You haven't said how much time the device runs.
The longer it runs, the more energy it dissipates.
Let's consider 1 hour of run time:
800 watts = 0.8 kilowatt
800 watts = 800 joules/sec
In 1 hour . . .
(0.8 kilowatt) x (1 hour) = 0.8 kilowatt-hour .
(800 joule/sec) x (1 hour) x (3600 sec/hour) = 2,880,000 joules
