Pre-heating saves little or nothing, since the cost of the electricity PLUS the cost of the heating hardware is never recovered.
You can just let the car sit and idle till the same temp is reached and see similar savings on the commute, with similar heating cost. Generally that means letting it idle till the first time it shuts off.
This was analyzed and repeated many times over at the Prius forums. If you drive in open loop mode for 2 minutes you suck down several times more gas than if you let it idle for the 2 minutes it takes to warm up to closed loop. The average MPG for the trip is higher than if you just take off cold. The gain is comparable to using a block heater.Whats the theory behind that, how is it better than just driving away immediately? Gonna try that btw, Thanks!
And voids your hybrid system warranty, engine warranty, etc.