I have one on my
B7800 and if it's 30 f or below outside, I use it.
For most of my engine starts, the outside temps are usually in the 20s. I've started it before in the 20s, without the use of the block heater and it starts harder(turns over more before starting) once started it seems to struggle to idle and it clatters pretty badly. I know....I know.... it's a diesel and that's how diesels perform in cold weather. It just sounds like it's a real strain on the engine.
With the block heaters warmth the engine acts more like it's being started in the summer. Engine turns over much less before it starts, it idles like it is suppose too and it is a lot less noisy. Maybe the block heater is not necessary, but I feel better about it just because the engine acts better with the block heaters use.
I plug my block heater in about 15-30 minutes before I start my
B7800 , depending on the outside temp.
I knew of one guy who left the block heater plugged in 24/7, when his tractor was not in use and his tractor was not started on a daily bases either.
That all changed, when the guys wife saw the electric bill.