Installing solar pv replaced buying power from the grid not anything to do with buying gas. Driving EVs is what caused the OP to not have to buy gas and that can be done with or without solar panels.
In economics there is a concept called "Opportunity Cost". I think this what is in contention. Opportunity cost is the consequence of taking one course of action versus another. Thus, I could put my money in a bank with a CD yielding perhaps 1%. So in a year's time it would yield $100 on an investment of $10,000. Or I could put that same money into a different investment that may yield $200 per year. If I decide on the CD my opportunity cost would be $100. Of course there are many reasons one may want to forego the $200 investment, but the exercise helps to analyze each investment.
In a sense both sides have a point. For example, someone buys a TV set on sale. The retail price was $500. The savings on the sale amounted to $100. The purchaser says, "I saved $100". The other person says, "You didn't 'Save' any money, you spent $400". As far as the purchaser was concerned his opportunity cost of purchasing the set for $500 versus for $400 is $100.
So it seems that if putting in a PV system and driving an EV versus not putting in a PV system and driving an ICE causes the person to "save" $300 per month, then that is the opportunity cost of the two courses of action. On top of that however, one needs to weigh the opportunity cost against the total investment to see if the course of action is still financially advisable.
Reserved 4/20/10, Ocean Blue Ordered SL 9/30/10, ESVE Installed 11/22/10, Delivered March 8th, 2011.