Photo by Jason Reed

How Many Solar Panels Do I Need? 

To figure out how many solar panels you need, you must consider the amount of power your home uses, the hours of sunlight your site gets, and the size and efficiency levels of the solar panel system you want to install. 

For the most accurate information we recommend speaking with one of our energy consultants, which you start by filling out our form here.

If you have already received quotes on a solar power system and have heard different numbers from different solar system installers then you may be wondering how many panels you really need. 

We often hear from confused folks who heard from one energy consultant that they need a certain number of solar panels while others recommended more or less – and yet somehow the offset percentages were the same. There are many reasons why that might be the case. The language of solar energy can be confusing, but we are going to help you understand what it all means.

In this guide we’ll give you information that will help you figure out how many solar panels you might need for your home and how to make the best decision for which solar system is right for you.

Information to Gather Before Designing Your Solar System 

Whether you’ve decided to opt for solar installation for new construction or for your existing home, it is a good idea to think through your needs and gather together some information before you reach out to any solar contractor you are thinking about working with.

The size of your new home solar system should be based on your specific energy needs and location. You won’t be able to figure out how many solar panels you need based on the amount of electricity used by the “average home.” 

According to the U.S. Energy Information Administration, the average amount of electricity used by residential power customers in 2019 was 10,649 kilowatt hours (kWh), or about 877 kWh per month. But that is an average of all the homes in America, including the state with the highest annual usage (Louisiana at 14,787 kWh per home) and the lowest (Hawaii, which used an average of 6,296 kWh per home). 

The questions and answers we have provided below will help you gather everything you need to get an accurate estimate of the number of solar panels you’ll require to power your home: What is Your Typical Energy Usage?

• What is Your Typical Energy Usage?

• How Can I Find My Usage History?

• How Can I Plan for My Future Energy Needs?

• How Much Roof Space or Ground Space is Available?

• How Many Peak Sunlight Hours Will My Solar Array Get?

• What is the Amount of Wattage Produced By Each Solar Panel?

• How Efficient are the Photovoltaic Panels I Want to Purchase?

• What Percentage of My Total Energy Use Do I Want to Offset with Solar?

• What About New Construction? How to Plan For Solar in a New Home

• How to Estimate the Number of Solar Panels that Will Meet Your Needs

• Example System Size Calculation

• Online Solar Panel Estimators and Other Tools

• Is it Possible to Make Changes to Reduce the Number of Solar Panels Needed?

• How Much Do Solar Panels Cost?

The first step to figuring out how much solar power you’ll need to power your home is researching how many kWh of electricity your home regularly uses. 

Your monthly electric bill is the first place to look to begin figuring out how much power your solar system will need to generate. 

Don’t rely on a ballpark estimate of how much energy you use. The more accurate your understanding of your typical energy usage, the better you’ll be able to determine the number of solar panels you might need. 

When putting together a proposal for a new solar energy system, we always begin by looking at your electricity bills to figure out your energy consumption patterns. Your most recent power bill is a good place to start, but don’t stop there. One month of usage data does not provide enough information to get a clear understanding of your energy needs. 

We recommend that you dig a little deeper into your electricity usage history and start thinking about your future needs too. Home energy usage tends to go up and down according to seasonal changes in the weather and with changes in our lifestyle and the size of our household.  

If possible, look at a full year's worth of past utility bills (two years or more is better) to find the average number of kilowatt-hours (kWh) of electricity you’ve used on a month-by-month basis. 

The more historical data you can provide a solar company, the more accurately they can design a solar power system that will meet your needs.

Your energy provider should be able to help you gain access to your power usage history. Every power company will have a different way to access that information so you’ll need to check out their website or call their customer service number to find out. 

Here in North Carolina, where Sugar Hollow Solar is located, Duke Energy is the primary grid energy provider. If you are one of their current customers, here is what you’ll need to do to find your power usage history online: 

1. Navigate to duke-energy.com.

2. Log into your online Duke Energy account (or create one if necessary)

3. Look at the Billing & Payment menu 

4. Click on Billing & Payment History 

5. Click on Export Data and download your history for the longest amount of time possible. One year is good, but two years is better. 

You’ll then have a spreadsheet that can help us or another solar energy company estimate your energy needs, which will directly impact the number of solar panels you will require. 

They say history repeats itself, but in electrical usage terms that is not always the case. A panel count based on your history will only be helpful if you continue to live your life the same way you are living now. 

At Sugar Hollow Solar, in our proposal process we always make a point to ask homeowners if they have any big life changes coming up in the future that will impact their energy usage. There are lots of ways that your usage can change: 

• Are you planning on purchasing an electric vehicle that you’ll need to charge at home? 

• Is the size of your family changing one day soon? 

  • If you are on the verge of having a new baby or having a child go off to college, your energy usage is about to change quite a bit. Those changes will alter the calculation for how many solar panels your home will need. 

• Are you planning on replacing your heating and air conditioning system? 

  • Changes to your heating or cooling system often have the greatest impact on your home energy usage. HVAC systems typically use more electricity than anything else in a home.

• Are you updating your air conditioner, hot water heater, stove, fridge or other major appliances to versions that use less energy or versions that don’t rely on electricity? 

The major takeaway here is that your history is only the first place to start estimating your power needs. You’ll need to consider your future energy needs as well.

Once we have an idea of your energy needs from your past electricity consumption patterns and your estimated future usage, we’ll look at the available space for your panels, whether on the roof or the ground. 

The square footage of your roof is not enough information to accurately size a solar system. 

Even if you know the exact size of the site where you want to place your panels, it is likely that not every inch of that will be suitable for solar. 

You must also consider the number of hours of sunlight that roof or groundspace receives. If it doesn't get enough hours of sunlight then it cannot be used for solar. (More on that in the next section). 

To determine whether a site is suitable for solar panels, we’ve got to look out for objects that will shade the panels from the sun. Are there chimneys or other permanent rooftop structures that will cast shade on your roof panels? What about neighboring homes or other buildings? Are there any trees that will shade part of the roof seasonally?  

Once you figure out how much of the site is really useful for solar panels you can then start thinking about where those panels should be placed to maximize energy production.

The size of the solar panel system you install will be limited not only by the size of your roof, but also by the number of hours of sunlight that the roof receives. 

A minimum number of peak sunlight hours will be required for solar panels to work productively enough to meet your energy consumption needs. In simplest terms you can think about peak sunlight hours as those hours of the day when the sun’s rays beam down with maximum intensity (which is when your panels will produce the most energy). 

There are apps online that will calculate how many hours of usable sunlight your state or region gets per year based on past weather patterns – but it isn’t your state or region you are solarizing, it is your home or business.

Sugar Hollow Solar does not have a recommendation for a tool you can use to calculate the sunlight hours available on your property. In our opinion most of the basic tools that you will find online for free are not detailed or accurate enough to provide reliable data for most neighborhoods in the U.S. Solar professionals have access to advanced tools that can accurately calculate the number of daylight hours that your array would receive. Find a solar company that you trust and ask them to take a look at your property.

The number of hours of sun your roof gets is not the only consideration. We also look at the orientation of the site (is it southern facing or east/west facing?) the pitch of your roof, the amount of shade and other factors specific to the site.

Not all solar panels produce the same amount of electricity. If you are seeking a quote from a solar installer, ask them about panel wattage.

When you sit down and talk to a solar energy consultant, they may talk to you about solar panels, rails, inverters, mounting clips, conduit lines, and other types of specifications. But most of those won’t impact the number of panels you will need to install. What you need to pay close attention to is the wattage.

Right now solar panels for home systems vary from 340 watt panels to 420 watt panels. This is one common reason why you might see different numbers of solar panels in different proposals.

Let’s say you get two different proposals and both recommend 10 solar panels, but one shows a greater offset percentage and higher cost. If you are seeing different figures for offset percentage and cost from proposals based on the same number of panels, it is most likely because the proposed solar panels carry different power levels.

Also, keep in mind that solar panels come in different sizes as well as wattages. But the size of the panel does not always indicate how much power the panel can produce. Sometimes more watts can be achieved by smaller sized, but more highly efficient solar panels. (We’ll talk about solar panel efficiency in the next section).

The big takeaway here is to ask about watts when you talk to the energy consultants you are getting proposals from.

In addition to the wattage of the solar panels you are considering, you will also need to think about the efficiency of those panels. 

Did you know that very little of the sunlight that hits a solar panel can be converted into electricity? About 70% of light energy from the sun is impossible to convert into electricity. The energy that cannot be turned into electricity is lost as heat.

A solar panel's efficiency percentage indicates the amount of usable energy that the solar panel is able to produce from sunlight.

The higher a panel's efficiency percentage, the more light it can convert into electricity.

If the same amount of sunlight shines on two solar panels with different efficiency ratings for the same period of time, the more efficient panel will produce more electricity than the less efficient panel. As you are considering proposals for solar panel systems for your house, the efficiency rating of those panels will impact the number of panels you will need.

Even high-efficiency panels will have less than a 30% efficiency rating. A majority of modern solar panels are measured at between 15% and 22% efficiency.

As you are shopping around, you should also consider the projected efficiency of the panels you are looking at in the future. All solar panels degrade and lose production capacity over time.

Right now there are residential solar panels that come with 25 year warranties and are projected to produce between 80 and 85% of their capacity in year 25, but there are also panels that will achieve 93% of their power in the 25th year. Ask your solar energy consultant to talk through the differences between these options.

Your offset, in solar terms, is how much solar energy your home will be using versus grid energy. You may want to go completely off-grid but that may not be possible.

Let’s say that my home currently uses 10,000 units of grid energy. If I then install a solar panel system that can make 8,000 units of solar energy – my offset percentage will be 80%.

It is possible to achieve 100% offset, but that is rare right now. You’ll need to talk to the solar energy company you are working with to discuss what offset percentage you can achieve based on the available useful space for your array.

Your utility provider’s solar rules might also impact the number of panels you will want to install. You’ll need to check with your provider as different utilities have different policies concerning solar power. Here in the Carolinas the utilities we work with most often are Duke Energy Progress, Duke Energy Carolinas & Blue Ridge Electric.

For folks that are planning to install solar panels in a newly constructed home, there are some extra steps you can take during the design/pre-construction phase to maximize the efficiency of your solar system.

Here are a few talking points you can use with your architect, contractor, and electrician to optimize your new residence for solar power. Ideally the home will have:

• 180 degree orientation with 34 degree slope of roof

• Minimal shade interferences

• True south orientation 

  • If your builder and architect are planning for an east/west orientation, optimal solar capture will occur through lowering the slope of your roofline. Simply put, make your roof flatter as your roof line moves further away from true south.

• Chimney stacks placed on the north side of the home

  • Placing your chimney stacks on the north side of your roof allows for more panels on the better solar producing south side.

• A standard electrical meter 

  • not a "combination meter"/"combo meter"

Another thing to consider is the kind of HVAC system you plan to install. Remember that heating and cooling systems typically use more energy than anything else in a home. If your new residence will be heated by an electric heating system, you will require more electricity than a home with a gas furnace would.

You should also consider your typical thermostat settings. Do you like your home to feel like the North Pole in July or the Bahamas in December? If so you're going to use more energy and require more solar panels to achieve a higher offset.

If you want to figure out the number of solar panels you need to power your home on your own, follow these steps:

1. Calculate your annual household electricity consumption. You can find this on your utility bill. Look at the bar graph for the last 12 months and add up each month. This will give you an estimate of energy your home has used for the last year. 

2. Then take the household consumption number and divide it by an average quality production factor for your region. For the Western North Carolina region you can use a factor of 1.26. 

3. Take this number and divide it by typical panel wattages. That will show you how many solar panels you might need to power your home. 

Remember as you finish this calculation that the number you get is only a rough estimate. Not all homes are created equal. The exact number of solar panels you'll need will be determined by how much space is on your roof, how the roof is oriented towards the sun, and how much shade impacts your roof.

Also note that some panels produce better than others even if the wattages are the same. Here at Sugar Hollow Solar we typically have two or three different solar panels to choose from.

For example, if these are your numbers: 

• Household consumption: 12,000 kWh

• WNC Quality production factor: 1.26  

• Typical solar panel wattages: 340W & 420W

Then this is how you would do the math to calculate the number of solar panels you will need:

12,000/1.26 = 9523.

9523/340W = 28 solar panels

9523/420W = 23 solar panels

If you are looking for additional help and are not ready to call a solar contractor for an estimate, there are tools you can find online, some simple for informational purposes and others highly technical, that will take all the information you have gathered and give you an estimate of the number of solar panels you might need (or how many will fit on your roof).

One tool you can use to calculate the kWh per year that your solar panels might generate is the PVWatts Calculator from The National Renewable Energy Laboratory–which is a laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. If you are a DIY person designing your own solar system then that might be a tool for you.

But if you are using other types of free solar estimators or tools, remember that most of those calculators run on an ideal scenario – a south facing roof with no shade. Based on those perfect conditions, a calculator might tell you that you only need 300 square feet of roof space to cover your energy needs. But those calculations won’t reflect it if your roof faces east and your pitch is not ideal or if there's a large oak tree shading half the roof for much of the day.

By all means, use the calculators if you want a general idea, but it is best if you have a seasoned solar power professional come out and take a look at your property.

Sugar Hollow Solar offers free custom quotes for solar panels in Western North Carolina and Upstate South Carolina. Contact us if you would like for one of our experts to come estimate the number of solar panels you will need. 

Though you can’t change the number of solar hours your property gets – you can make changes to reduce the amount of energy your home uses.

Your heating and cooling system is typically the biggest energy hog in your home. You can use less heat or less A/C to reduce your consumption or install more or better insulation to reduce loss of heat and cooling produced by your HVAC system.

Another big step you can take to reduce your energy consumption is to switch over to using only Energy Star rated appliances. If your dishwasher, washing machine, dryer, and refrigerator are all energy efficient, your electricity usage will go down and the number of solar panels will too.

Of course there are simpler and cheaper ways to reduce the amount of electricity you need to power your home (and your carbon footprint):  

• Lower the maximum temp on your hot water tank.

• Wash your clothes with cold water instead of hot water.

• Hang your laundry out to dry instead of using an electric dryer.

• Set your thermostat up to maximize energy efficiency. 

• Caulk cracks and gaps near windows and doors.

• Switch from incandescent bulbs to LED bulbs.  

For other tips read our blog on how to kick the coal habit, shrink your carbon footprint, and start saving with solar.

The size of your budget may also have an impact on the size of the solar system you want to  install. If you are trying to figure out your budget, learn about what kind of tax credit you might get, discover available rebates, or explore whether you will save money over the long run by installing a solar system, Sugar Hollow Solar has published other plain language resources that can help answer those questions for you including:

• Will Solar Energy Save Me Money?

• Solar Incentives, Tax Credits and Rebates

If you have any questions that we have not answered here, or if you want to arrange a free quote please contact Sugar Hollow Solar and we will be happy to help you.