How Many Solar Panels Do I Need?


Exactly how many solar panels do I need, anyway?

The average US residential solar installation needs about 20 solar panels, though that number can range anywhere from 8 to over 50. Why such a big difference?

Because the number of solar panels you need is dictated by several factors, including your electricity usage, the environment where the system will be installed, and the size and orientation of your roof.

The real question you are really trying to answer here is: how much solar power do I need?

Figuring out the number of solar panels you need is fairly basic, but there are a few surprising factors that must be taken into account during your calculations.

Number of Solar Panels Depends on Energy Use

First and foremost, the number of solar panels you install depends on your energy usage. Most homeowners want to produce as much electricity as possible, and at least enough to cover 100% of their energy usage. So, for example, if you used 10,800 kWh of electricity per year, you would want to install enough solar panels to produce 10,800 kWh per year (figuring out your monthly and yearly electricity usage is pretty straightforward from your power bill).

Every solar panel is rated to produce a certain amount of electricity — measured in watts — at any given instance. This is known as the solar panel’s ‘nominal power’ and most residential solar panels have a nominal power rating of 270 watts. This means that if the sun is shining and you have perfect weather, in one hour that panel would produce 270 watt-hours of electricity (equal to .27 kilowatt-hours).

Simple math to determine the number of solar panels you need

Going back to our example above, if you use 10,800 kWh per year and one solar panel produces .27 kWh in a single hour, you would think that you could do some simple math to figure out how many solar panels you need:

  1. Divide your total yearly energy use (10,800) by the total number of hours in a year (8,760 hours) to find your hourly energy use: 1.23 kWh
  2. Then simply divide your hourly usage (1.23 kWh) by the production of a single panel (.27 kWh) to find the total number of solar panels you need: 4.6, rounded up to 5 solar panels.

Five solar panels would be a pretty small solar system, and this sheds light on the fact that we have a few issues with our simplified calculation above.

First off, the sun doesn’t shine 24 hours a day, so your solar panels can only produce energy for 4 to 6 hours a day, depending on where you’re located. We didn’t account for this in the above example.

Secondly, numerous components of the solar system and environment, including shading, snow, and temperature affect the overall production of your solar installation. When solar companies estimate the number of solar panels a homeowner needs, they always account for these ‘real world’ issues.

To help make sense of all this, let’s discuss things in a little more detail.

Solar Panels in the Real World

As shown above, we can’t just use simple math to figure out how many solar panels you need.

Starting with your energy use is a crucial first step, but there are numerous other factors that affect how much energy a solar panel can produce:

  • Solar Irradiance – Measured in kWh per meter, this is a measure of how strong the sunlight is where the panels will be installed. Sunny Arizona sees about 6.5 kWh of electricity per square meter per day, compared to 3.75 kWh in the state of Washington, so a single solar panel in Arizona will produce much more electricity than the same panel in Washington.
  • Equipment InefficiencyInverters, wiring, and wiring junctions aren’t 100% efficient and a small amount of electricity is lost from the moment the electricity is created by the panel to it going to your house for use. All of this add up to about 4.5% in efficiency loss.
  • The age and dirtiness of the panel – As solar panels age or become dirty, they become less efficient. Solar installers take this degradation into account when estimating the total energy production from solar systems.
  • Weather – Cloud, storms, and haziness can all block sunlight from hitting solar panels, decreasing production
  • Heat – For every one degree Celsius that the solar panel rises above 65 degrees, it loses .4% efficiency.

Kind of a lot, right?

In fact, all of these small factors decrease the efficiency of the system by about 14%! When sizing your solar system, these inefficiencies are taken into account by your solar company and your system is sized to cover these losses.

You’re probably wondering how a solar company can take all this into account for each solar system they install. It can be done by hand, with research and lot of know-how, but thankfully, there are now programs like the National Renewable Energy Lab’s PVWatts Calculator that do the hard work for you. Users simply punch in the address and proposed system size and PVWatts looks at local irradiance levels and weather then estimates how much electricity the system will produce. Easy right? Go try it!

Your Roof Can Affect the Number of Solar Panels

Before you go calculating how much electricity you need, there are a few other considerations that can affect the size of your solar system.

The most common reason homeowners aren’t able to cover 100% of their energy use is limitations brought on by their roof – either shading on the roof or the size or orientation of their roof.

Even just a small bit of shade on a couple of panels can drastically decrease your overall production. You would think that if 20% of your panels are shaded, you would lose 20% of your production. Unfortunately, this is not the case. Since all the panels are interconnected and most inverters treat the group of panels as one large panel, if there’s shade on just a few panels, production for the entire system can drop dramatically – meaning it’s best to avoid shade where possible.

If there’s shading in a small section of the roof, the installer can simply avoid that section of the roof. If there’s a large amount of shading though, and you can’t trim or cut the trees down, you’ll likely have to relocate the panels to a different section of the roof – which might be smaller or facing a direction where solar panels won’t produce as much energy.

Another common issue that affects the number of solar panels is the orientation of your roof. Here in the United States, the best orientation for solar panels is due south, directly facing the sun, and at a 90 degree angle to the sun. As you move further away from south (facing east or west), energy production decreases.

In western Oregon, for example, a solar panel facing directly east or directly west will produce about 25% less than a panel facing directly south. Production decreases significantly if panels are facing north, since they aren’t hit by direct sunlight, though they’ll still produce electricity due ambient light. Solar companies will install on the north side of the roof if there’s no other options, but generally steer clear of this approach if possible. Three north-facing panels won’t produce nearly as much as 3 south-facing panels – so if there’s no other place to install other than facing north, solar companies must install more panels to make up for that lower production per panel.

The difference between north and south-facing panels is so drastic that in Oregon, where the state provides financial incentives for going solar, they have a rule that eligible solar installations can’t include north-facing panels. Oregon provides fairly generous financial incentives for going solar, so they want to make sure the money is being used wisely by ensuring that panels are producing as much energy as possible and not limited by their placement.

What If I want to Produce Over 100% Solar Power?

Homeowners often want to know if they can produce more electricity than they use. Maybe they want to increase their savings through a net metering agreement, or are about to have children, or want to put in a pool. They want to be absolutely positive that their solar system will cover their energy use in the future.

Almost all utilities have rules around what percentage of your electricity usage can be offset by the solar panels. For most utilities, its 100%. However, some utilities, like Xcel Energy in Colorado, allow homeowners to offset up to 120% of their energy use!

If you’re interested in making you solar system even bigger than what you currently need, be sure to call up your utility and ask if that’s allowed. Going bigger is really a financial decision that each homeowner needs to individually assess to decide if that makes sense for them – do the math and compare the higher costs for the additional panels and installation against how much they would get back through net metering or the value of having a larger system.

Let’s Look at an Example

All of this information might be spinning around in your head by now. Well, let’s clear it all up with an example.

Meet Martha. She lives in Phoenix, Arizona, and wants to install solar. She loves her air conditioning in the summer and uses an average of 1200 kWh per month, which adds up to 14,400 kWh per year.

Using PV Watts, we find that Phoenix sees irradiance levels of 4.25 kWh per square meter in December all the way up to 8.04 kWh in May. PV Watts recommends accounting for inefficiency in solar system wiring, inverter, shading, and dirtiness of panels by decreasing solar panel production by 14%.

After taking all of these inefficiencies into account, we find that an 8.25 kW system will produce 14,259 kWh annually – a little less than we want. However, if we install 53 270-watt panels, that raises our production to 14,310, which would cover over 99% of her energy use. Not too bad!

Before we start celebrating though, we need to make sure 53 panels will even fit on her roof. Solar installation companies actually send a technician to customers’ homes to climb up on your roof and take measurements so they can know, without a doubt, how many panels can fit. After assessing her roof size, it looks like Martha will just be able to squeeze in those 53 panels on the south facing section of her roof! If they hadn’t fit, Martha might’ve had to move a few panels to an east- or west-facing roof and added a couple more to stay at her 99% offset.

As is often the case, figuring out how many solar panels you need is a fairly simple process, but the devil is in the details. In most cases, the solar installer will perform these steps for you before you sign an agreement. However, using this guide and the online resources mentioned above, you can create an estimate for yourself, allowing you to have an idea of the process and how many solar panels can fit on your house before you even contact an installer.


  • by Ryan Austin
  • |
  • October 12, 2016
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