I used a mechanical timer for many years. It was rated for 1000 watts I think. I would set it to be on for about 2 hours prior to normal departure time. That was enough to have the temp gage almost touch the first line when I fired up. I would get just enough heat to keep the ice off the windshield when I rolled out of the driveway.
"what for block heater you use on the boat diesel, humm?"
Actually the Cummins 6BTA had a thermostatically controlled block heater. The element was in that same location as the truck one, but it was connected to a thermostat on the opposite side of the engine. I had the one set for 120 F (and 120V). . they (Fleetguard) also make one set for 70 F. I'm told the effecient way to go is order the 220V in 70 F and it will use half the power. I didn't care, I didn't pay for elec at the marina.
Currently I have a 250 watt Wolverine oil pan heater pad to install. One of those roundtuit jobs. LOL