If you’re considering home solar power, it’s natural to have questions about where the solar panels will go — which is probably your roof.
How many panels do you need to meet your electricity needs? And, will your roof fit that number of panels? How do roof angles, solar panel ratings, and shade impact that?
The good news is every home solar system should be a custom-designed system that takes your roof, your energy usage, and more into account. The better news is you don’t have to figure any of it out on your own.
Solar panels work for most homes and most roofs. This article will answer many roof-related questions you may have.
How Many Solar Panels Can Fit on My Roof?
There are a few rules of thumb that can give you a general idea how much roof space is needed for solar panels. These guidelines can also help determine how much roof space you have available for solar panel installation.
Generally, every square foot of roof space has the potential to generate about 15 watts of solar energy. Thus, a solar panel installation on a small home might need about 200 square feet of roof space, while a larger home can require more than 1,000 square feet of roof space to properly offset electricity usage.
To offset an average amount of energy usage by the average American home, you’ll typically need 18-24 panels. That is, of course, if the panels are of a standard rating, their positioning is optimal, and the location gets adequate sunlight year-round. If you change any of those variables, the number of panels you need in your array is going to change as well.
If you want to get a sense of how many panels a roof can support, you don't need a fancy solar panel square footage calculator. Here’s an easy calculation you can do.
Multiply the square footage of your roof by .75 to account for the required solar setback. (More on that below.) Take that number and divide it by 17.5, which is the average square footage of the standard solar panel size. The resulting number is the maximum number of solar panels you can fit on your home’s roof.
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How Much Power Can My Roof Generate?
Many solar panel arrays, as we said above, have 18-24 panels. So let’s take 24 panels as an example. Then, let’s say they are 400-watt panels as it’s a common rating. Pretty simple math here is 24 multiplied by 400 to total 9,600 watts.
Capacity, for a panel or a whole array, is measured in watts. The electricity produced is measured in watt-hours or, more commonly, in kilowatt-hours (kWh). If a solar panel produces 400W of electricity for one hour, it is producing 400 watt-hours, or 0.4 kWh. So 24 panels, with ideal power-generating conditions, could produce 9.6 kWh in an hour.
Learn more about solar panel output with more detailed math and examples. You can also go to the National Renewable Energy Laboratory’s PVWatts Calculator, enter your address, and let the calculator help.
How Close Can Solar Panels Be to the Edge of the Roof?
Most roof-mounted solar installations will need a “solar panel setback” for safety. This is one of the most common requirements for roof solar panels in local and state building codes. This setback is the open space between the edge of the solar array and the edge of the roof, and it provides an unobstructed pathway around your rooftop for emergency responders like firefighters to get better access to your home in case of an emergency.
The minimum solar panel setback varies from state to state, but generally, the setback will take up about 25 percent of your roof’s usable space. This accounts for two roughly 36-inch wide pathways that run along the edge of your roof, on a roof with just two basic faces. If your roof is more complicated than that, with multiple faces, or different shapes that come together at odd angles, your setback requirements may be different, which is why it’s important to work with solar professionals when designing your home solar power system.
Factors to Determine How Many Solar Panels You Need
When determining how many solar panels you need, it’s important to start with your goals and why you want to go solar in the first place.
Do you want to maximize your return on investment?
Do you want to save as much money as possible?
Do you want to reduce your upfront costs?
Do you want to reduce your carbon footprint as much as possible?
Most people want a balance of these goals and may have other priorities as well, so it’s helpful to get a clear idea of your specific goals before you start designing a solar power system.
Once you have your goals in mind, you can determine how many solar panels you need to get there. This calculation is going to depend on how much energy your family uses, your roof area and specifications, your home’s location, how much sunlight shines in your part of the country, the efficiency of the solar panels you’re using, and if your local utility offers net metering. Plus, you need to consider your budget, because a large solar power system might produce more energy, but the initial installation is going to cost more.
Here are a few things you should think about when determining how many solar panels you need for your roof.
Energy usage
How many solar panels you’ll need, and thus how much roof area you’ll need, starts with an estimate of how much power your family uses in a given year. There are plenty of ways to determine your annual energy usage, but the easiest is to take a look at your monthly energy bills over a year. That will tell you how many kilowatt-hours of energy you used in a year. If you don’t know your own estimated energy usage, a good starting number is the average American home uses about 11,000 kWh of energy every year.
You should also consider potential changes to your family’s energy usage in the future. For example, if you buy a new electric vehicle, start working from home more often, or expand your family with a new child, your energy needs might change pretty significantly.
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Location
Different parts of the US get different amounts of sunlight. For instance, Juneau, Alaska gets 85 days of significant sunny conditions a year according to NOAA data. Yuma, Arizona gets 313 sunny days a year. The average across the US is 205 sunny days.
This impacts how much roof space is needed for solar panels. If you live somewhere with lots of sun, you might need fewer solar panels. But if you live in Juneau, you’ll need more solar panels to generate a similar amount of energy.
The direction of your roof also determines how many solar panels you need. In the northern hemisphere, southern-facing roofs are ideal, as they receive more direct sunlight and can use that sunlight to create more energy. If your roof does not face south, you may need either a more complicated installation to get your panels facing the right direction or more panels to make up for the difference in energy-creating potential.
Size and rating of your solar panels
Solar panels vary in efficiency. More efficient panels may be able to squeeze more energy out of a smaller space. If you have the space for them, less efficient panels could meet your goals for less money. That’s why panel specifications are part of determining the ideal system size for your roof.
While the efficiency of solar panels might vary, solar panel sizes typically don’t, as most companies have a standard solar panel square footage to make installation easier. The standard solar panel size dimensions are about 65 inches by 39 inches, which is roughly 17.5 square feet.
Your budget
Generally, larger systems are a great way to quickly offset your current electrical and fossil fuel energy usage. However, larger systems are naturally more expensive. While you may have the roof real estate for a large array, you might not have the financial budget for it, and vice versa.
Another thing to consider when figuring out your budget is whether your local utility offers net metering, and what rate they offer for that net metering. If you’re not familiar, net metering is when your utility company offers you credits for the extra energy your system produces and feeds back into the grid. These credits can be used to offset the cost of power you might use from the grid, such as at night or during storms if you don’t have a battery storage system. If your local utility offers a generous net metering policy, it may allow you to expand your initial budget and then make up that difference over time. Net metering often comes with restrictions on how large of a system you can install.
Is it Possible to Install Too Many Solar Panels?
Believe it or not, it’s not always beneficial to install as many solar panels as you can possibly fit on your roof. Adding extra panels that aren’t needed just increases the cost, and if you don’t have a way of capturing or getting credit for the extra energy you’re generating, then you're not getting a good return on that investment.
A good solar installation should offset as close to the exact amount of energy that you use as possible. That’s why installers ask for samples of previous power bills when designing a system. These power bills help them estimate your power requirements, and design a system that matches your specific needs. Some months you might use more energy than your system produces, and some months you might use less energy than you produce, but at the end of the year, the goal is to generate about the same amount of energy as you use.
That said, there are some instances where it makes sense to install more solar panels to generate more energy than you plan on using. The first is if you plan on installing an energy storage system to capture that excess energy. Solar battery storage lets you use the energy you generated during the day to power your home at night, and also gives you a backup source of power in case you have a blackout or other issue.
Another time you might want to generate more power than you plan to use is if your utility offers a strong net metering benefit.
How to Put Solar Panels on Your Roof
Your home’s roof space is just one of the factors that determine the optimum solar power system for your family’s needs. The arrangement of panels and the difficulty of the installation are determined by your roof, but you also need to consider your family’s energy needs, any future changes your family might expect, your local incentives and net-metering programs, and other individualized factors.
Palmetto can help figure out the precise number and type of panels that will work best for your roof and your family’s needs.To find out how many panels you can put on your roof, get started with a free solar estimate or contact a solar advisor today
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Frequently Asked Questions
What if my roof is too small for solar panels?
If your roof is too small, high-efficiency panels or ground-mounted panels may be a better fit.
Are rooftop solar panels a good fit for everyone?
No, while solar technology is becoming a better fit for more homeowners and steadily becoming more affordable, there are circumstances when home solar panels aren’t the right choice. A home with low energy bills or an incompatible roof design or material may not be a good fit.
Andrew GiermakWriter and EditorAndrew joined Palmetto in Charlotte in August 2024. He’s been a writer in journalism, then in business, going back to almost the 20th century. He’s lived in Indiana, Virginia, Pennsylvania, Virginia again, and now North Carolina for the last 12 years. He likes golf. Is he good at it? Not so much.
