How Many Solar Panels Do You Need?

How many solar panels you would need to install on your roof depends on your energy needs, how much sunshine you get, the nature of your roof, and the sizes and capabilities of the panels to be installed. The math is simple, but the number of variables makes it complicated. Knowing how many panels you'll need will determine whether or not installing solar on your home will be a good investment.

Variables to Consider

First, let's determine how much electricity you want to generate, which may be the easiest question to answer. Then we'll weigh the variables that go into determining how big of a solar system you'll need, including available sunlight, available roof space, panel size, and the efficiency of solar panels.

Electricity Needs

If you plan on hooking your solar system up to the electricity grid, then all you will need to do to determine your electricity needs is review your electricity bills for the past year and find your annual electricity usage, measured in kilowatt-hours, or kWh. Over the course of the year, you will use more electricity during certain times of year (such as for air conditioning in summer), and other times when you use less. In states with net metering programs, your utility company will credit you a portion or all of the excess electricity that you generate but do not use, and apply that credit to those months when you use more than you generate.

Plan ahead. Solar panels have a long lifetime—25 years or more. Consider that you may have higher or lower electricity needs in the future: children may move in or out of your home, and you may purchase an electric heat pump to replace an oil-burning furnace, or replace a gas-powered car with an electric one. But if you overbuild, you may not get credit for all of the energy you produce.

Available Sunlight

To reduce the amount of solar panels you need, get the most out of them by installing them in the place that receives the most amount of year-round sunlight. Nothing replaces an onsite estimate from a solar specialist with specialized equipment for measuring sun exposure, but the PVWatts Calculator from the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) or Google's Project Sunroof can give you rough estimates of how much usable sunlight is available on your property and how much electricity can be produced by installing panels there.

Available Roof Space

You'll want to know how much of your roof is able to receive sufficient sunlight to make it worth the cost of putting panels there. That depends on the dimensions of your roof, its orientation, and how many hours of sunlight it receives per day. The more sun, the more likely you are to receive a return on your investment. Too little sun, and a rooftop solar system might never pay for itself—in which case, consider a community solar farm as an alternative. That way, you can still generate all the electricity you need without installing anything on your own property.

Panel Sizes

Solar panels come in different sizes, different costs, and with different levels of power and efficiency. The larger the available space on your roof, the more you will be able to use larger panels, which may not be the most energy-efficient option but are more cost-efficient because the area they cover will require fewer structural supports. For smaller spaces, more energy-efficient panels make more sense. 

Efficiency

Efficiency is determined by the percentage of sunlight that panels can ideally convert into electricity. Solar panels decline in efficiency about 0.5% per year. Even if the degradation rate was double that, solar panels will still operate at approximately 80% after 25 years. New technologies increase that efficiency all the time, though of course the most efficient panels will be more costly. Investing in the most efficient solar panels probably only makes sense if you have limited rooftop space.

How to Calculate the Number of Solar Panels You Need

To calculate the number of solar panels you need, you need three figures: your energy needs, the power capacity of each panel, and the panels' production ratio. Power capacity is determined by the manufacturing process, while production ratio is determined by the environment those panels are installed in.

Power Capacity

The specifications on a solar panel will include its power rating, measured in watts, with “watt-peak” being the highest possible power output of the panels. Don't expect your panels to always achieve watt-peak, however, since this is its output under ideal conditions: full sunlight, perfectly clean panels, operating under maximum efficiency. A typical panel might produce anywhere between 250 and 400 watts. Under those ideal conditions, for example, in over five hours of direct sunlight, a panel that produces 300 watts of power will produce 1,500 kWh of energy. When you are comparing solar systems, price comparisons are usually made in dollars per watt. Those costs have declined dramatically over the past decade. Including installation costs, a 22-panel rooftop solar system cost on average $7.53/watt in 2010. By 2020, that price had fallen to $2.71/watt.

Production Ratio

A panel's efficiency is only one factor in determining how much energy it can actually produce. Environmental conditions, such as the amount and intensity of sunlight, cleanliness of the panels, and ambient temperature, affect the production ratio, which is the ratio of how much estimated energy a panel can actually produce compared to its rated "watt-peak" capacity. Depending on where you live and how your roof is oriented, a production ratio might be anywhere from 1.1 to 1.8. If, for example, you live in a sunny spot in Southern California, your production ratio might be 1.6, whereas if you lived in regularly overcast Seattle, your production ratio might be 1.3. 

We're Ready to Do the Math

Get your calculator out. To determine the number of panels you would need to produce 100% of the electricity you need, divide your annual electricity needs by the production ratio by the power capacity of the panels (annuals needs / production ratio / power capacity). Here are two examples:

If you're the average American, you consume 11,000 kWh of electricity per year. If you live in Southern California and your production ratio is 1.6, you will need a solar system that can produce 6,875 kWh over the course of the year. If you decide to purchase panels with a generating capacity of 320 watts each, you will need 21.5 panels to supply your annual energy needs.

If you live Seattle, consume the same 11,000 kWh annually, and your production ratio is 1.3, you will need a solar system that can produce 8,461 kWh over the course of the year. Perhaps you have a smaller roof and have decided to invest in higher efficiency panels that can produce 360 watts each. You would need to purchase 23.5 panels to supply your annual energy needs.