.
The number of solar panels you need to run your home depends completely on how much electricity you use in a year. You can find this out by getting a copy of your electric bill and looking for the section that tells you how many kilowatt hours you were billed for. Once you have that number, you can use our calculator to quickly find out how many solar panels you need to generate an equivalent amount of solar electricity.
This part scares off some people because it seems a little bit technical, but it’s really not difficult at all. Our solar calculator takes care of the math, and we wrote an article that goes into detail about how to read your electric bill. You just need to understand a few concepts, and then you’ll be all set. Here’s a few terms:
A watt measures electrical power. One kilowatt is equal to 1,000 watts.
Okay, so what is power? What it comes to electricity, power (or wattage) tells you how much electricity is used by something. For example, think of light bulbs. A 40 watt incandescent light bulb is dimmer than a 100 watt light bulb because it uses less power. A big air conditioner uses more power than a small one, and so it will have a higher wattage rating.
Some more examples: ten 100 watt incandescent bulbs is equal to one kilowatt. A refrigerator, when running, draws about half a kilowatt. A space heater or your hair dryer is about 1.5 kilowatts.
A kilowatt hour measures how much electrical power you’ve used over time. This is the measurement that you care about when it comes to your utility bill. To calculate kilowatt hours, simply divide the number of kilowatts by time.
For example, if you turn on a 100 watt light bulb, after one hour it will have used 0.1 kilowatt hour. If you keep it on for 10 hours, it will have used 1 kilowatt hour.
While we’re at it, we’ll explain that volts are a measure of electric potential, which you can think of as being analogous to water pressure.
Let’s think of this in terms of garden hoses. Voltage is like water pressure: with very low pressure you might be able to stop the flow of water with your thumb, but you can’t with high water pressure. The water pressure forces your thumb out of the way.
Wattage is like the diameter of the hose: a wide fire hose lets more water through than a narrow garden hose.
Finally, kilowatt hours is like the gallons of water that your hose has put out over time. For example, if you leave a garden hose running for an hour, you might be able to fill a kiddy pool. But if you want to fill a big inground swimming pool, you’ll need to leave it running for a day or more. The big pool takes more gallons of water, in the same way that a big central air conditioner uses more kilowatt hours of electricity than a window air conditioner.
How much electricity do you use in your home: a kiddy pool amount, or an inground pool amount? This will determine how many solar panels you need to power your home.
To find out, look at your electricity bill. It will tell you how much power you use in a month, and hopefully also how much you used in the past year. To help you decipher your electric bill, we wrote an article that explains this in depth.
Once you know how many kilowatt hours you use in a year, you can use our calculator to find out how many solar panels you’ll need to generate an equal amount of electricity.
Why is it important to look at it over the year? Because electricity usage is seasonal. For example, if you live in the southwest and have air conditioning, you probably use a lot more electricity in the summer than you do in the winter. If you only looked at your electric bill in December, you would underestimate how many panels you really need.
Just because a solar panel is rated for 300 watts doesn’t mean that you should expect it to generate 300 watts when it’s sunny. In fact, most panels will realistically produce about 75% of their maximum rating in the best case.
There are a lot of factors that determine how much power your solar photovoltaic system will produce. These include:
The angle of the sun in the sky relative to the panels determines the intensity of the light they receive. This angle changes during the day as the sun moves across the sky, but it also changes seasonally: the sun tends to be higher in the sky in the summer than in the winter. Finally, your latitude on earth is also a factor, because the closer you are to the equator, the higher in the sky the sun will be. Most people mount their panels flat on their roof and therefore don’t control the angle at which they sit, but you need to know these variables to know how much power your panels will generate.
Your climate determines how much sunlight reaches the ground. A climate that tends to be cloudy and rainy gets let sunlight than one that is dry sunny at the same latitude.
The efficiency of your panels determines how much sunlight gets turned into electricity. Solar panel efficiency ranges from about 15% to 23%. Premium panels made of monocrystalline silicon rather than cheaper polycrystalline are more efficient, but also more expensive. Read our guide on solar panel specifications to understand more.
Shading is a major factor in your power generation. If you have any kind of shadows on your solar panels from trees, nearby buildings, or different parts of your own roof or chimney, this can have a major impact on your power generation. If this is the case, you should consider using microinverters or power optimizers with your system, because they can help minimize the negative effect of shading. We wrote an in depth article that explains the many different types of inverters.
Inverters also have efficiency ratings, although most are between 96-99% efficient, so inverter losses are usually not a major factor.
These are a lot of factors to consider. Fortunately, our calculator does all this complicated math for you. Just enter your zip code, which gives us your location on earth, and give your best estimate for your roof orientation. We handle the rest. Our calculation takes into account the solar angle, climate, and typical inverter and solar panel efficiencies and gives you a really fast but good estimate of how much power you can expect to generate, and how many panels you’ll need.
Not only that, but it will let you know what the federal, state, and local incentives are and what you can expect to pay for a system based on the average cost in your area.
If you want to learn more, the best place to start is to read our complete guide to buying solar for your home. It covers all the technical and financial aspects of buying solar for your home.
Once you’ve made the decision to go forward, you can use our free service to get quotes for a home solar installation. We prefer to work with local installers who have a long service record, not fly-by-night installers who have only been in a business for a couple years.
Still have something on our mind? We’re happy to answer any solar-related questions you have. Just send us an email or send us a tweet at @thesolarnerd.