The only trouble with use a heater is that you're robbing Peter to pay Paul, in other words maybe you get an extra 2mpg but you're paying electric to heat the car so one way or another you're paying extra for the savings.
The amount of energy that is wasted in extra gasoline burned during the warm-up cycle is a whole lot greater than the amount of energy needed to heat up the engine. The waste is due to the fact that when the engine is cold (and in this context even 80 or 90 degrees is "cold") gasoline condenses out on the cylinder walls. To make up for this, the car's system makes the mixture excessively rich. The unburned gasoline is flushed out through the exhaust valves on the exhaust cycles. In olden days you could smell it in the exhaust. Nowadays it gets burned in the catalytic converters.
So, a block heater is a pretty efficient way to reduce the cold start gasoline wastage.
A whole lot better way is to get a PHEV and avoid most of the cold starts altogether!
Although I'm a strong believer in block heater benefits, with the Energi, I don't see the need.
Edited by Smiling Jack, 29 October 2015 - 09:37 AM.