The short answer: Why bother? The shorter answer: Why not?
Toyota says the Prius is neat because you don't have to plug it in–it charges itself when running on gas. But what if you could plug it in? If you could fully charge a set of supplementary batteries overnight from household current and do the next day's errands purely on volts? (The gas engine will stay off under 35 mph if you have enough battery life and you're gentle on the throttle.)
CalCars, a California eco-car consortium that has converted a handful of Priuses to plug-in models, is about to issue a free do-it-yourself instruction dossier with photos, a video and a shopping list of all the components you need. The hardware will cost you about $3,000, a third of it for a trunkload of lead-acid electric-bike batteries (so Costco runs are out). It´s not a weekend project, but anyone comfortable working with high voltages can do it. The mod won´t void your car´s warranty entirely, but don´t expect the dealer to help if you screw it up.
So for the price of a perfectly good used Corolla (plus the Prius´s $22,000 price tag), you have a plug-in hybrid that can take a trip of less than 10 miles on electricity alone. But you can still
go as far as your gas engine will take you-averaging as much as 75 mpg, according to CalCars. Is it worth the bother? That´s up to you.