So lets do a quick operating cost analysis on the Volt. At 200 mpg our 90k miles have burned about 450 gallon regular gasoline. That has cost about $1575 if we assume an average cost of $3.50 per gallon which is realistic for the past four years in California. 

Had we driven this distance with a Minivan at 20 mpg we would have burned 4,500 gallon at a cost of $15,750. So just on fuel we have saved around $14,175 over 4 years!

Now you will rightly say that electricity is not free, either. In fact, California is notorious for its high cost of electricity! So lets look at the electricity cost over the four years that we were driving the Volt. Here the magic of Net Energy Metering (NEM) and our time-of-use rate plan shine. Before we got the solar panels we used about 6000 kWh of electricity annually. We installed a 4.9kW solar system in April 2015. The system was projected to produce about 7500 kWh per year. In the first four years the Solar system actually generated (8119+7835+7808+7566) kWh. Since the solar system is sized to provide more electricity than our house needs there would be a surplus if we did not have to charge the Volt. The Volt can drive about 3 miles for each kWh. Since we drive so many miles per year we are actually net users of electricity as the table below shows in more detail. In the first year (bill 4/2016) we had the Volt only for 6 months. Therefore the imported electricity was only 736 kWh. In the following years our net import went up due to a combination of increased driving distance and slight reduction in the solar generation. Solar panels age, and also some dirt and dust reduces the production. In fact, we have never cleaned our panels and instead relied on rain to do this job for free. In total over 4 years we have spend $394 for 16,119 kWh from SCE. The average cost per kWh is about 2.4 cents. So the effective cost per mile to drive electrically is 2.4 cents/3 miles=0.8 cents. Even in the last year where our import was more significant we only paid less than 2 cents per mile. Sweet!

Now I owe you an explanation why our electricity is so cheap. The combination of NEM and a time of use tariff creates a substantial trading gain. Our excess production during daytime gets credited at for example $0.40 per kWh, and at night we can import electricity at $0.14 per kWh. In essence, the grid is a battery that costs us almost nothing and allows us to multiply our exported electricity by a factor of almost 3. Why would anybody buy his own batteries under such conditions?

So lets summarize our electricity cost and savings. Before we had the solar panels our annual bill was around $1000. So without the solar panels we would have paid $4000 over the 4 years vs. $394 with solar panels. So we saved about $3600 on our electricity bill over 4 years. The combined solar savings and gasoline savings add up to a total of $17,775. The cost of the solar system was $22,000. After the 30% federal tax credit the net cost comes to $15,400.

This essentially means that our investment has paid off after less than 4 years! Even without the tax credit we would be in the black after less than 6 years. And we have reduced our carbon dioxide emissions massively! And these benefits will continue for many years into the future! The above calculation does not even take the reduced maintenance cost on the vehicle into account.