This Is How Many Solar Panels You’ll Need to Power Your Home

This formula will help you figure out how many panels you’ll need to reach your energy goals.

Power outages, soaring electricity costs and environmental concerns are all making residential solar power more appealing.

But how much does it cost to install a solar panel system on your roof? Solar costs can range anywhere from $15,000 to $50,000. The 30% federal tax credit coupled with available state and municipal government tax credits, tax exemptions, rebates and other incentives can further reduce the cost of buying and installing solar systems.

To figure out your actual costs, you will first need to know how many solar panels you need to power your home. A reputable solar installer can do this math for you. But if you want to get an idea of costs ahead of time — or want to check the installer’s math — we will walk you through it step by step.

Here’s how to figure out how many panels can support your energy needs and what other factors that can affect that number.

Factors that affect how many solar panels you will need

The number of solar panels you’ll need depends on a variety of factors and is going to vary drastically by household. A few factors affecting the amount of panels you’ll need are:

• Wattage per panel
• Panel efficiency
• How much energy your solar panels are producing
• Your household’s energy consumption
• Size of the panel
• How many hours of peak sunlight you receive
• The condition and shading of your roof
• Your own personal energy goals and needs

How to calculate how many solar panels you will need

Jake Edie, an adjunct professor at the University of Illinois Chicago suggests taking the following steps:

Step 1. Review your monthly electric bill: It’s important to determine how many kilowatt-hours of electricity you consume monthly. As an example, we will use 1,500 kWh every month.

Step 2. Convert monthly energy use to daily use: Given 1,500 kWh is consumed per month, to ascertain the daily usage, we need to divide this figure by the average number of days in a month, which is roughly 30.42 days (365 days divided by 12 months).

Hence, the average daily use = 1,500 kWh / 30.42, approximating 49.3 kWh daily.

Step 3. Determine peak sunlight hours: This factor varies based on location and climate. For this example, let’s assume that this home receives an average of about 5 peak sunlight hours per day.

To calculate the total daily energy production required, divide the daily energy consumption by the number of peak sunlight hours. This gives the amount of energy your solar panels need to produce per day.

Energy production required = 49.3 kWh per day / 5 hours, which equals 9.86 kW.

Step 4. Calculate the number of panels: Lastly, you’ll need to determine the wattage of the solar panels you plan to install. The average solar panel efficiency in the US is rated between 250 and 400 watts. For this example, we’ll use a rating of 350 watts.

By dividing 350 by 1,000, we can convert this to kilowatts or kW. Therefore, 350 watts equals 0.35 kW.

Step 5. Determine the required number of solar panels: Divide the daily energy production needed by the solar panel’s power output.

Number of solar panels needed = 9.86 kW / 0.35 kW per panel, which equals 28.17 panels.

This hypothetical homeowner will need approximately 29 solar panels to generate enough electricity to match their current usage from the electric company. While this calculation may seem straightforward, there are many factors that can affect the effectiveness of solar panels, such as shading, roof orientation and seasonal variations in peak sunlight.

We highly recommend that you seek the guidance of a professional solar installer who can assess your circumstances and provide a tailored solution to meet your needs.They should be certified from the North American Board of Certified Energy Practitioners, which is the solar industry standard. CNET also has a well-researched list of best solar companies.

Other factors that affect how many solar panels you need

There are a variety of factors to take into consideration that will help you and an installer determine how many solar panels you need to power your home. Here is a breakdown:

Solar panel wattage

One big part of a solar panel’s performance is its wattage, and it will affect how many panels you need. The higher the wattage, the more power a panel can generate.

Most residential solar panels have ratings of 250 to 400 watts. The most efficient solar panels on the market are 370- to 445-watt models. The higher the wattage rating, the higher the output. In turn, the fewer panels you might need.

For example, you might buy a solar panel with a listed output of 440 watts. You’ll need to multiply the panel’s wattage by how many hours of sun you get every day to understand how much energy it will produce.

If you don’t have much space, you might want to invest in solar panels with higher efficiency and wattage ratings since they’re equipped to generate more energy per panel. But they’re also more expensive, so bear that in mind if the solar budget for your home is tight.

Output efficiency

If your roof has limited space for panels, you’re going to want to get the most performance per square inch of panel that you can, Corda explained.

“Scientists and technical developers of solar panels have been working hard for decades to try to make each solar cell on the panel able to convert more of the sun’s light to electricity than before, or to make them more efficient,” Corda said.

As she explains, currently, the most efficient panels on the market have anywhere from 18% to 22.8% efficiency, with most panels hovering around 20% efficiency. So the higher the efficiency, the fewer solar panels you might need.

In reality, a more efficient solar panel will require fewer panels overall for your home, assuming all other factors are equal.

Production ratios

A production ratio for solar panels helps you determine how much energy you can get from a panel. The production ratio, or performance ratio, is an important measure of the effectiveness and efficiency of a solar system. It compares the actual output of the system to the output it would produce under ideal conditions. This ratio takes into account factors that reduce output, such as temperature, dust, snow, shade, aging of the panels and inefficiencies in the inverter.

The performance ratio is expressed as a percentage, with a higher ratio indicating that the PV system is producing a greater percentage of its theoretical output. For example, a performance ratio of 80% means that the system is producing 80% of its rated output in real-world conditions. The higher the production ratios, the fewer panels you might need.

Panel size

There are three main sizes for solar panels: 60-cell, 72-cell and 96-cell. The 60- and 72-cell panels are more common for residential installations are generally about 3 by 5 feet, or 15 square feet.

Where you live and hours of sunlight

The more hours of sunlight your roof is exposed to, the fewer panels you’ll probably need to install. This is based on the direction, pitch and orientation of your roof, the weather and how much shade covers the roof. It also depends on the time of year and where you live.

“In the winter [the solar panel] produces less than in the summer. So your energy production from solar will change throughout the year and then the usage within your home will change depending on what appliances are using electricity,” said Justin Draplin, CEO of Eclipse Cottages, a sustainable home technology and development company.

“So if you live in a really hot climate, then during the summer months, your electrical bill is going to be a lot higher to cool your home versus if you’re in a cold environment, your electrical bills are going to be a lot higher in the winter,” he said.

How much shade your roof gets always plays a factor in how many solar panels you’ll need for your home, Corda said. If your roof is covered by large oak trees or a chimney and gets a lot of shade, this will bump down solar panel output. In turn, you might need more panels to power your home. But if your roof doesn’t get much shade, your solar output will be higher for the same space.

Another thing to consider is whether your state or city incentivizes solar energy, Coleman said.

“Is it worth it to maximize your solar production to quicken your payback period thanks to the value of solar energy you’re able to produce?” Coleman said. Certain states, like California, are altering their net metering programs, fundamentally changing the amount of credit a homeowner receives for their solar panels. “This historical net metering program brings new considerations for how many panels and what size system you should consider, and how storage factors in,” Coleman said.

Additionally, you may also want to consider whether your area is at high risk of power outages, Coleman said. If this is the case, consider buying a battery to supplement your solar panels in the event of an outage.

Roof type and condition

The orientation, angle, shape and type of roof will affect the number of panels you can reasonably fit into a given area, Corda explained.

A home without a complicated roof structure, pitched at a 10-degree angle and south-facing is best for solar panels.

“That would be an ideal roof for solar because you’ve got it tilted, it’s facing south, and the pitch of the roof is neither flat nor very steep, which is ideal for putting panels on there to capture as much energy from the sun as possible,” Corda said. A house with a more complicated roof structure won’t be able to fit as many panels, she adds. For instance, Spanish tile-roofs are considered solar unfriendly and require special attachments.

Cost and budget

While powering your home on solar energy can save you money, it does require a serious investment upfront. The costs to power your home on solar and your budget will determine how many solar panels you can afford.

Currently, the average cost for a home solar panel system is around $3 to $4 per watt, according to various industry surveys. Based on this figure, a 5-kilowatt size system would be $15,000 to $20,000 before any tax breaks or incentives kick in.

Solar isn’t just for wealthy people who can afford to shell out $20,000 — Coleman said that low and middle-income Americans can reap the most benefits from home solar systems by reducing their energy burden. With the help of leases and PPAs, customers can purchase solar power at a reduced price and redirect the electricity expense to pay a clean energy company instead. “In a market with high interest rates, leases and PPAs are likely the least expensive way to go solar,” he said.

Whether you are paying cash or financing, knowing what you can afford will play a factor in how many panels you add to your home.

Annual electricity usage

To know how many panels will meet your energy demand, you’ll need to know your annual energy usage. You can log onto your account online, review statements, you’ll see how many kilowatt hours of electricity you use. “You’re going to want to look at your patterns over the course of a year — if not the last couple of years,” Corda said.

Once you have that number, you’ll know how much solar power you need to generate to cover your needs.

Besides recent use, factor in the future energy needs, Corda points out. For instance, do you anticipate purchasing an electric vehicle? Or do you plan on growing your family? Or are you and your spouse going to be working from home more? If so, then your energy needs will go up in the future years. On the flip side, if your teens will soon leave the nest to go to college, then you can expect your energy usage to taper off.

Your personal solar goals

Determining your personal solar goal is figuring out what you want to achieve with your solar panel addition. Living completely energy independent and off the grid would mean more solar panels. “If you want to power your whole house, you have to really oversize it to make sure you have enough power in the winter, even though you’re going to be over producing in the summer,” Draplin said.

Adding battery storage will also play a factor in how many panels you need. With solar battery storage, you can essentially bank energy and store it for later use when you’re producing excess energy.

If your goal is to lower your energy bill or reduce your carbon footprint, then maybe you won’t need as many panels, Draplin said.

Figuring out the number of solar panels you need is only part of the equation. Learn more about the benefits and costs of home solar from CNET:

• How Do Solar Panels Work?
• Do Solar Panels Save Money? Yes. Here’s How
• 4 Cheaper Solar Energy Options to Use at Home

Solar panel FAQs

Correction, Aug. 18: This story originally presented some statements as direct quotations that were actually paraphrases of what the individual cited had said. Those passages have now been rendered appropriately as paraphrases.