Last updated on February 19th, 2018
The US EPA estimates that electric cars use about 300 watt hours per mile which is roughly 3 miles per kilo-watt hour (kWh).
Generally speaking a 4 kilo-watt solar electric system will produce for 6 hours per day giving you a maximum of 4 kilo-watts times 6 hours or 24 kWh of electricity.
You will lose energy putting it into the car’s batteries, taking it out of the car’s battery, and turning it into mechanical energy with your electric motor.
Roughly speaking these efficiencies are 85%, 85% and 85%.
Thus the 24 kWh produced by solar electric system is reduced (24 * 0.85 * 0.85 * 0.85) to approximately 15 kWh by the time it is used to move your vehicle along the road.
At 3 miles per kWh you can drive a maximum of 45 miles with your electric vehicle assuming you do not use heating or the air conditioner.
Of course, the vast majority of “electric” car owners do use the A/C and the heater, so the distance one can drive is less.
Electric cars use energy in lots of other ways that are often not included in the calculations. For example, electric cars will heat their batteries in cold weather, and often times, owners will pre-heat the interiors of their vehicles in the morning while they are still “plugged-in”.
So how much extra energy does an “electric” car use maintaining the temperature inside the vehicle?
It varies with the temperature outside. Generally somewhere in the range of 1 to 50%. In other words, the miles you can drive are reduced by a little to a lot depending on how cold or warm it is outside.
So if you can dedicate enough of your roof space to 4000 watts worth of solar panels and your commute is less than 20 miles each way you can run your “electric” car on solar at least on days when the sun is shining.
And while I generally talk down “electrics” cars because the vast majority really run on oil, coal, or natural gas, I am all for electric cars that run on Hydro, Wind, Solar, or Nuclear.
The problem is that “electric” cars running on oil, coal, or natural gas, back at the power generating plant, consume a lot more resources and pollute a lot more than light duty diesel vehicles with SCR/DEF systems.