ford-sunpower-solar-carSo, you’re thinking of solar-charging an electric vehicle but you’re not sure how big a solar system you need.

Figuring out what size solar system you need to fuel an electric car isn’t too difficult. You just have to crunch a few numbers and take into consideration how much sun you get where you live.

Read on for a some quick pointers from SolarChargedDriving.Com on sizing your solar system for an EV:

  • First, calculate how many miles you drive per year.

  • Second, divide the number of miles you’ll drive annually in your EV by the number of miles you’re likely to get per kWh in your EV. The typical miles per kWh range for mainstream production cars such as the Nissan LEAF, Chevy Volt and Mitsubishi iMiEV has been between 3 and 4 miles per kWh. We suggest cutting the difference in the middle and plugging in 3.5 miles per kWh into your equation.
  • Third, consider how much of your total home electric use – including the draw of your electric car(s) — you want your solar system to cover. Do you want to cover 100 percent of your total home electric use? 75 percent? Do you want to simply be able to say that your system covers all the miles you drive in your electric car, but not the electricity used by other electric devices in your house?
  • Fourth, consider the solar insolation where you live, meaning how much average daily sun you get where you are. The same-sized home solar system will produce differently if you live in Seattle than if you live in Phoenix. We’re using AltEStore.Com’s Solar Insolation Data for U.S. Cities in our sample equation below.
  • Fifth, take the number of kWh you want to produce in one day and divide it by the insolation value where you live. Allow for normal energy losses and inefficiencies in a solar electric system. Do this by increasing by 30 percent the number of kW you calculate running the kWh/insolation value equation.

Below is a sample solar system sizing equation for an EV for Denver, Colo., Seattle, Wash., and Phoenix, Ariz.:

  • 12,000 miles = annual EV miles driven

  • 12,000 miles ÷ 3.5 miles per kWh = 3,429 kWh
  • To create enough electricity to power your EV for a year you need a home solar system that produces 3,429 kWh annually, or about 9.4 kWh daily.
  • 9.4 kWh ÷ 4.87 (average daily insolation value in Denver, Colo. from AltEstore.com chart) = 1.9 kW
  • 1.9 x 1.3 (add 30% for solar system inefficiencies) = 2.5 kW
  • Thus, you will need a home solar system of about 2.5 kW in Denver in order to produce enough electricity to drive 12,000 of what we at SolarChargedDriving.Com call Sun Miles©
  • In comparison, plugging in the average daily solar insolation values from cloudier Seattle — 9.4 kWh ÷ 3.57 = 2.6 kW x 1.3 (adding 30% for solar system inefficiencies) = 3.4 kW — you’ll need a 3.4 kW solar system in order to produce enough electricity to drive 12,000 Sun Miles© per year.
  • Finally, in Phoenix, which gets even more sun than Denver, 9.4 kWh ÷ 6.58 = 1.4 kW x 1.3 (adding 30% for solar system inefficiencies) = a 1.8 kW home solar system to generate 12,000 Sun Miles© per year.

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In fact, based on our own experience with a 5.59 kW home solar system in Aurora, Colo., the numbers above – which we calculated based on an equation suggested by AltEStore.Com are a bit conservative.

According to AltEStore.Com’s numbers, our 5.59 kW system would produce about 7,700 kWh per year. REC Solar, which installed our system, predicted 7,900 kWh per year. In fact, across two years, we’ve been averaging about 8,600 kWh per year.

Of course, in the end, it’s probably better to use more conservative numbers to calculate the size solar system you need to fuel your EV, as you’ll be happier when your system produces more fuel than you expected rather than less.

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