Solar power can save you thousands of dollars over time, while your home would be using less electricity generated from fossil fuels. In addition, solar panels can offer you a payback period of less than 8-10 years with favorable conditions, which is great when you consider that many top solar panel brands are rated for 25-30 years of service.
However, to achieve the best results, you first must determine the ideal system size (expressed in kilowatts) and the number of solar panels needed for your home.
There are three main factors that determine how many solar panels are needed to cover your home’s electricity needs:
- Annual electricity consumption of your home – Larger homes tend to use more power, but small homes can also get high bills due to inefficient appliances or energy-wasting habits.
- Local sunshine – Solar panels are more productive in sunny places, and you will need less of them to cover your electricity needs.
- Solar panel brand and model – Most residential solar panels have similar sizes, but the latest high-efficiency models often have a higher power rating. As the wattage of solar panels increases, the number of solar modules needed to meet/augment your home’s power needs is reduced.
When going solar, the best starting point is contacting an expert and getting a professional assessment of your home and power bills. That said, you can estimate the number of solar panels needed for your home, based on the three factors described above – in this guide, we will explain how.
Step 1 – How Much Electricity Does Your Home Use?
According to the Residential Energy Consumption Survey from the Energy Information Administration (EIA), the average US home uses around 11,000 kilowatt-hours per year. However, this can be lower or higher depending on your electrical appliances and energy usage habits.
The electricity needs of buildings also depend on the local climate, and homes in southern states tend to use more kilowatt-hours due to air conditioning. The US EIA has also calculated the average consumption per region:
- South = 13,895 kWh per year
- Midwest = 9,567 kWh per year
- West = 8,525 kWh per year
- Northeast = 8,211 kWh per year
Larger homes tend to use more energy, but there are exceptions. For example, a 1,000-sq.ft. home with incandescent lamps and old window-type air conditioners will likely use more power than a 1,500-sq.ft. home with LED lighting and highly efficient ductless mini-split air conditioners.
To estimate the annual electricity consumption of your home, you can add your power bills for the last 12 months. Make sure you add the kilowatt-hour values and not the paid amounts, since electricity prices are constantly changing.
- If you don’t have a record of your power bills for an entire year, you can ask your electricity provider to send them by email.
- In many cases, you can simply check your bills online with your account information.
- However, don’t multiply your latest bill by 12, since your consumption estimate will not be accurate – energy usage varies depending on the season.
Electricity consumption tends to be higher during summer, and lower during the winter months. This happens because air conditioning systems run with electricity, while most residential heating systems burn natural gas.
Step 2 – How Much Sunshine Will Your Solar Panels Receive?
Once you have determined the electricity needs of your home, the next step is getting reliable data on local sunshine rates. Two homes can have the same electricity usage, but the number of solar panels required can vary depending on their location.
For example, a home in a sunny state like New Mexico will need fewer solar panels than a similar property with the same electricity consumption in Massachusetts.
The Global Solar Atlas from the World Bank Group is an excellent resource, where you can get accurate solar data for the exact location of your home. You simply need to click on the location where you plan to install solar panels, and the map will display the solar radiation available. The value you need is the specific photovoltaic power output, which is measured in kWh/kWp:
- The kWh/kWp unit means “kilowatt-hours of annual electricity production, for each kilowatt of solar generation capacity installed.”
- For example, if the Global Solar Atlas displays 1,600 kWh/kWp for your location, and you plan to install an 8-kW solar system, you can multiply these two values to obtain 12,800 kWh – this is the estimated annual solar electricity production.
However, to calculate the number of solar panels needed, we will follow the opposite procedure. You already know your home electricity consumption, and the specific PV power output for your location. By dividing these two values, you can calculate the solar capacity that would produce the kilowatt-hours needed, and from this value you can estimate the number of solar panels.
Estimating the Capacity of Your Home Solar System
This procedure is easier to understand with a simplified example. Assume you have already added your last 12 power bills, and your consumption is 16,000 kWh per year, while the Global Solar Atlas displays a specific PV output of 1,600 kWh/kWp for your location.
- Solar capacity needed (kilowatts) = 16,000 kWh / 1,600 kWh/kWp = 10 kW
Sunny states like California and Texas have many locations that reach or exceed the solar productivity value of 1,600 kWh/kWp. However, when installing solar panels in Northeastern states like Massachusetts or New York, sunshine rates are more modest.
To visualize how solar radiation affects the photovoltaic capacity needed, let’s repeat the calculation for a place where the Global Solar Atlas displays 1,300 kWh/kWp.
- Solar capacity needed = 16,000 kWh / 1,300 kWh/kWp = 12.3 kW
In this example, both homes are using 16,000 kWh per year. However, Home #1 needs a lower capacity of 10 kW thanks to the extra solar radiation, while Home #2 needs 12.3 kW to compensate for the reduced sunshine. The second solar system is 23% larger, even when both homes have the same electricity consumption.
The exact number of solar panels will depend on the wattage offered by the model you choose. For example, you can reach 6 kilowatts using 20 solar panels with a capacity of 300W, or 17 more efficient panels with a capacity of 355W.
Step 3 – Calculating How Many Solar Panels You Need
At this point, we have determined your annual electricity consumption by adding your last 12 power bills, and we used solar radiation data to estimate the kilowatts of solar capacity needed. The wattage of solar panels depends on the brand and model, and the exact number of panels will depend on the specific product you pick.
Below we will estimate how many solar panels are needed to reach 10 kW capacity, using products from three popular brands in the US – LG Solar, Hanwha Q Cells and SunPower.
- LG 340N1C-V5 = 340 watts (66.4” x 40”)
- Hanwha Q Cells Q.PEAK DUO BLK-G8+ = 350 watts (68.5” x 40.6”)
- SunPower A-Series A390-BLK = 390 watts (72.2” x 40”)
The following table calculates the number of solar panels needed to reach 10 kW of capacity, and includes the roof area needed for the photovoltaic arrays.
|Solar Panel||Amount Needed (10 kW)||Module Size||Roof Area Covered|
|LG 340W||30 panels||18.44 sq.ft.||553 sq.ft.|
|Q Cells 350W||29 panels||19.31 sq.ft.||560 sq.ft.|
|SunPower 390W (390W-420W)||26 panels||20.06 sq.ft.||522 sq.ft.|
The available roof space is an important factor to consider, since it may limit the number of solar panels you can install. Also keep in mind that shading is the #1 obstacle for solar panels since it greatly reduces their productivity. Even with a large roof, shadows cast by surrounding trees and buildings can limit the usable area.
Monocrystalline and Polycrystalline Solar Panels
There are two main types of solar modules that dominate the residential solar market: monocrystalline and polycrystalline.
- Monocrystalline modules have solar cells made of single silicon crystals, while polycrystalline modules have solar cells made of multiple crystals fused together.
- Typically, monocrystalline modules are considerably more energy efficient. If you compare equally-sized solar panels of both types, the monocrystalline panels will generally have higher overall wattage.
- Having a higher power output, monocrystalline panels use the available roof space more efficiently. They also typically have a higher price than polycrystalline panels, but you will typically need fewer monocrystalline panels to reach the required capacity for your project (measured in kilowatts).
When it makes sense to pay extra for high-efficiency monocrystalline panels
There are high-quality solar panels of both types available on the market, but monocrystalline panels are recommended when there is limited available rooftop space. You will be able to reach a higher system wattage with less space and fewer panels. When space is not a limiting factor, both types of panels can offer an excellent return on investment.
How Much Does a Home Solar System Cost?
The exact price of a solar power system will depend on its capacity, local installation costs, and the product brands used. However, according to research data from the Solar Energy Industries Association, US homeowners can expect to pay $3,000 per kilowatt of solar capacity.
Based on SEIA data, a 6-kW system will cost around $18,000, while a 10-kW system will cost around $30,000. However, you can claim back 26% of your total investment as a federal tax credit, and many states also have incentives from local governments and utility companies.
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