How Many Solar Panels Do You Need? - EmPower Solar

How Many Solar Panels Do You Need?

When considering solar, the first question our clients usually ask is – how much will a solar panel installation cost? This simple question has a somewhat complicated answer, since cost will vary based on how much energy you need, how ideal your home is for solar, and the efficiency of the panels that you choose.

For this exercise, let’s assume we’re looking at an average home on Long Island: 1,500 – 2,500 square feet of living space with 2-4 members of the household using oil or gas for heating and air conditioning for cooling in the summer months.

This average household will have an electric bill of around $210 each month. That means that it uses around 12,000 kWh per year. Kilowatt hours are used to measure energy, and are based on watts, which measures energy consumed over time. Example: the average electric dryer uses between 2 and 6 kWh per load. When you consider how many loads of laundry you do each month, it can really add up!

Efficient household Average household High energy needs household
$100 monthly bill $210 monthly bill $400 monthly bill
5,714 kWh per year 12,000 kWh per year 22,857 kWh per year


Now let’s focus on that average household using 12,000 kWh per year. The number of solar panels you’ll need to produce that much energy varies on a few difference factors, including shade, orientation, and pitch of your roof. For example, if you have a south-facing roof without shade, you’ll need fewer panels than a house with a roof that faces north, east, or west and has peaks and dormers. The same goes for measuring nearby trees: if your house is more shaded, you’ll need more solar panels to produce the same amount of energy. The formula we use to bundle all of these factors together is called total solar resource fraction (TSRF). The higher your TSRF, the better situated your house is for solar panels!

Here are a few examples from some recent clients across the spectrum with bills around $200.

TSRF 75% TSRF (some shade, using a small south roof and mostly west roof) 80% TSRF (a little shade, using south, east and west roofs) 99% TSRF (no shade using a southern roof)
Avg. electric bill $197 $221 $192
Avg. annual kWh offset 11,217 kWh per year 12,660 kWh per year 11,019 kWh per year
Solar panels needed 27 415-watt panels 36 327-watt panels 18 415-watt panels
Roof space needed 800 sq. ft. 1,000 sq ft. 500 sq. ft.

** Assumes siting of solar panels on Long Island or New York City. Energy may vary based on national locations based on solar insolation and weather data


As you can see, homes A + B are pretty similar in terms of TSRF and monthly electric bill, but home A needs 27 panels and home B uses 36 panels. One big difference here is the panel type they chose. Home B had a lot of roof space available so they went with an average efficiency panels – 327 watts per panel. Home A was more roof-constrained so they had to use a more efficient panel to get the same amount of energy output on a smaller roof space.

Comparing home C to homes A and B really shows how a southern facing roof with no shade impacts the amount of panels necessary. This home needs half the square footage as home B and ten less panels than home A to produce roughly the same amount of energy!

So when you ask “how much does it cost?” our answer is, well it depends on how many kWh do you need and what your TSRF is. While this is not something we expect any average homeowner to know, our in-house design team lives and breathes these calculations and are here to help! If you send us your PSEG bill, we’ll crunch the numbers for you and share a few different savings plans to help you explore which is best for your family.

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