I'm an EE by training and I'm embarrassed to admit I can't for the life of me recall how to set this fantasy problem up.

Pretend we have a PHEV with enough battery on board to go 300 miles at 75 mph (ignore wintertime issues). I'll assume 3 miles per kWh so then I've got 100 kWh of battery on board. I know, 100 kWh of battery is insane for what I'm talking about but let's start there for the heck of it. With a proper equation we can easily change the battery size.

To extend range I'll add only a 10 kW generator that I can run whenever I choose. Assume I've got a rather large gas tank. Assume I don't need no stinkin' battery buffer!

Also assume I've got a very big bladder.

I start driving with a full battery, full fuel tank, and empty bladder.

I'm going 75 mph so I'll be able to go for four hours before the battery is depleted. But at the start of the trip I also started the generator, so at the end of the 4 hours I've added back 40 kWh. That will move me another 120 miles. At the same 75 mph another 1.6 hours have passed and the generator has added back 16 kWh. That gives me another 48 miles of range ..... blah, blah, blah.

So how long before I'm down to attempting to run the car on nothing but the generator? I know this should be a simple linear-math equation but I'm getting nowhere trying to set this up.

Thanks in advance!