How to Calculate Number of Solar Panels Required

A solar panel system is a significant investment that should last for decades. But before jumping into formulas and calculations, first focus on a home and land assessment and professional input.

My wife and I live in a rural area of East Texas, where wind or thunderstorms can easily knock out our electricity. During significant weather events, the grid in our region has been down for days. In 2014, when we decided to install solar, we’d reached an age where dragging a generator and cables around was no longer feasible.

Today, soaring energy prices are the reason many people consider making their own electricity. Regardless of your reason for moving toward installing your own power plant, putting in a properly designed solar energy system to meet your needs and goals is key.

Getting Started

Before you start counting solar panels, do your homework on whether your location works for solar. Whether you plan on hiring someone or installing a system yourself, free online sites such as Google Earth give you a great view of your home and property. They allow you to get a true picture of the space you have available for solar panels.

The latest software offered by reputable installers can give you an almost perfect picture of how many panels you need, where they should go to make the most electricity, and what you should do on your property to maximize production.

You can do your own computations and reach the same answers, but software designed specifically for solar installation is hard to beat. It is expensive and typically worth paying an expert for the information. It was worth it to us to hire someone to ensure we bought the right panels and equipment and that calculations were accurate.

When you look for a solar contractor, you want one who is local, thorough, and experienced. My wife and I also wanted someone who was certified in solar.

The North American Board of Certified Energy Practitioners is a national solar installers network. I recommend having at least one certified person on your installation team.

We interviewed two local solar installers. One sold three packages from which customers could select a small, medium, or large system. They offered no visits to our home or any assessment of our usage.

We chose the other installer because he met with us and evaluated everything about our home and land. He had 10 years of experience, guaranteed his work, and was available for questions, repairs, or upgrades.

His pre-assessment included looking at one year’s worth of our electrical use, using software to calculate how much sun we receive from the southern sky, and whether we needed to consider removing any trees to make sure the amount of electricity we wanted could be created.

Next Steps

Once you have determined that your home or land can accommodate solar, assess your annual electrical usage. Divide the total annual electrical usage for one year by 12, and that is your average monthly use. This amount gives you a starting point.

Panels vary by quality and efficiency. If you trust your installer, trust what they recommend, but also do your own homework. Contractors know what gets the best results, and happy clients make good references.

The panels we chose were the best available, which were 15 percent efficient, meaning that each panel converts 15 percent of the energy it receives from the sun into electricity. Today, the best panels on the market convert about 23 percent.

We learned that most panels are built to withstand bad weather, including a degree of impact. Our panels have survived hailstorms that our roofing shingles did not. Like efficiency, panel durability varies, so be sure to take that into consideration.

The equipment that ties into your panels and operates your system should be selected based on whether you are grid-tied (your panels work in conjunction with the electrical grid); grid-tied with batteries; a grid-tied system with batteries and a generator; or you are completely off-grid.

I do not recommend going off-grid unless you have no other option. If you are off-grid, the number of panels and batteries you’ll need will increase exponentially, and as a result, so will your maintenance and costs.

Also, having an experienced installer becomes more critical as the complexity of your goals increase.

We chose a grid-tied system with batteries and a generator because we wanted power in any situation. Since we opted to not rely on our panels to operate our HVAC system, our installer was able to fit a battery backup system within our budget that, when needed, kept the critical smaller loads such as appliances, lights, and computers running.

A net zero goal was the deciding factor on the number of panels required for us. Net zero is making the amount of power that you use.

For us to reach net zero, we needed 50 panels. We had room for 24 panels on the roof of our home, and the other 26 were ground-mounted behind our house.

If you are going to install your own solar, you should approach which panels and equipment you buy the same way a professional does.

1. Determine how much access to the sun you have facing south. With good performance, you can still install on any roof due east or west, but you lose about 6 percent of your annual yield compared to south.
2. Determine exactly how much electricity you consume yearly, monthly, and weekly, and buy the most efficient panels your budget allows.

Our installer did all the design work to calculate what our available installation areas would provide for us annually in kilowatt-hours (kWh). These calculations accounted for our local weather, hours of peak sun in our city, and any shade losses we might expect. Then, he divided the number of kilowatts by the performance of the panels we chose and how many hours a day it was projected they would generate power.

We needed just over 12 kilowatts, which 50 solar panels could generate.

The size of your system will depend on the average number of hours of sun you receive each day — defined by where you live geographically — and your goals and climate. A common misconception is that locations with lots of cold weather and short summers will not support a solar panel system.

Solar panels work fine in cold climates, but heat and cold do affect efficiency. Extremely hot temperatures can actually impede the productivity of solar panels, which should be factored into the equation.

Where you put your panels could be a matter of necessity or preference. You may only have room on your roof or need a ground mount set up.

Panels can be mounted on your roof or the ground with little difference in performance. The ground-mounted cost premium is only around 20 percent. The ease of snow removal from the ground mount and ability to track the sun may be important considerations if you live far from the equator.

Solar panels may look similar, but they are not made equally. Our panels came from Canada, but panels are manufactured all over the world. Companies have different levels of quality, durability, and, most importantly, how much of the sun’s energy they can convert into electricity to power your needs.

Supply chain issues may affect which panels are currently available, so if you decide you only want solar panels from a specific company in a specific quantity, you may have to wait and pay a bit extra.

If you determine your location works for solar, collaborate with your power provider, do the calculations, and buy the right panels in the right number. Whether you install your panels or hire an installer, you’ll enjoy the fruits of your solar labor for many years to come.

John Moore and his wife Terry live on a 10-acre homestead in East Texas. He is a syndicated newspaper columnist and book author.