How Many Solar Panels Do I Need?

how many solar panels

“How many solar panels do I need?” is typically the first question most of us have around solar.

Even I, the author who has worked in the industry for years, continue to ask myself that question all the time: how many solar panels do I need for my home? And of course, many factors affect that answer: How many solar panels do I need if I install this super-efficient AC system? What if I add more insulation? What if I never turn on lights? Avoid washing dishes? Stop bathing? You get the idea.

The size of the solar installation is the basic building block to calculating your financial savings, as it gives you an idea of your total installation costs.Every home and family is different, so you can’t just look up information for your specific situation on a website. It requires some basic information on your own life, as well as some easy math. But you know what the easiest way to answer the question is? Just get estimates from a few installers. They know your local area, 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 and get a free quote from multiple installers.

Installers will always provide an estimate of size, cost, and savings, so reach out to local solar companies now to learn more about what they can do for you.

If you keep reading, we will show you how to estimate yourself, but if you really want to know how many solar panels you need, talk to the pros by entering your zip code in the form above!

The Average Home Needs 28 Solar Panels

Before we get started figuring out how many solar panels you need, let’s take a look at what the average homeowner needs.

According to the US Energy Information Administration, the average US home uses 10,404 kWh of electricity each year. With 1 kW of solar panels producing about 1,600 kWh per year on average in the US (more on this later), the average home would then need a 7.5 kW solar installation, after taking into account various inefficiencies in the system (more on this later too!)

With the typical solar panel around 275 watts, the average home would then need about 28 solar panels to cover 100% of their electricity use with clean solar energy.

Of course, you aren’t average. No one is. Statistics can mislead if you’re not careful. Case in point: Let’s say I have $100 and you have $0. Together, we have an average of $50 each, but in reality I’m much richer than you are.

Solar is the same way. 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, while you live alone. (Wow, you’re broke and alone in this scenario. Sorry about that.)

The best way to find out just how many solar panels you need 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.

Reach out to installers right now to find out exactly how many solar panels you’d need to cover 100% of your energy use.

How Many Solar Panels Do I Need?

Okay, you’re not average. Now let’s figure out exactly how many solar panels you need for your home. And actually, 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 with your local solar irradiance
  3. Increase installation size to account for system inefficiencies
  4. Find the number of solar panels by dividing installation size by solar panel size

Like you can see in Step 1, 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 want to install enough solar panels to cover 100% of their electricity use. So if you use 10,404 kWh of electricity each year (the national average), you probably want to install a solar installation that produces around 10,404 kWh of electricity each year.

Of course, 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 1: Find Your Total Electricity Use

The first step to figure out exactly how many solar panels you need is to find your total annual electricity use. Most solar installers recommend sizing your solar installation so it covers 100% of your electricity use, so our first step is finding that information!

Utilities make this step incredibly easy. Just open up your online account and look at your last 12 months of usage. Below you’ll find the author’s own electricity use for 2017.

Like I mentioned above, I’ve wondered how many solar panels I 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.

I made my energy use into the nice little chart below. This isn’t required, but it is fun.

You can see that my household uses very little electricity, hovering around 200 kWh/month! That jump in July is when we couldn’t handle the summer heat and had to turn on the AC.

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.

Considering the average annual electricity usage in the US is 10,404 kWh, your usage will likely be much higher than my own. (I live in a small house.)

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 electricity 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 Oregon and Washington. You’ve probably heard about just how popular solar installations are in southern California and Arizona – now you know why!

Thankfully, there are many resources to help us out here. Take a look at this map from NREL, which estimates how many kilowatt-hours of electricity you can expect your solar installation to produce per kilowatt of solar panels.

With this map, it’s easy to figure out how big your solar installation needs to be. Just multiply 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. I live in Denver, CO, and NREL estimates that a solar installation in Denver produces 1,900 kWh for each kilowatt of solar panels each year. So:

  • Author’s home: 3,112 kWh / 2,000 = 1.6 kW

I’d need a 1.6 kW solar installation to cover all my electricity usage. If you needed to produce 10,404 kWh (again, the average usage in the US), you’d need:

  • Average home: 10,404 kWh / 1,900 = 5.5 kW installation

After you find your annual electricity use, just locate your area on the map and do the simple math above!

Step 3: Increase Install Size to Account for Inefficiencies

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 mathmagics, we can then calculate how much larger our systems need to be to account for these system losses:

  • Author’s home: To account for this 14% loss, we need to increase the installation size from 1.6 kW to 1.8 kW (1.6 kW / 86% = 1.9 kW)
  • Average home: For the average installation, we need to increase from 5 kW to 6.4 kW to produce the same amount of electricity (5.5 kW / 86% = 6.4 kW)

Step 4: How Many Solar Panels Is That?

Now we’re at our last step. We know our system size, 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:

  • Author’s home: For my 1.9 kW installation, I’d need 7 solar panels (1,800 watts / 275 = 6.9, rounded up to 7 panels)
  • Average home: For a 6.4 kW system, you’d need 24 panels (6,500 / 275 = 23.3, rounded up to 24 panels)

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

  1. Find your total electricity use over the last 12 months
  2. Calculate system size with your local solar irradiance
  3. Increase installation size to account for system inefficiencies
  4. Divide installation size by solar panel size
Postlude: Do You Have Enough Roof Space?

The last step in sizing your solar installation is to make sure you have enough roof space to install all those solar panels!

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.

Most homeowners have ample space to install 10 – 20 solar panels, but you can be limited by:

  • The dimensions of your roof area – 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 do 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 typically 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 Does Solar Cost?

Now that you know the size of the installation, it’s very simple to estimate solar panel cost. As of 2018, installing solar costs about $3/watt, so:

  • Author’s home: For my 1.9 kW installation, I’d need to spend $5,700 out of pocket. If I take advantage of the federal income tax credit, my total investment drops to $3,990. Not bad!
  • Average home: A 6.4 kW installation would cost $19,200 before the federal income tax credit, and $13,440 afterwards.

So you can see that figuring out exactly how many solar panels you need, as well as cost, is actually a pretty simple process:

  1. Find your total electricity use over the last 12 months
  2. Calculate system size with your local solar irradiance
  3. Increase installation size to account for system inefficiencies
  4. Find the number of solar panels by dividing installation size by solar panel size

Of course, these are very basic estimates. It certainly gives 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.

Image Source: CC license via Flickr

Be first to comment