it is a dc block heater - the pump, you can buy these in any auto shop for the ac varient.
Basicaly these systems run full bore till you turn on the truck and disconect them, but this was made by thermo king so there'd be a logic board to cycle the heater and ensure the pump was on befor heater was activated.
It can be used as an anti freezing system, and start assist system by prewarming the block on the diesel/propane run refer units.
800w is a good figure, not too low not too high for a engine warming system.