Charging a Car Using Solar Power

(Why it's not practical)

Since I live off grid and drive a plug-in electric hybrid (2013 Chevy Volt), people sometimes assume that I charge it at home. This is far from possible. Here's why:

To charge an electric car, you typically need at least 3,000W of continuous power. (It's possible to charge slower, which I'll get to later)

On a good summer day, my solar system produces a peak output of 1,500W. On a winter day or a cloudy day, it produces next to nothing. The total maximum yield for a day of solar can be between 2-3 kWh. I rarely see that, because the system shuts off once the house battery is fully charged.

My car battery capacity is about 12 kWh, which means it would take 4 days of full sun to charge it completely.

Electric cars are designed for relatively fast charging; they're not designed to trickle charge or have a charge cycle turn off and on randomly. It's possible to buy a programmable car charger that can limit the current, but the lowest rate would still be about 1,200W, making it impossible to charge directly off my solar system (except at noon on sunny summer days).

My house uses a 5kWh battery (less than half the size of the car's battery). If I had an inverter big enough to power a standard car charger, my house battery would be depleted in 2 hours. Even in slow charging mode, the battery would drain in 4 hours, and the car would still not be charged.

So far, the math isn't working out for charging the car from off-grid solar. But anything is possible if you throw enough money at it. What would it take?

If I wanted to charge the car as quickly as possible (3kW for about 4 hours) without having to deplete the house battery charge, I would need an oversized solar system: 6,000 W might work. I'd need a large inverter to output 3kW continuously, and a larger battery to handle intermittancy. If my household demand is about 1kWh per day, and I don't want car charging to interfere with that, let's assume I'd need a battery that's about 15-20kWh capacity.

Cost (in 2024 dollars):

• 6kW solar system: $2,300
• inverter: $2,000
• 15kWh battery: $3,000
• car charger and wiring: $1,000
• labor: free...

Total: $8,300

So, it is possible, but it would cost much more than the car is worth, require more solar panel area than I have, and involve expensive ongoing maintenance (replacing the battery every 5 years).

Since the Volt is a hybrid, we can compare this cost with the cost of gas. At today's gas prices, I'd have to drive about 100,000 miles to break even on the solar charging system...

By the way, my electric hybrid has a puny battery worth only about 40 miles per charge. An all-electric vehicle has a battery 10 times larger. Factor that in to the off grid battery capacity calculation above.

Conclusion

Although it is possible to charge an electric car using off-grid solar, it's impractical because of the enormous up-front costs and complexity of the system. People who have electric cars and grid-connected, rooftop solar can boast that they're "100% solar" only because they rely on the grid to cover intermittent generation (for instance, at night). Even if their total energy demands and production balance out at the end of the month, such that they're not paying an electric bill, they're depending on the fossil-fuel powered grid to act as a giant battery.

As a rule of thumb, for every electric car that's put on the road, assume that at least 10kW of new solar panels (about 25 physical panels) need to be added to the grid, to cover the power demand. And grid-scale batteries would need to be added, just as in the off-grid scenario.

Considering the staggering numbers involved, it's easy to see why the so-called "energy transition" is not happening.