Time to quote myself - posted this on the block heater thread:
I just got [the block heater] installed (1000W), and have been testing the effects. Two days ago I had the car out early & cold, took almost 7 miles to warm up. Until then the MPGuino read about 45mpg avg - this is at 35MPH in 5th gear. Warm gets around 65mpg.
Yesterday I plugged the heater in, but got excited and hopped in after an hour. That's just not enough time. The mileage was not much better, and the engine barely started to warm up after 5 miles.
This morning I let it go 45 minutes, started it briefly to circulate the coolant, then let it go 45 minutes more, drove off - the heat wasn't really there but the mileage sure was. Needle barely above cold, full temp inside two miles. Right away I got 65-70MPG @ 35mph in 5th. I think starting it briefly helps a lot.
I have now added a grille block to keep the heat in.
I will continue testing, at 2, 2.5 and 3 hrs. However, if this is going to pay for itself in gas saved, you have to price the kilowatts. A 1000 watt heater uses 1 kwh every hour. The price of electric here ends up about 9.5 cents per kwh, with taxes etc.
So my jaunt this morning cost me about 14 cents in electricity. That much gas would go about 10 miles @50mpg......
Or looking at it another way, eliminating the warmup period of 7 miles improves mileage for that distance from about 45 to 65mpg. That saves 1/20th of a gallon, about 13 cents worth. Just about break-even, slightly favoring the unheated method. And I didn't run the heater long enough to thoroughly warm the engine, I suspect that point comes around 2.5 or 3 hrs, costing 25-30 cents.
The equation looks more favorable when either gas goes up or electricity goes down. With electric at maybe 5 cents and gas around $4.00, looks more like this: 3 hours of heater on would cost 15 cents, vs gas saved worth 19 cents.
Not a big savings, to be sure - at that rate, it would take about 4 winters of use to pay for the heater.