O'Canada

There generally isn't a net savings. The purchase of the heater plus the cost to heat the engine and generally it is on longer than it needs to be, so it can be wasteful, rarely adds up to the savings of the warmup cycle with the engine. But if you want higher gas mileage and are willing to trade a slight increase in electric costs, then why not?

The Prius like all cars with cats needs to heat up the catalytic converter or else it is useless. Most cars including the Prius run rich and hotter until it is warmed up, and then start running normally. In a 15mpg vehicle, it is not really noticeable. In a 50mpg Prius, you get mid-30's to low 40's during the warmup cycle until S4 (Stage 4) is reached when everything is up to the correct temperature and the Prius is operating normally. In climates where it is really cold and the engine is cold, this takes longer. This is most evident in short trips. If it takes 2-3 minutes to warm up and you only have to go 4km, then the car may just warm up by the time you turn it off. So you will be getting mid 30's to low 40's mpg. That's 6.7 to 5.2 l/100km. But once you are up to temp and you can drive around normally, you will be getting mid 50's which will slowly drag your average up, or in your case low-4l/100km and drag the average down.

 

Just curious, if you don't mind me asking...how big is the employee discount for Toyota?

 

For your original question, I found the block heater pretty much non-effective in Edmonton on the GII. I got a GIII without it (in Alberta) which is supposed to be not possible. It would be a waste of money in my opinion. The Prius has no trouble starting with a 20 HP starter motor and a 200V battery boosted to 350ish volts! When it's cold enough to -need- a block heater on other cars the wimpy one provided has no effect. You need at least 600W for a block heater to be effective when it's cold enough to need it. The one provided is half that.