The math of adding plugs to a regular prius

If you add a plug to a regular prius, you will charge 0.5 kwh (40% of 1.3kwh for nimh, 70% of 0.7kwh for li-ion for maximum battery life), or save 2oz/50ml of gasoline per charge of the battery. Which granted is pathetic and impractical, but the idea of filling 50ml of “gas” from the comfort of your own home just seems so ridiculously cool – to me anyway.
At ~1500 watts on a common household socket, that’s 20 minutes of charging without a specialized charger.
Also, showing off my absolute ignorance in electric physics here but suppose you charge the battery pack from a 5v 2A (10wh) usb port… it would take at least 50 hours for a full charge and you would burn off all that energy in a minutes drive, but still, you would technically be charging a full size 3000lb car on a fucking usb port. Which again, is ridiculously cool.
Now for the economics: If you charge it and drive once a day, you will save 1 gallon of gas every 30 days. If you charge it twice a day, you will save 1 gallon of gas every 15 days. But of course, the hybrid system doesn’t know you want to drain the battery and may leave you with a semi-charged battery at the end of your trip, in which case you’ll be able to charge it less.
With this information, plus the cost of electricity and the cost of adding a plug in mind, we can eventually calculate the payback period of this extra cost. But suppose the car has a useful life of 20 years, you charge it twice a day from maximum allowable discharge – and with intense use and charging at every opportunity, best case scenario you save 24 gallons every year. That’s 480 gallons over the vehicle’s lifetime. If you start today, you can expect to save $48 on gas in the first year assuming $2 gas – which isn’t nothing, granted. But is it even worth the effort of plugging and unplugging? Is it worth the cost of parts and labor (time if you diy)? Is it worth the risk of getting zapped by your custom mod? If you’re willing to spend that kind of effort, you might as well just get a real plug in.
Another aspect the math above doesn’t cover: The 2014 Accord Hybrid has a 1.3kwh lithium ion battery, and upcoming 2017 hyundai ioniq hybrid is expected to have a 1.6kwh lithium-ion battery. Both would have around 1kwh of usable capacity (less for honda and more for hyundai), but basically, double usable capacity is double the fuel savings of my figures above, and potentially saving an additional 50gal/ $100 on gas a year… never mind the inevitable reanimation of higher gas prices… means a plug should make sense on many of the hybrids currently considered by their manufacturers to be non-plug ins, even if they don’t have the plug in ev range.

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