Because it also heats the engine as well maybe? Also I guess a car loses heat much faster than your stationary cabin because the flow of air around it as you drive is very good at taking the heat away. Also there is no insulation anywhere.

Anyway, from its spec sheet it says it can actually use up to 0.6L/h at full load:

https://www.webasto-comfort.com/fileadmin/webasto__media/web...

Full load is way too much unless you're arctic trucking. The lowest setting is plenty for my sun room until it gets below 20f or it's especially windy. This uses about a gallon in 6 hours.

But you're right, without any insulation, the moving vehicle will lose heat much faster. The room has terrible insulation but at least it has bare brick for the bottom 3 feet which probably retains a lot of the heat. I suspect with proper insulation your fuel usage would be drastically reduced.

OP's rate of usage equates to using 2.4 gallons in about 23 hours. Assuming you are only using yours during the few hours a day the sunroom is occupied, several days sounds about right.

Yes i run it 6-8 hours a day when I'm using the room. It really surprises me how effective and cheap it is vs a space heater. A solar powered heat pump would be even cheaper, I suppose but this solution was only about $200 to build. I doubt a heat pump would last long enough to pay for itself unless i could figure out away to cheaply acquire and install it.