Can solar on the roof really power your EV?

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Sono Motors prototype claiming around 30 km/d from the sun – Image from Sono Motors website

Sono Motors prototype claiming around 30 km/d from the sun – Image from Sono Motors website

With China, India, Norway, UK,  France and California planning to ban manufacture and sales of new internal combustion engine cars, and electric vehicles (EV) on the rise, will a solar roof become popular add-on option for plug in EV’s, and how many kilometres a day can they provide?

solar car 2 solar car 3

Panasonic’s components for the Toyota Prius Prime PHV 180w – Source: Bloomberg via Toyota

Below are the three summary points aimed to answer this question, but I welcome more experienced tech writers and engineers to add meat to the bone. This article is a follow up from my previous story on experimental solar cars and solar assisted electric vehicles (SAEVs), and the just completed World Solar Challenge.

  • How many solar km/day can we expect now and in the future for electric cars?

The solar roof on the Prius can generate enough electricity for 3-5km/day. Several groups are targeting between 10-30km/day with more specialised light weight 4 seater electric cars.

  • Does a solar roof make economic sense (for the bean counters) ?

Solar roof pricing isn’t yet available, but if a 200W roof can generate about 300 kilowatt hours (kWh) per year and if charging were to cost 40 cents per kWh over the payback period, that’s about one hundred dollars a year in avoided charging cost. If the upfront cost of the roof was $500 that’s about a 5 year payback. And that’s a lot of ‘ifs’.

  • Isn’t it smarter to put the solar on your roof if you own a house?

For now yes, but if costs come down and solar output increases the answer in 5 years might be to do both.

From development underway, there seem to be two distinct types of emerging solar assisted electric cars:

  1. The first like the Prius above we’ll call the ‘ SAEV 1.0’. Around 180-200 watt panel on the roof of a conventional EV will power accessories or deliver a small number of kilometres.
  2. The second, let’s call it the ‘SAEV 2.0’, will be a purpose-built EV with fully integrated solar on several surfaces of the car (Sono and LYO below). They will cost considerably more than a standard EV because of the light weight design but the sun could provide up to 30km/d from the sun as targeted by Sono. And this all depends on where you live and where you park the car.

In the comparison below I’ve only looked at smallis EV’s around the size of the Nissan Leaf or the smaller GM Bolt and will leave bigger cars to another time because they take more energy to drive each kilometre.  I’ve included a table and a glossary at the end of this article. So let’s get into the detail.


Panasonic are making the 180 watt roof the Toyota Prius Prime for the Japanese market. Panasonic say their roof will make the equivalent amount of energy for 3-6 km/day but for now the energy will be stored in a low voltage battery for heating/cooling/radios etc. Panasonic are progressing the design so when the low voltage battery is full, it will send electricity larger battery that drives the car. Tesla, Ford and Fisker have all produced prototype solar rooves (SEAV 1.0) but have put their solar roof plans on hold for the moment.


Alta Devices, a division of Hanergy are developing roofs, similar to Panasonic/Prius for selected future Audi models but two start-ups, Sono Motors and Light Year (LY), inspired by solar racing are building cars from the ground up (SAEV 2.0).

solar car 4 solar car 5

Audi e-tron concept with Alta Devices/Hanergy solar roof and the Fisker Karma with a solar roof

Sono estimates 30 km/day from the sun which could be a theoretical upper limit for a light weight production commuter car. Making some big assumptions I expect the engineering challenges for Sono, LYO and other young hopeful companies attempting to achieve these targets could need to include:

  • Increase solar capacity by using the bonnet, boot, roof etc. to around 600 – 900 watts,
  • Reduce weight of the car to less than the BMW I3 which is about 1.2 tonnes,
  • Reduce aerodynamic drag to less than the Tesla 3 which is around 0.4 CdA m2
  • Reduce rolling resistance by using larger, narrow wheels like the Ecopia’s on the BMW I3,
  • Do as many of the above to get more km/kWh (Renault Zoe is around 10km/kWh)

For comparison data see the table at the bottom of the article 

solar car 6   solar car 7

Sono Motors and Light Year’s prototype cars above, Sono claims 30 km per day of travel from the sun

This link to a short article suggests that if you can fit around 350 watts of solar on the very light BMW i3, it could deliver about 10 solar km/day in sunny latitudes like California. Any solar assisted car would get more solar kilometres if you live between latitudes such as 30oN -30oS than if you live in latitudes from 30o to 40o.

solar car 8


If you own a house and the roof has better orientation and less shading than the roof of your car is likely to have during the day, the solar on the house will have more output per watt than the car. Residential solar has been around for about 10 years so price per watt is lower but as automotive solar roof production increases this price difference could reverse. So for now home solar is a good bet but in 5 years it will be cost effective to have solar on your electric car as well.


At the moment the price of a new EV is roughly twice the price of the same sized conventional car making EV’s too expensive for most people, but in 5 to 10 years EV’s are predicted be down to the same price as normal cars.  But how do the numbers add up for an EV with a small solar roof, or a fully pimped solar EV?

SAEV 1.0 – For a small 200 watt solar roof the extra cost of about $500 it could pay itself off in 5 years. With automotive battery costs around $600/kWh, a car company could offer a slightly smaller battery pack that would offset the additional cost of a 200 watt solar roof, so the solar roof could be included effectively no additional cost to the customer, and provide savings of around $100 per year to the owner.

SAEV 2.0 – For a fully wicked solar car, putting the solar component to one side for the moment, the plug-in charging cost of this car should be half that of an equivalent EV because of their light weight and efficient design, saving up to $2k per year. Then if you live in a sunny part of the world and were able to get 30 km per day from the sun and you used it 5 days a week, you could save another $312 per year of charging costs**.  So that could be $2,312 effectively in your pocket each year. But in addition to these purely cost related issues, you gain a couple of key benefits. One is being able to be more independent of charging infrastructure and the second is that you’re getting on board with a whole new breed of transportation.

(**30 km times 5 days a week, times 52 weeks a year is 7,8000 km. If in 5 year’s time, electricity cost you $0.4 per kWh and the car used 1 kWh per 10 kilometres, every 1 km will cost you $0.04 to charge. And 50,000km per year x $0.4/kWh x 5 km/kWh = $4k/year for a std EV) 

Appendix table with rounded and indicative figures only so go easy on me.

electric + hybrid cars chart


W One Watt is the amount of power that delivers one Joule every second.

kW One kW is 1000 Watts. Approximate power to run a small toaster

kWh One Kilowatt for one hour. Similar to running a small toaster for an hour.

SAEV Solar Assisted Electric Vehicle

ICE Internal Combustion Engine

EV Electric Vehicle

PHEV Plug in Hybrid Electric Vehicle

Solar km/d Kilometres per day generated from the roof of an EV with solar cells.

David Woodgrove works in the solar industry, follows solar car racing, is passionate about SAEV’s and tinkers in his shed.   

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  • George Darroch

    These are very optimistic assumptions. But I’m glad there are people driving this forward.

    Even 5-10km of driving is not insignificant, considering that the average car in Australia does about 35km per day.

  • Sam Thebikekid

    I can see this haveing real potentiol, but defenatly not a silver bullet. My rough calcs give pritty similer numbers. on the order of 20 -30 km per day whith decent sun should be pretty achieveable. 150-200wh/km car, 20% efficent solar pannels, 3-5 M^2 of pannel aria. will defenatly not replace grid chargeing but could be valuble.

    • Sam Thebikekid

      being able to reduce the number of system component’s and converters needed is really nice- PV- MPPT, Battery. not PV- MPPT- inverter- rectifier- DC DC- battery. It’s also a lot less ” battry intensive” than if you where say chargeing at home from a house battery at night. shure- PV wll get hit by shadeing and poor orentation, whitch isnt ideal. whith some goood engeneering it may even be possable to cut out some of the cost associated whith steel panneling by replaceing it whith PV.

      • EnGee

        cost associated whith steel panneling by replaceing it whith PV.
        Good idea – even with amorphous semiconductors.

  • EnGee

    $500 for a car top panel. I don’t know why the panels are still so expensive. The price of everything Chinese is coming down, and with automation and the low price of raw materials used, i.e. Silicon and Aluminium they should be much cheaper.

  • Jorome

    Bicycle much simpler to use, park, get through gridlock, and finance

    • Rod

      Yep, chuck an electric motor on it and anyone, regardless of fitness levels and work attire can ride for transport.

  • Ian

    Why do cars have to weigh so much? Not knocking solar panels on the car’s surfaces, but the low hanging fruit is probably about reducing the different types of resistance like wind resistance, bearing resistance, rolling resistance and vehicle inertia.

  • Mike Shackleton

    Unless sprayed solar coatings become a thing (solar coating instead of normal automotive paint) I can’t see why this would be pursued too seriously. For one, a solar panel on the roof of the car is not optimally oriented to take advantage of the sun, nor do most people leave them parked in locations during the day to have sun falling on the panel surface. There’s a good reason why having the panels on the roofs of buildings is the best idea!

    • solarguy

      Well sure Mike, but think of this. A/C use sucks up power in any car. If 200w can offset that then that’s a range extender in anyone’s book, especially on a long drive and could make all the difference. Also it could be used to pre cool the car 5mins before getting in on a hot summer day.


  • Chris Fraser

    900 watts may be just enough power to overcome drag and rolling resistance, once a vehicle gets up to 120 km/h. For a cruising EV, it might just delay a recharge on motorways which are not shaded during the day – by several hundred km.

  • Miles Harding

    It started off badly by getting the definition of the joule wrong. That would be 0.5 Joules by 1/2 m v^2.

    • Davd Woodgrove

      Thanks Miles