So here’s the math, with some minor rounding to keep the sums simple.

The sun radiates an effectively unlimited amount of energy. It spreads it out in all directions and it is very far away. By the time the sunlight gets to the surface of the Earth, on a sunny day, at noon, at the equator, there is about 1000 Watts per square meter available for us to convert to useful power.

On a typical car, once you eliminate the windows, which you need to see through, there is maybe two square meters of surface looking up at the sky where you can stick solar panels, so there is a maximum of 2000 Watts to be had.

The best commercially available solar panels, in ideal conditions, can convert a maximum of 20% of the sunlight to electricity, but affordable ones get less. These ones are typical, and give about 100 Watts per square meter, or 200 Watts for our available area.

Ideal conditions means being at the equator, at noon, no clouds and the panels at right angles to the sun. We aren’t at the equator, it usually isn’t noon, clouds happen, the panels are at whatever angle the car happens to be pointing, and the sun moves across the sky anyway. Let’s generously say that all these non-ideal factors cut us down to 100 Watts.

The average electric car conversion uses 200 Watt-Hours per mile. If we assume that this is bimbling along at a useful 30 miles per hour, the car would be consuming 30 x 200 = 6000 Watts. The solar panels can only provide 100/6000 = 1/60th of the power we need to push the car along a suburban street. So they are pretty useless for powering the car in real-time. That is not to say that if you built a specially light car, with low friction and drag, and spread it out so it had lots of area for solar panels and then drove it somewhere really sunny, it wouldn’t work. In fact they race cars like this in Australia. It just isn’t something you are going to get registered at the DMV and run down to the shops in.

So what about using solar panels on the car to charge the batteries? That can be done, but is pretty ineffective. If we park it at work and let it charge for 8 hours at 100 Watts we can put about 800 Watt-Hours into the battery – which adds a whopping 4 miles (at 200 Watt-Hours to the mile) to the range of the car. Every little helps – you might think.

But this only takes into account the electrical characteristics. Solar panels have a physical aspect too. They are big, heavy and flat. The weight of our example panels is about 100lbs, plus mounting bracketry and extra electronics. The electric car conversion is already wallowing under the weight of the batteries – so we’d need to lose about two Trojan T105 batteries to make way for them. Even allowing for a 50% maximum discharge, these batteries represent 1350 Watt-hours of stored energy – nearly 7 miles of range. So if we kept the batteries and charged them before we left home, our overall range would be 3 miles longer than if we replace them with panels. Yes, the power is free, saving about 10 cents a day (assuming off-peak overnight charging with the new smart meters).

The other physical aspect to consider is that solar panels are big flat things. Little round cars (we are looking at the VW Beetle remember) don’t have lots of flat surfaces to let big flat things blend in, so our streamling will be kaput. This means it will take more then 200 Watt-hours per mile to push the car down the road. It won’t take much additional drag to have the car use more energy pushing the panels through the air on the way to and from work than they can generate all day long, at which point you need to buy more power from the power company than you did before.

Even if you could save that 10 cents a day, those two square meters of panels are priced at $1420. It would take about 39 years for them to pay for themselves.

So when they come up with cheap, flexible (paint-on?) solar panels, it would’t hurt to add them, but in the mean time the funds are better invested in things like regenerative braking or a smart charging sytem that lengthens battery life. Solar panels on the roof of the house, which has enough surface area to make in impact and offsets peak usage in the household (we were dinged up to 44c/Kwh last summer) makes sense – sticking them on a little electric car does not.

The primary components are:

• Donor vehicle (engine optional)
• Electric motor
• Controller for the electric motor
• Adapter plate to mount the electric motor
• Coupler to connect the motor to the transmission
• Batteries
• Charger for the batteries
• Mountings for the batteries
• Miscellaneous potboxes, shunts, contactors, wiring and connectors.

Step one is to decide on the application of the vehicle – what will it be used for? EV conversions, at least the affordable kind we are thinking on, are limited in speed and range. We live in a fairly small California town without much in the way of gradients, so an EV with the ability to make two across town round trips a day would make it an adequate runabout for the elder teenager who just hit driving permit age.  That is 25 miles on the flat with a max speed limit of 45 mph and mostly only 30 mph. Not unreasonable, and eliminates possible teenage drag-racing tendencies.

What vehicle to use?

A 1971 to 1974 Volkswagen  Super Beetle.

Why?

• Beetle conversions are common – full conversion kits are available.
• Beetles are common – the largest production run of any car, over 22 million from 1937 – 2003. While in some parts of the world they have largely rusted away, here in California they putter on forever.
• Huge parts availability.
• Easy to work on – the whole body comes off the frame with about 20 bolts.
• Lightweight.
• No power steering – without a constant running engine in an EV there is nothing to drive the pump.
• No power brakes – without the Infernal Combustion Engine intake there is nothing on an EV to generate the vacuum.
• Affordable. There are always several runners available on the local Craigslist for under $2000. They are not old enough or rare enough to be collectible.
• Big parcel shelf behind the rear seat to put batteries in.
• Super Beetle (1971 on) has lots of room in the front for more batteries.
• Super Beetle has strut suspension that should be easier to beef up to support the battery weight than the torsion bar suspension on the standard Beetle.
• Pre-75 Beetles are exempt from smog, making the paperwork simpler at the DMV.
• They are CUTE!

There are some drawbacks though:

• Drum brakes. Upgradeable to disks, and there are kits to provide vacuum boost electrically. With the extra weight of the batteries this is a big concern.
• No airbags, crumple zones or any modern safety gear beyond seat-belts. Hopefully mitigated by limited speed.
• At 26-29 years old there will be wear, tear, rust and fatigue to deal with.