The first step in any homeowner’s solar journey is determining how many solar panels it takes to power your house. The average household needs between 17 and 21 solar panels, but this total fluctuates based on several variables, such as your average electricity usage, home size, and local climate. A professional solar installer can help you determine how many panels you need, but our guide includes a basic formula to get you started.

How To Calculate How Many Solar Panels You Need

You’ll need to know three key factors to calculate how many solar panels you need:

  1. Your annual electricity usage
  2. Your solar power system’s estimated production ratio
  3. The wattage of the solar panels you’re considering

You’ll then divide your annual electricity usage by the estimated production ratio, and divide that number by your solar panels’ wattage. We’ll break down these factors and the formula in more detail below.

Annual Electricity Usage

Your energy consumption is the amount of electricity your home uses annually. This number is measured in kilowatt-hours (kWh) and includes all lights and appliances, such as air conditioning units, water heaters, and refrigerators. The U.S. Energy Information Administration (EIA) reports that a U.S. residential customer’s average annual electricity consumption was 10,632 kilowatt-hours (kWh) in 2021, with an average of 886 kWh per month.

Production Ratios

A solar panel system’s production ratio is its estimated energy output over time (in kWh) compared to its actual system size (in W). You might assume this is a simple 1:1 ratio, but complex variables come into play, such as the amount of sunlight your home receives versus the amount of shade.

For example, a 10-kW system that produces 14 kWh of electricity in a year has a production ratio of 1.4 (14/10 = 1.4). This applies to a home in a sunny state such as Arizona, where there are large amounts of sunlight all day, year-round. If your home is located somewhere like New York, where it can be cloudier and there’s more rain, this production ratio might dip to 1.2. 

This is a complicated ratio, so we advise you to contact a professional for the best results. 

Solar Panel Wattage

You might think that the best solar panels are all built the same, but that’s not true. There are different solar panels for different projects with different energy outputs. A panel’s wattage is how much electricity it emits, and most residential solar panels range between 300 and 450 watts of power. The higher the wattage, the fewer panels you’ll need. 

The Formula

The actual formula a solar installation company will use to quote you a solar panel system is as follows: 

  • Number of panels = system size/production ratio/panel wattage

If we use a simplified version of the numbers we listed above, we get the following:

  • Number of panels = 11,000 kW/1.4/300

This formula equals approximately 25–27 panels. However, you may require fewer or more solar panels depending on your specific home. The National Renewable Energy Laboratory (NREL) maintains a calculator to help you estimate your needed system size.

Other Ways To Calculate How Many Solar Panels You Need

A simple alternative to the formula above is to look at your monthly electric bills and determine how much energy you use. Then, multiply your energy usage by the hours of sunlight your home gets, and divide that result by the wattage of the panels you’re considering.

Factors That Influence How Many Solar Panels You Need

Though the three factors above can give you a general idea of how many solar panels you’ll need, other factors can influence that total. Read about some other important considerations when building a solar panel system below. 

Roof Size and Layout

If your roof is small or unusually shaped, you’ll need to calculate the number of solar panels you need carefully. Homeowners with large roofs may choose to sacrifice efficiency and buy larger panels to achieve the ideal energy output. Those with smaller roofs must use a smaller number of high-efficiency panels to produce enough energy to power their homes.

Your roof’s layout and design also determine your solar panel system’s size. For example, chimneys or unique roof designs might mean you can only install a certain number of panels.

Hours of Sunlight

The amount of energy you’ll get from your solar panels is directly related to how much sun your home receives. If your home is in an area with limited sunlight, it will require more panels. 

Solar Panel Type

The type of solar panel you choose affects how many solar panels you order. For example, you’ll need more polycrystalline panels than monocrystalline to power your home because polycrystalline panels are less efficient. These are the three main types of solar panels:

  • Monocrystalline: These are the most efficient and cost-effective solar panels. They’re most commonly offered by solar panel companies, and you can order less of them than other types given their high efficiency of 15%–25%. 
  • Polycrystalline: These panels are less efficient than monocrystalline panels, with average rates of 15%–17%. You’ll have to buy more panels to match the output of a monocrystalline system, but polycrystalline solar panels cost less than monocrystalline ones. 
  • Thin-film: Thin-film solar panels have a broad efficiency range of 10%–20% and are used for more specialized projects, such as converting a shed, recreational vehicle (RV), boat, or small guest house.

Solar Panel Efficiency

Solar companies use efficiency to measure a solar panel’s ability to convert sunlight into usable electricity. Efficiency ratings are expressed as a percentage to help you quickly compare different solar panels’ performance. For example, if a solar panel has a 25% efficiency rate, 25% of the sun’s energy it captures is converted into usable electricity. A solar panel’s solar cell composition, roof placement, and design all affect its efficiency.

Opting for more efficient solar panels will cut costs long-term because you’ll receive more energy savings over time and a higher return on investment (ROI).

Our Conclusion

Though the formulas provided in this article can give you a general idea of how many solar panels you’ll need, they don’t provide a definite answer. This is because the number of solar panels you need is highly individual and depends on various factors, such as your roof layout and the panels’ size, efficiency, and cost.

We recommend contacting a solar installer that services your address. A professional can evaluate your home and tell you exactly how many solar panels you’ll need in addition to solar batteries and other components. Solar installers can also help you access the federal tax credit and local incentives to reduce costs and maximize your savings.

Use our tool below to connect with reputable solar installers that offer solar panel installation in your area.

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