How to Size Your Solar Energy System for Future Use

Solar panels and electric cars are a match made in heaven ­– when you install a solar energy system on your home, you can use it to bothpower your home and charge your electric car for emissions-free transportation. The cost of solar is falling rapidly, and companies from Tesla to Nissan are manufacturing electric cars for your daily use. Now, the ability to install a solar PV system large enough to power both your home and your car is an option within reach. But even withincentives and rebates available for both technologies, most homeowners still can’t afford to install solar and buy an electric car at the same time. Luckily, it’s easy to install a solar energy system today that takes your future electricity consumption into account, if you take a few additional factors into consideration.

How Much Electricity Does an Electric Car Use?

Before you can make a decision on the size of your solar energy system, you need to determine how much electricity your car will use in the future. In addition to helping you size your solar energy system, knowing your electric car’s mileage rating can help you quantify the amount that you’re saving by switching to an electric vehicle.

Since electric cars don’t run on gasoline, the EPA rates them based on how many kilowatt-hours (kWh) it takes for the car to drive 100 miles, which they convert to a “miles-per-gallon equivalent” (MPGe). You can use U.S. Dept. of Energy's Fuel Economy website to find and compare the kWh/100 miles and MPGe ratings for all of the electric vehicles on the market in the United States.

Once you know EPA’s fuel economy rating for your chosen vehicle, you can easily calculate how much extra solar electricity you’ll need to charge your car. Here’s an example: the 2014 Nissan Leaf, an all-electric vehicle, has a combined fuel economy rating of 30 kWh/100 miles – this means the Leaf requires 30 kWh of electricity to drive 100 miles. If you drive 25 miles on an average day, that means you’re using approximately 7.5 kWh of electricity per day – or just over 2,700 kWh of electricity in a given year. This is the “extra” amount of electricity you’ll need your solar energy system to produce.

Armed with this information, you can work with your solar installer to design a solar panel system that will generate sufficient power to cover both your home and your electric car. But if you’re not ready to make the investment in both at the same time, you’ll need to install a solar PV system that can grow as your electricity use increases. 

How to Size Your Solar Energy System for Future Use

First things first: don’t put off going solar just because you might want to get a bigger system in the future. If you wait to install solar, you could miss out on state and local financial incentives – plus, you’ll have to continue paying for electricity from your utility every month. By sizing your solar energy system for future usage and ensuring your system is “add-on friendly”, it’s easy to find an option that generates enough electricity to power your home today and can charge your electric car in the future. Here’s how to do it:

Install an inverter that can handle more power. The default option for inverters is known as a string inverter. With string inverters, multiple solar panels are arranged into “strings,” which feed the power they produce into a single inverter. Typically, solar installers will include an inverter that can handle the expected output of your solar panels, but no more. If you know how many more panels you’ll need to add to your system later on, you can install an inverter that can handle the capacity of your existing panels plus the new ones you plan on adding after purchasing your electric vehicle.

Install microinverters with your solar panels. If you opt for microinverters instead of the default string inverter, each of your solar panels will have its own inverter. With microinverters, you can easily add extra panels to your system down the line without having to worry about whether your existing inverter can handle the additional electricity your new panels will generate.

Install a second, smaller solar energy system. So long as you have enough space on your roof, you can add a second system to your home whenever you need it. Homeowners can claim the federal tax credit for solar more than once, so you’ll still save significantly on your purchase.

Determine your future use, and build a bigger system to match. If you know that your electricity use will increase in the next year or two and have access to enough financing, you can build your solar energy system based on your future electricity use. This isn’t always an option – some utilities won’t approve systems that go significantly beyond your historical electricity use, so be sure to talk to your solar installer about your options first.

Another option is to “make room” later down the line by implementing energy efficiency upgrades to your home, which has the added benefit of reducing your overall energy costs. Consider switching out lightbulbs, installing a programmable thermostat, or upgrading your appliances to free up some of the electricity your solar panels generate for future use in an electric vehicle.

It’s worth noting that the strategies above work not just for electric cars, but also for any other additions or changes you make to your house that will increase your electricity usage. If you’re considering adding an electric heat pump system, electric water heater, or an addition on your home, you can expand your solar energy system to take your future electricity use into account.

Comparing Your Solar Options Can Help You Save

If you’re thinking about going solar, it’s important to know all of your options – you can save up to 20 percent just by reviewing multiple offers. To get started, find out just how much you can save with EnergySage’s Solar Calculator, or register your property to start receiving custom quotes from pre-screened solar installers near you.

Vikram Aggarwal is the founder and chief executive of EnergySage, the online solar marketplace. EnergySage simplifies the process of researching and shopping for solar. By offering shoppers more choices and unprecedented levels of transparency, EnergySage allows consumers to select the option that provides the best value for them, quickly and easily.