How to Calculate Solar System Size for Your Home

The average-sized newly built single-family home in the U.S. (which is around 2,480 square feet) will likely need to install a 9 to 10 kW rooftop array to meet projected electricity demands.

The majority of online solar marketplace quotes use a 5 kW model to estimate the cost of going solar. Data from actual installations suggests that most homeowners install arrays significantly larger than 5 kW though. Why? How big of a solar array do you need to offset all your electricity use?

The short answers to these questions are:

1. The average U.S. household uses around 11,000 kWh of electricity annually and will need an 8 kW system to meet 100 percent of those needs
2. It’s complicated! Working out the right size for your rooftop array means knowing how much energy you actually use and how much electricity solar panels can produce where you live.

NB: The average-sized newly built single-family home in the U.S. (which is around 2,480 square feet) will likely need to install a 9 to 10 kW rooftop array to meet projected electricity demands. Here is a quick guide to buying solar panels based on the square footage of your home.

How big of a solar array do I need?

If you want to get deeper in the weeds beyond just the average electricity consumption based on square footage, you need to figure out how much electricity you currently use.

You’ll then need to estimate how efficiently you can create that electricity on your roof.

Working out how much electricity you use

If you’ve lived in the same home for a few years, you should be able to use your historical utility bills to determine your average annual electricity use. Some utilities offer an online summary that shows your yearly electricity use.

If you don’t have an annual figure for electricity use, take the last 12 months of utility bills and add up the kilowatt hours (kWh) used each month. As a guide, most homes in the U.S. use around 11,000 kWh each year. If you’ve added up your monthly data and get something wildly different, it’s a good idea to double check your math.

Once you have a figure for annual electricity use, which will be in kWh, you can use this to determine how big a system you need.

What if you don’t have 12 months of utility bills?

If you don’t have any consumption data to rely on – let’s say you’re building from scratch to house renters or a new family – you’ll need to take a different approach. This might mean relying on average electricity use for the home’s square footage. In general, the bigger the home, the more electricity its occupants use.

The U.S. Energy Information Administration (EIA) states that the average house in the United States between 2,000 and 2,499 square feet in size uses 11,606 kWh annually, or 967 kWh per month. This works out to around 5.16 kWh per square foot annually.

As a guide, expect the following household annual electricity use in the U.S. based on square footage:

Most homes in the U.S. use more electricity in summer and winter months, to cool and heat the home. The more space you have to cool and heat, and the more extreme the climate, with higher highs and lower lows, the more electricity you’ll use overall.

Online solar calculators

Various online tools are available to help you calculate the right size array for your home. You can usually toggle settings to work out an array sized to offset all or some of your annual electricity use. If you’re planning to go off-grid entirely, or really want to slash your utility bill, you’re looking at sizing your array to meet at least 100 percent of your current annual electricity draw from the grid. A key exception is where you intend to install solar on a new home that’s far smaller than your current home.

The best calculators use data specific to where you live. This is important because the productivity of any given solar panel will vary depending on where it is located. These kinds of calculators use what are known as production ratios, total insolation hours, or capacity factors.

Production ratio

An estimate of kilowatt hours produced annually by a set of solar panels, divided by the wattage of the panels, for a given location.

The higher the production ratio, the more efficiently the panels will produce energy in that location. In sunny California, for example, a typical solar panel production ratio ranges from 1.4 to 1.8. In Delaware, the production ratio is more likely to be just 1.3. This means you’ll need a bigger array to produce the same amount of electricity in Delaware as in California.

Other calculators use what’s known as the capacity factor. This is the ratio of energy generated over a year (usually) divided by the installed capacity. Below is an example of how that works in practice.

As you can see, most 10 kW arrays in Seattle would produce 10,000 kWh annually. In Phoenix, the same size array would produce more than 16,000 kWh. This means many homeowners in Phoenix can get away with installing a smaller array to meet household needs.

Total solar hours and kWh per kW installed

For our calculations at Leaf Score, we typically use the SunWatts calculator. This has more granular data for average daily solar hours in every state in the U.S. and even for different parts of each state. It also offers a figure for kWh per kW installed capacity at each of these locations.

The figures at SunWatt are based on data from the PV Watts database from the National Renewable Energy Laboratory (NREL). The PV Watts tool lets you toggle an incredible number of parameters, including the tilt, azimuth, module type, albedo, and more. For its purposes, the SunWatts calculator assumes the following input standards:

• 19% of greater solar panel module efficiency
• Fixed rooftop mount (no tracking)
• 20 degree tilt
• 180 degree azimuth
• 98% inverter efficiency
• 12% standard system losses or 15% in snowy counties.

This is really getting into the weeds, though, so let’s look at some examples of how this plays out for the more extreme (sunny versus not sunny!) cities in the U.S.

As you can see, to produce the same amount of electricity each year, homeowners in Seattle or Anchorage may need a rooftop solar array that is nearly twice the size as for homeowners in Tucson or San Bernadino.

How useful is average electricity consumption data?

One thing we’ve noticed while constructing our cost of solar models is that average electricity consumption per household varies greatly across different states and even within those states. It also varies greatly between households.

There are lots of reasons for this variation. For instance, residential electricity consumption in Colorado is lower on average than in most other states because Colorado residents typically heat and cool their homes (and water) with natural gas rather than electricity. Annual electricity consumption in Colorado is around 7,500 kWh, or 625 kWh monthly, according to the U.S. Energy Information Administration (EIA) estimates. With Colorado Springs, CO, getting 5.72 sun hours, this could mean the average home here needs just a 4.79 kW array.

If you’re a Colorado homeowner considering installing solar, you might assume it’s fine to install a 5 kW array or smaller. However, if you use electricity to heat your home and water or plan to switch from natural gas to a heat pump system, you’re likely to need a larger array. The same goes if you plan to charge an electric vehicle at home.

According to government data, the average home uses more than 2,000 kWh of electricity to run air conditioning each year.

In general, the appliances that will quickly bump up your annual electricity consumption include:

1. Air conditioners, space heaters, and other heating and cooling equipment
2. Water heaters – though heat pump water heaters are much more energy efficient
3. Light fixtures – switch to LED bulbs to save energy
4. Washing machines and dryers – wash full loads and line dry to cut energy use
5. Home entertainment systems – games consoles, DVRs, TVs, and powerful computers accounted for a staggering 61 billion kWh of electricity in American homes in 2020 alone!
6. Ovens and stovetops – induction is the most energy-efficient way to cook
7. Dishwashers – different models and different settings vary greatly in energy consumption
8. Fridges and freezers – nearly a third of U.S. homes have two or more refrigerators!

How big a solar array do you need per square foot in the U.S.?

Let’s look again at our figures for annual electricity use in the U.S., according to a home’s square footage. This time, we’ll also use the average figure of 4-6 sun hours to determine the size of solar array per square foot.

By our calculations, the average-sized home in the U.S. will need around an 8 kW array to satisfy 100 percent of its occupants’ electricity demands. Note, though, that the average size of new single-family homes in the U.S. has risen from 909 sq. ft. in 1949 to 2,480 sq. ft. in 2021, but that the number of occupants per home has decreased.

The energy efficiency of homes has also improved over the last few decades, thanks to better insulation and construction techniques. However, most homes now have far more appliances and electricity demands than even 10 years ago.

All in all, it’s very tricky to meaningfully apply averages when it comes to household electricity use.

Utility caps on array sizes

In many states, utilities place restrictions on how big an array an individual homeowner can install. This is typically capped at a size expected to provide 100 percent of a household’s annual electricity use. In some areas, homeowners can install systems to provide for up to 120 percent of their previous 12 months of consumption. If you plan to install an electric vehicle charger or expand your home, the utility will usually allow for a greater system size to meet projected needs.

The reason utilities do this is to try to prevent customers from feeding too much surplus energy into the grid. Instead, utilities need to be able to predict fairly accurately how much energy will be produced on a given day and take action to ensure output from other sources (such as by having coal-fired power stations on standby).

If homeowners could install huge solar arrays that far outstripped their personal needs, the utilities would need to manage all that unexpected surplus energy somehow. This would likely require utilities to add infrastructure to maintain the safety of the electric grid.

Where I live in British Columbia, Canada, the provincial utility, BC Hydro, didn’t initially cap the size of home solar arrays. It also offered net metering at a fairly generous rate. The result was that a small number of customers installed massive solar arrays and made a lot of money from the utility. Now, BC Hydro caps system sizes and doesn’t offer net metering at such generous rates.

Final thoughts on how big of a solar array you need

As I said at the top, the short answer to what size solar array you need is “It’s complicated!”.

The best way to work out an appropriate array size is to look at your electricity usage and factor in your location and climate.

Other things to consider include:

• The size of your home – a bigger space requires more heating, cooling, and lighting (and vacuuming!)
• What your home is built from and how – older homes are often less air-tight and require more energy for heating and cooling
• The number of household occupants – the more people the more energy used!
• The number and type of appliances – more, bigger, and less energy-efficient appliances consume more electricity
• How often you use those appliances and how you use them – washing full loads of laundry and dishes is more energy-efficient overall.

It’s also worth noting that calculations using the SunWatts tool assume a rooftop array made up of modules that are 19% efficient. If you use the most efficient solar panels available (Sunpower’s X series), you can likely install a smaller array overall. Conversely, if you use much cheaper, less efficient panels, you’ll probably need a bigger array.

If you’re thinking of going solar at home, look for a reputable installer that runs models based on your actual electricity usage and any expected changes in use. If they offer a quote based on average numbers only, beware, especially if these are averages for the U.S. as a whole.

Even if a solar installer offers a quote based on average figures for where you live and the size of your home, chances are these won’t apply to you. After all, if you’re interested in solar energy, there’s a strong chance you’ve already taken steps to make your home more energy efficient and to reduce your electricity consumption.