Originally Posted by Tony Athens
If these are high wattage coolant immersion heaters, then I would be very cautious about leaving them plugged in all the time regardless of a thermostat or timer, and especially so if "original from decades ago".. I went that route up to about 10-12 yrs ago and after seeing so many issues with them in the "Boat Environment", I went the other direction. Very low wattage run all the time.
They are definitely coolant heaters. Mine are 120v so if the chart is correct they are 500w heaters. We love in the south (SC) so we don't have many cold nights. I am guessing i should turn the block heaters on for the cold nights and a few hours before cranking.