Potential customers interested in solar generally talk in terms of how many panels will be installed on their roof. In order to know how many panels we will install, we find out how big your electricity bill is and design a system offsetting a certain percentage of your electricity needs up to 100%. The average electricity bill in America for a household is around $120 based on a price of $0.13 per kWh of electricity.
The average residential solar installation is just over 7 kW. This would reduce the average American household's electricity bill by around $100 for an 80% offset. The average installation would therefore be around 24 panels. How do we estimate this? How big is the average panel? Will 24 panels fit on the average roof in America? Let's take a closer look.
How many panels are in a kW?
A kW is 1,000 watts. Solar panels generally range in size from 265 to 340 watts. A good estimate of how many panels are in each kW would be 3.5 panels. This would indicate an average panel size of around 285 watts, which is well within the range we are working with. If you install larger panels that are 340 watts then only three panels per kW would be needed.
Solar energy systems are designed to utilize the space provided. A solar company will use a certain panel size to provide uniformity for your design. There is not really a decided advantage one way or another in choosing smaller or larger types of panels since the pricing is still by the watt. The specific size panels that are ultimately chosen are determined with the intent on fitting them on the roof in the best geometrical shape. So there is really no discount for buying a larger or smaller panel size.
The average panel is typically 5' by 3.25' and equals around 17 square feet. It is easier to round up to 20 feet. So for a kW you are looking at between 60-70 square feet of coverage. For a 7 kW array, we are looking at anywhere from 420-500 square feet of roof space.
What is the average sized-roof?
As of 2013, the average home size is around 2,600 square feet. In comparison, in 1983, the average home was around 1,725 square feet. The average home size would therefore have a roof size of approximately equivalent to around 1,600 square feet. All things being equal, if there were no obstructions such as pipes, chimneys, or skylights on top of a room, a typical 1,600 square foot roof could accommodate around 80 panels. This assumes no shading issues.
According to a report by the National Renewable Energy Laboratory: Rooftop Solar Photovoltaic Technical Potential in the United States: A Detailed Assessment, around 26% of all rooftop space for small buildings are suitable for solar. This would mean that for the average house with the average roof space with the average amount of suitable room for solar would call for 20-21 panels. This should be taken with a grain of salt, however, since every house is different, and not every house has suitable space for solar due to shading issues, roof pitch, or obstructions.
When there are shading issues, it will generally invalidate the ability to install solar on a house. Otherwise, if your residence has a good sightline to the sun with limited obstructions, your roof will have no problems fitting as many panels to get a solid offset of your electricity bill.
The average-sized roof has plenty of space to fit enough panels to get your required offset. If you have a smaller house or a house with a unique roof design, we have to take that into consideration. But a good rule of thumb to follow along is that for every 1 kW of solar you are installing, you can expect to install 3.5 panels.