Can I add solar panels to an existing system?

Homeowners with an exiting solar array sometimes want to add more panels to generate more electricity. Here's the different ways it can be done.

Photo of a nice house with solar panels.

If you have a home with a solar array, it’s pretty common to find yourself using more electricity than you did when the solar panels were first installed.

Solar homeowners tend to be interested in other green technologies such as electric vehicles, heat pumps, and induction cooking. If you installed solar panels years ago and then, say, bought a Tesla, you may find that your solar panels no longer fully cover your electric bill.

Of course, there’s no rule that your solar panels need to cover 100% of your home’s electricity needs. Even if you generate only half of your annual electricity usage with solar, that’s still a significant financial and environment benefit.

But if you do want to upgrade your solar array to add more power generation, it is possible. By adding new panels and possibly upgrading your inverter, you can expand your system to keep pace with your electricity usage. The feasibility will depend on considerations such as having enough roof space and the type of inverter you have. You may also find that the project isn’t cost effective. With those caveats in mind, many existing home solar systems can be upgraded.

How much more solar electricity do you need to generate?

If you’ve already purchased an electric vehicle, added a heat pump, or completed the home upgrade that will be sucking more electricity, you should be able to take a look at your electric bill to find out what the impact on your electricity usage is.

If you haven’t made the purchase yet, you’ll need to do a little product research. With an electric car, that’s pretty easy. Just look up your vehicle’s MPGe rating on fueleconomy.gov. If you know how many miles you drive a year, just multiply the kWh/100 mi rating by the number of miles you drive.

For example, a Tesla Model 3 consumes 24 kWh/100 miles driven. If you drive 15,000 miles a year, that works out to 3,600 kWh per year.

How many solar panels will you need to add?

As a solar homeowner, you’ll be familiar with the proposal process with a solar installer. After an initial consultation, you’ll get an estimate of how solar panels you’ll need to meet your electricity requirements.

Getting multiple quotes is the best way to get an accurate estimate of how many additional panels you’ll need to add to your system, but a quick way is to use the solar calculator on this website. While it’s less accurate than an analysis by a professional installer, it’ll give you a pretty good idea of how many panels you’ll need. This can help you decide if have the roof space to make it work.

How you expand your system depends on the inverter

The inverter in your system converts DC electricity from the solar panels into AC electricity that your home can use. When it comes to system expansion, there are really two types of inverter: microinverters and string inverters.

String inverters are central units that perform the energy conversion for the entire array. This includes both conventional string inverters and those that incorporate power optimizers, such as the SolarEdge HD-Wave series. A string inverter has a maximum input/output capacity, beyond which you won’t be able to connect any more solar panels.

If you have a string inverter, you’ll be able to add more panels to it only if there is spare capacity. If the inverter is already operating at maximum capacity, you can still expand your system, but you’ll need to either purchase a second inverter or upgrade the existing one. In most cases it will be less expensive to get a second inverter, but an installer can talk you through the options.

The better situation is to have a system based on microinverters. With microinverters, each solar panel is paired with its own inverter. To expand this type of system, you can simply add more panels and microinverters. You’ll only be limited by the size of your roof.

How much does it cost to expand an existing solar array?

Unfortunately, it isn’t cheaper to add panels to an existing system than it is to install a brand new one. In fact, because your expansion probably won’t be as large as the original system, it might be more expensive on a per-watt basis. A larger system is generally cheaper than a small one, so your per-watt price will likely be higher than it was for your original system.

This is because much of the work for the solar installer is “soft” costs, which includes things like labor, permitting, and overhead. The installer will likely need to do most of the same work they did for the original installation, which includes applying for permits and scheduling a day or two of work for the installation crew (known in industry parlance as a “truck roll”).

While the 26% federal tax credit and other incentives that may be available can generally be used for your system expansion, you might find that the higher per-watt cost of a system expansion means the project won’t have a good financial payback.

Most solar installers have a minimum project size

If you only want to add a few solar panels to your array, you might discover that many solar installers won’t take the project. Very small projects often don’t have enough profit margin to make them worthwhile for companies. When speaking with installers, you may find that many have a minimum project size of 3 or 4 kilowatts, which is about 8-10 panels.

One way around this is to oversize your array to meet the minimum size, but at the same time utility companies often have a maximum size limit on the array to prevent homeowners from generating more electricity than they use. Because of this, you might find yourself unable to size your system expansion to fit within the minimum and maximum sizes imposed by the installer and utility company.

You might not have the ideal location on your roof for extra panels

Your original solar array should have been placed on the best part of your roof for solar panels. This means south-facing with minimal or no shade. If you want to add more panels, you may find that the ideal location on your roof has already been used, and you’ll be forced to consider east, west, or even north-facing panels.

While west-facing panels are often desirable because of utility prices that vary during the day, east-facing isn’t as good and north-facing is usually the worst choice of all.

It is possible to install panels on the north side of a roof, especially if the panels are tilted up using a method known as reverse tilt, but in general a north-facing array should be considered a last resort.

Should you use the same company that installed your original system?

While you usually don’t need to use the same company that installed your original system, it’s a good idea to at least get a quote from them. They will be familiar with the equipment, will have access to the monitoring system already, and there’s no risk of voiding the warranty of the original system.

Having a different company add a system expansion usually won’t void the original warranty either, but it would be a good idea to check.

Other than that, the process of getting prices on a system expansion is the same as a standard home solar installation. Be sure to get multiple quotes, and ask the usual questions.

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