The Prius, Echo, and Scion vehicles all share the same 1.5 L engine block so their respective warm-up times on a block heater should sort of follow this rough measure. After a little over an hour of block heater time:
- 5 C - traction battery temperature (ambient)
- 13 C - MG2 temp, the one furtherest away from engine in transaxle
- 18 C - MG1 temp, the one closest to engine
- 36 C - engine block temperature, ~31 C increase
After four hours, I've seen ~35 C difference between ambient and block temperature. Thereafter, the block temperature begins changing in proportion to the ambient (aka., it doesn't seem to get any warmer relative to ambient.)
My math suggests that based upon Huntsville AL rates, a one hour, block warm-up is about as long as one wants to run it based upon local electrical and gasoline rates. Any longer and the additional electrical costs begin to exceed our local gas savings from a shorter warm-up.
I have not tried to solve the equations but even 30 minutes is enough to see a savings. The best approach being a circuit you can "turn on" in the house when you are fixin' to get ready to leave.
FYI, this shows the typical warm-up curve for my NHW11, 1.5 L Prius engine:
The goal is to get to 70 C quickly so the car can flip into Stage-4 and saves about 2 of the ~5 minutes it takes to warm-up. The block heater nicely brings the temperature up to 40 C and then if you have a thermistor hack, it can all but instantly spoof the car into Stage-4. This is when the engine will cycle off as soon as it is able. <grins>
NOTE: The USA EPA is worried that people making modifications to their cars may change the emissions characteristics. In fact California saw legislation or a vehicle inspection rule that was going to require a full emissions certification for every after-market, do-dad sold for cars. But if located behind the glove box, a thermistor hack should be all but invisible except to the excessively curious. <GRINS>