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

posted by Adele on May 14th, 2010

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 a lot: 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 tack 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, 1BOG 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 1BOG 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 1BOG 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 1BOG’s group discounts and we’ll help you figure out the details.

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26 Responses to “Charging an Electric Car at Home: How many more Solar Panels do I need?”

  1. Brian Cole says:

    Weather is quite variable. That and other factors are going to cause notable lessening in array output from the ideal, The calcs for solar panel sizing only work if there is a constant 1000 watts/m2 available for those 5 hours, but that’s not realistic. Okay for a guesstimate, but not supportable in real-world situations.
    -brian

  2. If interested i’ve posted numbers showing our 6.5 kW solar system here powers a Tesla for about 72 miles, per day of sun. This is pretty good since our solar panels already paid for themselves (payback came at 7 years), this lovely electric car isn’t slow like a typical gasser (say a Porsche Boxster or BMW old-style gas car), our EV doesn’t need all that gasser maintenance like oil changes, etc, and we can go anyway much farther (240 miles per charge) than we need to–while our electric car (much) more fun to drive, boot!

    Here’s that post: http://www.wildershares.com/pdf/Solar%20Power%20for%20a%20better%20Solution.pdf

    We make about 24 kWh per day from sunlight: am driving tomorrow to LA on the ‘power of the sun’! : – )

    Rob

  3. Hans says:

    I’d say, the bit about “Payback period for additional solar panel cost” is misleading! The additional investment for photovoltaik is no business case if $0.11/kWh is your only compensation. Regardless of how you use the electricity. You’d achieve the same savings by charging the Leaf off the grid.
    Or did I misunderstand something?

  4. Rick says:

    Dear Author,

    The prior post is correct. You should not use the gasoline savings to justify the solar cells. The gas savings is needed to justify the extra investment in an EV versus a cheaper gasoline auto.

    You should use the cost of electricity from the grid saved to payoff the solar cell.

    $1,808 (cost of solar cells) / $127.16 (cost of electricity used per year) = 14.2 years to payoff the additional solar capacity.

    14 years is a very long payoff but if you want to go solar to charge your EV that is the correct financial analysis.

    Feel free to reply if I made a mistake, but I think this is right.

  5. Clayton from 1BOG says:

    Re: Hans and Rick

    Thanks for the feedback! This was an error on our part, and we’ve updated the post to reflect the appropriate payback time.

  6. evjuice says:

    Can you go ahead and calculate the solar panels needed for all available (and soon to be available) electric cars?

    rav 4 ev
    tesla roadster
    miev
    fisker
    smart
    think
    volt
    etc.

  7. Clayton from 1BOG says:

    Hi evjuice-

    I think the average numbers for the Nissan Leaf, Chevy Volt,and Mitsubishi iMiEV should about cover it for 99% of us (at least for now, or until we have time to write a followup). In the mean time, it’s easy to grab the specific numbers from each company website and use the above info to walk through the calculations.

    -Clayton

  8. I don’t think it’s wise to look for self-sufficiency. You can make averages, but very often, you don’t know how much you’re gonna drive tomorrow. Nor if it will be a sunny day. So unless your house is somewhere in the mountains, and you’re off the grid, I would advise to get the most solar panels you can afford, and that you have the space for, without giving any consideration to your electric car’s needs.

    Myself, I drive more than 1500 miles per month, there’s no way of doing that with solar panels, that are close to what I can afford.

  9. Quads4444 says:

    Using 11 cents per KWH grossly understates the marginal cost of electricity.

    I am in Long Beach California the cost of my last KWH used each month is usually at the ‘Tier 4 Rate’ which is 29 cents per KWH. So if I use solar panels to generate 1/3 of my own electricity, I will be saving at the most expensive rates, the expensive tier 4 and tier 3 rate levels.

    In the same way, in order to get a fair comparison between electric and gas, you must again use your most expensive electric rates because those extra KWH will be billed to you at the top marginal rates.

    So I personally would have to use 29 cents per KWH in my calculations, not 11 cent per KWH. This makes a big difference.

  10. Vincent Wolf says:

    The pay back in years is far less when you consider that most people drive closer to 12,000 miles a year and not 5200 miles. The actual payback period would be less than 6 years assuming typical inflation rates, more appropriate mileages, etc.

  11. Vincent Wolf says:

    Payback would probably be under 8 or 9 years assuming typical inflation rates.

  12. Christof Demont-Heinrich says:

    I disagree with some of the comments below that you “can’t” count kWh generated by a home solar system as offsetting gasoline costs. I’d say that depends. In our case, we’re pre-EV, but have PV. We’re overproducing kWh by the thousands with our 5.59 kW system and saving them to power an electric car (2,700 extra kWh in just 6 months). These kWh would never be used by us unless we got the EV. We’ll be ditching one of our 2 gas cars for an EV, and effectively using the banked kWh to power our EV. If we never got an EV, we’d never use these extra kWh, and our utility would pay us a pathetic 4 to 7 cents wholesale for each one of them. At 4 miles per kWh and $3 gas, and 25 mpg our 2,700 kWh are worth $1,300 in gas savings — because that’s exactly how we’re going to use them. This savings is a direct result of us: a) having a solar system; b) overproducing with that system; c) banking extra kWh for a future EV. Yes, an EV is more expensive than a gas car, but the fuel savings in our scenario, coupled with amazing out-of-pocket costs of $8,000 for 5.59 kW add up to big savings — http://solarchargeddriving.com/editors-blog/on-evs-a-phevs/321-re-doing-the-solar-math-on-the-nissan-leaf.html

  13. Mark says:

    I came across this discussion a little late, but why wouldn’t you use both the savings from the extra PV panels as well as the gas savings to figure the payback time? After all, the gas saving is money that you would have spent had you not bought the EV. So I am thinking the formula would be $650 (gas savings)+ $127 (extra PV panels-electricity cost savings)=$777- to off set the cost of the extra PV panels. It is probably even more as the electricity produced by the extra panels would likely have been charged at a higher tier (as was mentioned in a previous post).
    So in this scenario, it becomes $1808 (cost of the extra panels) divided by $777 (cost of gas and electricity saved) or 2.3 years payback.After that you are driving “for free”. Or am I missing something?

  14. David Yoder says:

    I’m
    Try an buy an electric car in 5yr. I wont to be prepard on how many solar pannels I need to charge the car

  15. David Yoder says:

    how many 45 watt solar panels would i need to complety charge a eletric car?

  16. Larry Frieders says:

    Cool. 14 years to recover the cost of the panels and another 8 to 10 years to recover the cost differential between gas and electric cars. If I switch today I’ll only be 85 when I break even. BTW, approximately 75% of our electricity is generated by burning fossil fuels. Nuclear is under 20%. We NEED more nuclear if you want to convince me that electric vehicles are cost effective and energy efficient.

  17. Mark T. Burger says:

    This looks like the right answer to my Nissan Leaf/Solar Panel Question.

  18. David Quijano says:

    The electric car is superior to what we currently use. That said, prices will have to come down before they are viable for most people. Also, range needs to be increased and people need to stop talking about how green these cars are. They are only as green as the source of your energy. Since most of us don’t have clean sources of energy, they are not green.

  19. Tyler Mclain says:

    I think we should put solar panels on top of the cars them self. Say you cover the entire top of a electic car with solar panels. it would power the car during the day as you drive. store the power in the energy in the batters like a wall charger. even on cloudy days they is still sunshine so it would always charge. and when out you could just stop and rest and let it charge itself. i know i cant be the first and its probably not as easy as it sounds but it must be possible. and even if it didnt charge them completely it would increase there range.

  20. Bob says:

    I keep seeing 10-11 cents being used for the cost of electricity. I know this is about the average cost of a kilowatt before delivery. If I use my bill as the basis for caluclation, I get more like 19-20 cents per kilowatt whick includes the delivery charges plus tax & other fees. Remember there IS a per kilowatt delivery charge by the utilities.

    • Shannon says:

      Hi Bob,

      Yep, as you mentioned, that’s just the U.S. average, which can be somewhat misleading, because in many states it’s way *above* that (i.e. parts of California and the Northeast), and in many states it’s actually way below (i.e. Southwest.) In places where the cost is as high as what you’re paying, the payback period on a home solar system does tend to be shorter, particularly in combination with the 30 percent federal tax credit on your system. If you haven’t already, you should definitely run a quick estimate using our free online tool http://1bog.org/solar-estimate . Or, if you live in one of the areas where we’re currently running a deal (http://1bog.org/deals), you can just sign up free on our site or call 877-444-4002 and we’ll do a quick initial evaluation for you over the phone.

  21. Joseph Varga says:

    Wait a minute. The car costs 30,000, the electrical panels cost more than 1800 as you need the electrical inverter, outlet for the plug, and any other items needed to provide the system for charging. Secondly, Solar panels supply 80 percent of their max for about 5 hours a day. Who is home during the day? The batteries on the charge decrease in storage amount as they get older. So lets be realistic, estimates are quesstimates and reflect agendas. If you buy an electric car for a 20 mile drive a day get a golf cart. The only reason one buys all this stuff is to fit a green agenda to satisfy themselves that they are lowering ther carbon footprint. Lets face it. this is not a good investment but a consious investment.

  22. Francis says:

    Yes, I do agree with you solar panels make a good payback compared to nissan’s leaf and the technology is getting better every year. So we should expect many more advancement in the solar panels which in turn should make it easier for electric car users.

  23. Thank you for such an informative post. I was initially a bit confused as to how much efficient and money saving these electric cars would get. But I do however have some queries though.All these calculations are based on ideal conditions. I have heard that the car batteries of these electric cars run out within an year or so. Therefore replacing them would mean an additional expense too.I don’t think that such a problem would arise so quickly with fuel-cars.

  24. adam says:

    Here in CA, we are given a baseline usage amount of 225 KWH as a “baseline,” probably fridge + lights. Additional usage is charged at greater rates up to $0.35 cents per KWH. I would Guarantee that the addition of a car would not be charged at 11 cents per KWH, rather closer to 30 or 35 (tier 3 and 4). Additionally, Your estimate on # of panels fails to account for shade, panel wattage, angle and orientation (I get 5.5 hours of sun but thats only part of the story). My235 watt panels (facing west) generate about 300KWH per year, so my math is needing 4 panels not 3…. However, winter production is quite a bit lower summer, and my added usage from my car will not be offset in winter months at all. I want to suggest people who have NO solar add 10 panels if they drive 12,000 miles per year and maybe 6-8 if they have a system and add a car. Otherwise good review.

  25. Al Tyler says:

    Wake up call & huge game changer here in San Diego (SDG&E) just raised our tier 3 and 4 electricity rates over 30%. My SDG&E statement for 1300 kilowatt hours this month as of Sept. 28 2013 was a $449.00. That’s right, nearly $450. And yes I did already inform SDG&E of the Nissan purchase back in early June 2013. I’m looking at my bill for this October (1st week only) and it’s already $276. At this rate our electric use will be over $500 monthly and we’re only driving the leaf 64 miles daily. Before the Leaf our rates averaged $130-140 monthly. And no I am no longer use central AC unit as of Sept.28 2013. $500 is what I was spending at the gas pump for ExxonMobile to fill 3 vehicles monthly! Now we’re spending that same amount on one added Nissan electric vehicle to our household … OUCH!!! This fall we’re switching back to our 2012 Nissan Murano (as opposed to the leaf) courtesy of SDG&E. Given the slight decline locally in gasoline prices, it’s actually cheaper to drive our 6 cyliner 2012 Nissan Murano as of October 2013.

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