Charging an Electric Car at Home: How many more Solar Panels do I need?

By Matthew Wheeland - May 14, 2010

*Last Modified: June 7th, 2014

Thinking about charging an electric car with your residential solar array?

Nissan-LEAF-electric-car

Pictured Above: Nissan Leaf

It’s a question we get asked often: how much bigger does my solar array need to charge a new electric car? Here are the two most important things you need to know:

1. It’s hard to add solar panels later, so make sure you buy the right number up front. Or, install your panels with microinverters for easy add-ons later.

Seems like it should be easy to add on another solar panel or two to accommodate an electric vehicle (EV), but unless you’re using new microinverter technology, solar arrays aren’t easy to upgrade. The standard inverter (which takes DC electricity from the panels and converts it to AC that can be used by your home) is sized for a specific number of panels, so if you add panels later, you may have to replace your inverter. It’s much cheaper to get it right the first time. We can help you figure out how much additional capacity you might need for an electronic vehicle.  Alternatively, microinverters make things much easier. You can add panels to your array later on — even panels of a different type, or on a different part of your roof.

2. Most of the new electric vehicles (Volt, Leaf, iMiEV) require about the same additional electrical usage – about 200-250 kWh / month.

We’re basing this number on the average efficiency for the Chevy Volt, Nissan Leaf, and Mitsubishi iMiEV — around 4-5 miles / kWh, and for an average driver in the US (12,000 miles per year). It also assumes you use the vehicle for all of your driving and you charge it exclusively at home. How much would an extra 200-250 kWh cost per month? Just multiply 200-250 by your local utility rate — US average is $0.11 / kWh, so this additional usage would cost you about $22-$27.5 / month.

And now, the details on home electric vehicle charging….

Last week, PURE was featured in the LA times article Driving in savings-neutral territory so far, written by customer and early electric vehicle adopter Susan Carpenter. She went solar through PURE and wanted to understand how much additional capacity she would need to charge her leased BMW Mini-E or a future Nissan Leaf.

We did some hard calculations for the article, which assumed a 20-mile per day commute in a Nissan Leaf, with all charging done at home. Keep in mind that a lot of variables will affect cost and payback time calculations, including:

  • Assumptions about electricity rates (this example assumes charging is always done at the baseline rate in a tiered rate structure).
  • Assumptions about charging locations (this example assumes that all charging is done at home).
  • Extra stuff that might offset costs, like feed-in-tariffs (payback time calculation here assumes that you don’t get paid for extra electricity generation).

Here are the calculations we did for the article (WARNING: Lots of numbers in here!):


1. Electrical cost for 20-mile daily (weekday) commute in the Nissan Leaf:

  • Weekday mileage: 20 miles per day x 5 days per week x 52 weeks = 5200 miles per year
  • Additional Usage: At 4.5 miles / kWh that’s about 1156 kWh per year or 96.3 kWh per month additional usage if charging exclusively at home. 96.3 kWh per month / 30 days per month = 3.21 kWh per day. Assuming 5 hours per day of good sunlight, 3.21 kWh / 5 hours = 0.64 kW.
  • Additional capacity: this means it would require a 0.64 kW addition to the solar array to accommodate your electric vehicle commute. With 200W solar panels, this is approximately 3 additional panels (0.64 kW = 640 W / 200 W = 3.2 panels).
  • Additional cost: We ran these numbers for Los Angeles – the last PURE price in the Los Angeles solar campaign was $5.56 / DC watt, so you’d be looking to spend an additional $1,808 after rebates and tax credits (note: we used some fancy spreadsheets to factor in all rebates and tax credits. Our solar estimate tool will do this for you as well, and we can walk you through it on the phone).

2. Payback period for additional solar panel cost (Revised 5/17/10).

  • Driving the Nissan Leaf 5200 miles a year will save 216 gallons of gasoline per year (assuming typical midsize car 24mpg).
  • If gas is $3.00 / gallon that’s $650 per year spent on gasoline.
  • Going back to the above calculations, the Nissan Leaf example adds 96.3 kWh usage per month to the electricity bill, or 1156 kWh per year.

If the (fill in the baseline electricity rate for your utility in Los Angeles here) average electricity rate in the US is $0.11 / kWh, then we’re looking at an additional cost of $127.16 / year.
So, $650 in annual savings – $127.16 in additional costs = $522.84 savings (cost difference per year between gas and fully electric).

You’d save $522.84 per year by commuting in the Nissan Leaf.

Payback time for additional solar panels: ~14.2 years

While 14.2 years might seem like a long time, keep in mind that solar panels are warrantied for 25 years of clean energy production for your home (for comparison, on strictly economic terms the Nissan Leaf will take 50 years to to pay itself off).

This example also inaccurately assumes that electricity prices won’t go up. Solar panels are a long term investment that save you money and cut out the biggest sources of CO2 emissions and pollution for most of us: home electricity and, in this case, driving.

A lot of calculations were completed here, so if we’ve made an error or you need additional information, please let us know! Special thanks to solar advisors Brad and Jonathan for assisting with these calculations!

If you’re considering solar for your home, then sign up for PURE’s group discounts and we’ll help you figure out the details.

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