How many solar panels will I need to meet my energy needs?
This is a frequent question when homeowners consider installing a solar system. While every family has different power requirements, there are common methods and criteria solar installers use to determine the appropriate size and power requirements of a residential rooftop solar energy system.
When determining what size system is appropriate, it is key for you, the homeowner, to know and understand the information your installer will evaluate as a part of the system design process. Below we walk through the data used by installers, the various factors involved in determining the size of a solar system, and give you a way to calculate an estimate for yourself so you can effectively analyze the bids you will receive.
Calculating System Size: The First Step
At the beginning of your solar buying process, a solar installer will evaluate your household’s annual electricity usage. This information will help them estimate how many kilowatts (kW) of energy are needed to fulfill your household power requirements. Annual energy usage data is available on your utility bill or from your local utility company. This is information an installer can easily pull from either source. They will then take that annual usage amount and divide it by the wattage generated by the proposed brand of panel.
Any significant changes you may be planning that impact your energy usage should be discussed with your installer. These changes could include purchasing an electric vehicle, switching from gas to electric appliances, or installing central air conditioning, battery storage, or a hot tub. All of these will increase your electricity usage.
Another consideration when designing your system is whether the state you live in offers net metering, a utility system that provides credit for excess energy generated by your solar system that is sent back to the power grid. Most solar installers design a solar system that offsets at least 90% of the energy consumed by your household.
These are only the first steps installers take to calculate how many panels you will need. There are several other key considerations.
The Role of Location and Geography
Another metric used by installers is the production ratio, which is defined by where you live, the orientation of your roof, and the amount of sunshine that hits it. The amount of sunlight exposure a roof receives determines the production ratio, and varies depending on which region you live in. The production ratio is the estimated amount of kilowatt hours of electricity per year that a solar system will produce, divided by the total wattage of the solar system. The higher the production ratio, the more efficient the system (not to be confused with the efficiency of the solar panel, which is different).
Production Ratio = Estimated kW hours per year of solar system / Total wattage of solar system
For example, California gets on average around 4.1 to 6 peak sun hours per day while Massachusetts gets on average about 3.7 to 4.1 peak sun hours per day. If you live in an area with fewer peak sunlight hours, you need a larger solar system. A typical California household consuming the national average of 10,632 kWh annually requires a 6.8 kW system to cover 100% of its needs. However, a household that consumes that same amount of energy in Massachusetts, a state with fewer peak sun hours, will need an 8.4 kW system to cover 100% of its energy usage. In America, production ratios are usually anywhere from 1.3 to 1.6.
Panel Size and Efficiency Ratings
The wattage of the solar panels and efficiency ratings are fundamental factors when calculating the number of panels required for the desired energy output. High efficiency solar panels made with premium quality materials, like those produced by Silfab Sola, may have a slightly higher price point, but will generate more energy than a lower quality, cheaper panel with low efficiency. Silfab Solar’s Prime residential panels have an efficiency rating of 21.5%. Meaning that of all the sunlight that hits a solar panel 21.5% is converted to electricity. Solar panel wattage also plays a role. The higher the panel wattage, the fewer the panels required for the system. The wattage of solar panels being produced is increasing constantly. On average residential solar panels on the market today have wattages ranging from 380 to 430W per panel.
Going back to the California and Massachusetts example above, the Massachusetts homeowner who needs a larger system to supply the same output as the California homeowner can use either a system with more panels or panels with a higher wattage.
Roof Layout and Space
The layout and dimensions of your roof are the ultimate factors for determining how many panels can be included in your system. A small or unconventionally shaped roof will allow for fewer panels, or panels with a higher efficiency rating to maximize the limited area. A large or flat roof that makes panel installation more accessible means lower efficiency panels or a larger quantity of panels can be used.
The orientation of the roof affects how much energy a system can produce. For example, the number of panels can be limited by the shading from surrounding trees, buildings, and obstructions on your roof like chimneys or unique design elements. Vents and air conditioning units can impact panel placement as well.
Calculating How Many Solar Panels
It is valuable in your solar purchasing process to calculate an estimate yourself so you can be informed when discussing the system design with your installer or multiple companies that are bidding for your business. The first step is finding three pieces of data: your annual electricity usage, the solar panel wattage produced by each of the panels you’ll be using, and the production ratio.
Next, plug those numbers into the following formula:
Number of Panels = System Size / Production Ratio / Panel Wattage
Using the averages listed above. We will use two numbers for the production ratio as both high (California) and low (Massachusetts) estimates.
Number of Panels = 10,632 kWh / 1.3 or 1.6 / 400 W
The two answers from the formula above, depending on the production ratio, are 21 400W panels for the low sun ratio and 17 400W panels for the high sun ratio. The total estimated system size in the example is 6.8 kW. Your estimate will, of course, be different.
Silfab Solar’s Calculator: The Simplest Method to Calculate How Many Solar Panels You May Need
The simplest method to calculate your panel quantity is to use Silfab Solar’s Calculator, which does all the math for you, and is accessible right on the Silfab Solar website. To calculate the needed system size simply enter your average monthly energy bill, average monthly energy usage, and your address. The calculator takes into account the additional details when determining a suitable system. Additionally, you can contact Silfab Solar directly and receive assistance finding a qualified local installer who can help you further.
Of course, there are a few more factors you and your installer will discuss when looking at system size. These include your budget and available financing, given that panel prices and overall installation costs can vary widely from state to state. Some installers offer financing or can help you secure it through a bank or a third-party financier. Additionally, your installer will know about State or Federal incentives and rebates available in your region. There is also the option of adding battery storage, for which there are separate state and federal incentives and rebates.
The decision to go solar comes with a certain amount of variability around cost, return on investment, and system size. Fortunately, there are fairly straightforward methods for getting an estimate of the number of panels needed so you can make the best decision.
Solar energy has many benefits such as lower electricity costs, greater energy independence, and a reduced carbon footprint. Getting bids and design proposals from three reputable installers is an important part of the process when assessing the suitability of solar power for your home. The more you know going in, the easier it is to make the right decision for you and your family.
For more information on where you can buy American made Silfab Solar panels and to learn more about potential solar savings visit this helpful section of Silfab Solar’s website – Solar for Homeowners.
Photo Credit: EFS Energy
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