So I have 18 solar panels on my garage roof, probably more than enough for my 2500 sq.ft. house in Placitas, NM.  Right now we have gas heat and hot water but when that eventually goes out I expect to move to electric.  The local power company also does “net metering” which means I sell extra power back to the grid for the exact same price I buy it for.  Basically I loan the grid my excess power.  A really good deal these days.

A friend pointed out that I’m using solar to charge my car, a Tesla Model 3.  I suppose.  He mentioned it must be pretty cheap.  But I wasn’t really sure.  I never did the numbers and it’s kinda hard to break out the part of my power bill that goes for the EV charging. Probably lots of ways to get a number but I did a spreadsheet to try to get a handle on it.

People have all sorts of different cars and driving habits and filling up with electricity is different from filling up with gas.  Our solar panels produce electricity whenever the sun shines, whether we use that electricity or not.  Gas is simpler it that it is stored energy.  You fill up a tank and then use it until you need more.

I figure I know what my panels cost (even if they are overkill for a single EV) so a good number is how many miles could I drive if I had bought gas instead of solar panels.  At $30k for the panels it’s a lot of gas and a lot of miles.  Of course it depends on the price of gas and the mileage of the car.  At 25 mpg and $2 per gallon it’s 375,000 miles.  At $5 a gallon it’s a mere 150,000 miles.  Ok, but it’s a start.

I was interested in payback time. How long would I have to drive my EV to get this same number of miles?  I’ll use my Tesla Model 3 numbers of 55 kWh battery and a 250 mile range.  I’ll also use 35 kWh per day production of my solar panels.  This is a real measured number, not some peak spec.  In fact I’ve been seeing 40 kWh but let’s be conservative.

Turns out these panels won’t even fully charge the Tesla in one day.  That sounds odd but we seldom drain the battery past half.  So at this rate, we can pay back the panels in 6.5 years with gas at $2 per gallon.  At $5 per gallon the payback is only a little over 2.5 years.

I think this is pretty stunning.  After a few years of EV charging a panel is all paid off.  These panels last decades, perhaps more than 50 years.  So once these are paid off you (and probably your children and grandchildren) will be getting “free” fill-ups.  That’s pretty wild IMO.

I’ve attached a copy of the spreadsheet below.

Update: so I’m not saying I am going to drive exactly 250 miles every day in my EV. This is just to see how quickly EVs (compared to gas) a solar panel breaks even. You can imagine a fleet of cars and a large solar installation. The general idea of payback time is the same.