I believe the OEM block heater wattage is 400w. If you run it for 2 hours, that's 0.8kwh, which costs about 13 cents where I live. Preheating the engine reduces wear, but even if we leave that out of the equation I'd need to save around 0.05 gallons of gas each trip to break even.
To put it in a perspective we're more used to thinking in, if I heat the engine 20 days per month I'm spending about $2.50, or a bit more than the cost of a gallon of gas here. I go through a tank roughly every 2-3 weeks and average maybe 60mpg in the winter, which is about 1000 miles per month or 17 gallons. I'd need to have an overall increase of 1/17 or 6% to break even with the block heater, which is ~3.5mpg for me.
Do I achieve this? If I'm making shorter trips, it's a lot more than that. Longer, can't say, the numbers are all small enough that they could easily disappear into background noise.
Last edited by Ecky; 12-09-2016 at 05:52 PM..