Here's my two cents. I'm a farm boy raised on a huge varity of diesels. Perkins, JD, Cat, IH,VW,CUMMINS, Detroit, both 2 and 4 stroke, and powerstrokes.
I don't plug in my cummins truck unless the temp gets below 15 degrees or so, and it only gets plugged in for 2 hrs max. Just 1/2 hour, to 45 minutes makes it start good.
Other engines it all depends on how well they start too. My cummins will start unassisted down to single digits, it doesn't like it though. Takes more time to build oil psi, and to idle smoothly, etc.
The service truck I drive (powerstroke unforunatly) struggles when it gets to the 20's, the teens it really needs plugged in. Still it only gets 1-2 hours before normal departure time in the morning.
I also work at the other end of the spectrum too, on machines that spend there whole lives away from 110 volt plug ins. Big batteries, 24v and a little ether will get most anything to start
Power is too expensive around here to leave engines tied to the wall all night long, pretty wasteful in my opinion.
Just my 2 cents worth though
