Recently purchased a Rav4 Prime plug-in hybrid for my wife. It gets about 2.8 miles per kWH of charge, and 38 mpg on gasoline. Using 13¢ per kWH (which is what I pay at home) to charge and $3.199/gal for gas, electricity costs about 56% as much as gasoline per mile. The car has a 400 mile range with a full tank and fully charged battery. There are four “free” charging stations in town, two at local libraries, one at the power company, and one at the Toyota dealer, but unless you’re going to stay a while, it’s not worth the wait. A full charge costs me about $2.00 bucks at home, so waiting a couple of hours for a free charge is no bargain, unless I got nothing else to do. Commercial charging stations are few and far between, but generally they would run a little more than a deuce. A full charge gets about as many miles as a gallon of gas, so any time I can get a full charge (from zero) for less than a gallon of gas, I am ahead.
Commercial charging stations would have to charge about 26¢ per kWH to cost more than gas at $3.50/gal.
So on your plug in hybrid...if not fully charged—but full tank of gas—what is the mileage difference? Have you taken any long trips with it to see what happens?