They do put out a lot of heat. I've seen a few people using these cheap diesel heaters for their shops, on YouTube.
I think one guy had a two car garage, that was not insulated! Must have been down south! He got the garage very warm!
Anyways, you might be able to find more information about the real BTUs on one of the facebook groups or even on YouTube.
If your shop isn't too large and has some insulation, you might be in luck!
I was expecting to get more heat into my HS Tarm boiler on my 1st test.
But the normal 1 deg loss per-hour wasn't slowed down much.
The big wood-coal fire-box was dirty with residue hanging on it's walls.
So, I cleaned most of that up and got a ash out of the clean-out pan area.
I want to run another boiler-heating test when the hotwater temp is abt 100F.
And run the heater at full speed, to see what level of heat can be scavenged from the exhaust pipe output.
I know the reasons for the 1 deg-loss-per-hour, and there isn't much that can be done about them. (One reason is excessive handwashing)!