How Many Solar Panels Do You Need

How many solar panels do I need for my home in 2021 is a very common question that is asked by many prospective solar panel homeowners.

The size of your solar installation is the first step to calculating your financial savings, as it gives you an idea of your installation costs.

Every home and family is different, so you can’t just look up general information online and assume it applies to you.

Fortunately, calculating how many solar panels you need is pretty easy.

All you need are a couple key numbers and some basic math skills.

But do you know what the easiest way to find out how many solar panels that you will need?

Just get a free estimate at UnderstandSolarPower.

Local installers know your area and your utility, and can jump on your roof and give you the exact answer you’re looking for.

If you want the most accurate estimate for how many solar panels you need, that’s how to find out.

A great way to find the best solar panel installers in your area, and to find out exactly how many solar panels that you will need for you home, is to enter your zip code below.

How to Calculate the Number of Solar Panels Your Home Needs

Calculating how many solar panels you need is actually a pretty straightforward process. We’ll go into each of these steps in more detail, but here’s the process step by step:

Remember that all the numbers above are just averages. Every home, family, and area is different. You and your neighbor might have the exact same house, but they might need 10 more solar panels than you because they have 5 kids and a grandma living with them.

The best way to find out just how many solar panels you need, and the best solar panels for your usage, is to talk to a few local installers. They’ll look at your home and energy use and find out exactly how large your installation needs to be to cover 100% of your energy use.

Now let’s figure out exactly how many solar panels you need for your home. Like we found above, answering the question of ‘how many solar panels do I need’ is just a handful of simple steps:

1. Find your total electricity use over the last 12 months
2. Calculate system size
3. Increase installation size to account for production loss
4. Find the number of solar panels

Let’s walk through each of these steps one by one.

Step 1: Find Your Total Electricity Use

The number of solar panels you install on your home is dictated by the amount of electricity you use – it’s the most basic piece of data when calculating system size. Most homeowners install enough solar panels to cover 100% of their electricity use. If you use 10,404 kWh of electricity each year (the national average), you want to install a solar installation that produces around 10,404 kWh of electricity each year.

So, the first step to figure out exactly how many solar panels you need is to find your total annual electricity use. Utilities make this incredibly easy. Just open up your online account and look at your last 12 months of usage. Above, you’ll see the author’s own usage over the last 12 months.

My house unfortunately isn’t a candidate for solar (it’s old with too many eccentricities), I’ve of course wondered how many solar panels I would need for my own home. So to satisfy my own curiosity and to provide an example we can all follow along with, let’s figure it out.

You can see that my household uses very little electricity, typically around 200 to 300 kWh/month. You also see that monthly usage varies quite a bit, as the summer heats up and we turn on the air conditioning. That monthly fluctuation is why adding up an entire 12 months of energy use is so important!

Now that you have your monthly usage over the last 12 months, just add it all up. If you’ve got Microsoft Excel or Google Sheets, this is incredibly easy. For my own home, my total electricity use over the last 12 months was 3,112 kWh. So, I need a solar installation that produces about 3,112 kWh of electricity each year.

Here’s my own energy use, compared to the national average:

Last 12 months of Energy Use

Your installation will produce less in some months and more in others, but if you’re enrolled in net metering, that’s not an issue. Under net metering, your utility will give you bill credits throughout the year for any electricity your solar installation produces which you don’t use in your home and goes to the grid. In a later month, if you don’t produce enough electricity to cover all your energy needs, you can pull from that credit to cover the bill.

Some very intelligent homeowners realize that, if they just install a solar system that regularly produces more than they use – let’s say sized at 150% to 200% of their usage – they could be making money from the utility. Obviously a little passive income is nice, but this isn’t really what net metering is about.

Net metering is an incentive to get you and your household running off the sun. It definitely helps you save money, but it’s not about making money. Utilities actually prevent this by putting a cap on installation size. Typically this is 100% of annual energy use, but some like Xcel Energy in Colorado increase that cap to 120% of annual use. Your solar installer will know exactly what your utility’s regulations around system size are.

Step 2: Calculate Your System Size

Now that you know how much electricity your solar installation will need to produce, let’s move on to the next step.

We need to find out how much electricity a solar panel can produce in your area. You might think that a solar panel would produce the same amount of energy no matter where it’s installed, but that’s unfortunately not the case.

Sunlight is more intense in certain areas of the world and solar irradiance is a measurement of how strong that sunlight is when it hits the ground. Solar irradiance is much higher in the desert southwest than in more northerly states like Oregon, Minnesota, or Washington.

Thankfully, there are many resources to help us out here. The map above, from the National Renewable Energy Lab, estimates how many kilowatt-hours of electricity each kilowatt of solar panels can produce in a 12 month period.

With this map, it’s easy to figure out how big your solar installation needs to be. Just divide your energy use by the number for your area. For example, we already figured out that I need to produce 3,112 kWh annually to cover all my energy use. In Denver, 1 kW of solar panels produces 1,900 kWh/year. So: 3,112 kWh / 1,900 kWh =  1.6 kW installation.

Looking at the map, the average production is around 1,600 kWh per kW, so the typical home would need a 6.5 kW system. This is remarkably close to NREL’s own calculation for the average rooftop solar installation size in the US, which is 6.2 kW.

Calculate Solar Installation Size

Step 3: Increase Install Size to Account for Production Losses

The above calculations take into account your area’s solar irradiance, but there are many other factors that can affect your solar installation’s electricity production out in the real world. Dirt, snow, wiring and equipment inefficiency, and solar panel age all play a part.

In their online solar calculator PV Watts, NREL uses a default production loss of 14%, so let’s use the same. With some simple math, we can then calculate how much larger our systems need to be to account for these system losses.

Dividing by 86%, we find I would need a 1.8 kW installation on my own home. For the average home, the system size jumps from 6.5 to 7.6 kW.

Account for Production Losses

Step 4: Calculate the number of solar panels

Now we’re at our last step. We know our system size, so it’s very easy to figure out how many solar panels you’ll need to install on your roof. The typical residential solar panel is around 275 watts, so just divide your total system size by the solar panel wattage.

For my own home, I simply divide 1.8 kW by 275 watts and find I would need 7 panels to cover my entire energy use. The average US home would need 28 panels.

Calculate the number of panels

And there you go. That’s it. Figuring out exactly how many solar panels you need is actually quite simple!

Step 5: What to Watch Out for When Estimating Solar Panels for your Home

The steps above are obviously simplistic, but it gives you an idea of the system size you’d need. The last step in sizing your solar installation is to make sure you have enough unencumbered roof space to install all those solar panels! Here’s a list of factors that could affect your installation’s size and placement. Your solar installer will take detailed notes on the size, placement, and effect on energy production of all of these factors:

• Roof space – Do you have enough physical space for 28 panels on your roof? Residential solar panels are about 3 feet wide by 5 feet tall, so you’re looking at 15 square feet of space needed per panel. For a 28 panel system, that’s about 420 square feet of solar panels. Shingle manufacturer Owens Corning has published a great little article on calculating your roof’s dimensions. For an even easier method, solar panel manufacturer LG publishes a very simple solar calculator that estimates your available roof space using Google Maps.
• Roof dimensions – Is the south-facing section of your roof large enough for all the solar panels you need? If not, you can break your installation into groups, installing as many as possible facing south towards the sun, with the rest facing east or west. Your total energy production will suffer slightly, but sometimes you have to do it and it’s a common tactic.
• Setback laws – The Internal Fire Code requires 3 foot wide open pathways up the roof to the eave for firefighters to safely access the home. We don’t need to tell you, this greatly decreases the amount of available roof space, though for a good reason. Some states and local jurisdictions have adopted slightly tweaked versions of this recommendation so they aren’t quite so limiting. Your installer will know exactly what your local jurisdiction requires.
• Vents, chimneys, satellite dishes, and anything else that you can’t cover with your solar panels – Satellite dishes can be moved and vents re-routed, but you’ll have to work around chimneys.
• Shade – This is an obvious one, but can be quite limiting. Your solar panels need sunlight! If you have a tree shading your prime roof spot, you can typically either move the solar installation to another section of the roof, cut the tree down, or trim the tree (though of course, you’ll have to keep it trimmed back semi-regularly). If shade is an issue, your installer will lay out your options and as well as your best course of action.

During initial talks with installers, they’ll give your roof a quick assessment for dimensions and any issues. Once you sign the paperwork, your installer will spend a couple hours on your roof taking detailed measurements of both the roof and any shading, as well as notes on any other possible issues.

How Much Do Solar Panels Cost To Install On Your Home?

Now that you know the size of the installation, it’s very simple to estimate solar panel cost. As of 2021, installing solar costs about $3/watt, so:

Calculate Installation Cost

Hopefully by now you see that calculating how many solar panels you need, as well as the cost, is actually a pretty simple process. Here are the four steps one last time:

1. Find your total electricity use over the last 12 months
2. Calculate system size
3. Increase installation size to account for production losses
4. Find the number of solar panels

These are very basic estimates. They certainly give you a ballpark figure, but to really answer the question of “how many solar panels do I need?” you will have to reach out to professional installers. A great first step is to enter your zip code below to get a free estimate from a solar pro.