The block heater heats the oil, not the antifreeze. Would help to warm up the engine quicker if you start it right away, but that's about it.
Electric heat heats only the heater core, not the entire radiator. If you start the engine then it flushes the entire core into the radiator and circulates everything. Used to be this case with the 2013 model (I have not verified it with the 2017 model), if you have the temp gage showing in my view, you can see it drop from 1/3 of the way up (with heater core hot from EV heat) down to nothing when the engine starts (due to circulating the entire coolant). At that point the engine and HV heat will work at 5+ KWH to warm the coolant back up.
The engine will not start even as low at 0 F. I have driven in 0F and used the entire battery with no heat on and the engine doesn't start. It only started for me when it was -10 to -7F outside, then it kicked in suddenly after using about 1/3 of the HVB for no apparent reason. Then ran mostly at idle for about 5 minutes on country roads speeds of 40mph or less then it quit. Started again a little later once more for about 2 minutes and quit, and then I was able to use the rest of the battery. I started at -10F and got to my destination at -7F. This is the only time the ICE ran without the heat on. If you turn on the heat then its a different story, will usually run if its below 30F normally unless you switch to EV mode that helps to lower the threshold but doesn't eliminate the possibility depending on the conditions. Bottom line is that you can still drive 100% EV mode and use the entire battery if the temps are 0F or above (without heat of course just heated seats).
On the way home it was 17F later in the day and was like a cake walk I almost felt hot in the car without heat around noontime, compared to driving at negative numbers early morning that was tough even for me a hard core guy driving with no heat.
I agree that grill covers would possibly slow down ICE radiator cooling, the car does have louvers that close but grill coverers are usually blocking everything in the front to hold the heat during the winter. What I don't like is that if you use the engine more continuously then you can heat the engine more than normally with the cooling air blocked up. For pulse and glide country roads should be OK as normally it doesn't get to full temp because of all the cycling anyways.
Edited by rbort, 11 January 2019 - 09:34 AM.