Before we got our Model S our electric bill averaged $85/month (we do not have AC and our heating is natural gas). We charge at night during off peak hours using an HPWC. Since last January our bill now averages $150/month. So far we have put almost 16K miles on the car.
So we are spending about $65/month to charge our car, and are saving hundreds of dollar a month that we used to spend on gasoline.
Our monthly electric bill is still so low that from a strictly economic point of view a solar PV system won't save us any money. But I still plan to do it someday.
You might be surprised. Even only $150 a month you might save money with solar. Or you might "break even" (loan payment for solar = old eletric bill) for the next 8-10 years, with free electricity after that.
In CA net metering is particularly good if you use TOU rates and charge at night. You get credited for extra you product during peak times at peak rates, but use most of you electricity at off peak rates.