Before we jump into it, I just want to point out that a microwave oven has two
power ratings. One is the amount of electrical power it takes from the wall outlet to do its job. The other is the "cooking power" it delivers into the chamber in the form of microwave radiation to heat the meatloaf. The utility power required from the wall outlet is always more than the cooking power radiated into the chamber.
But we didn't really need to go into all that in order to answer the question.
1 watt = 1 joule per second
1 minute = 60 seconds
(850 watts) x (1 (joule/sec) / watt) x (60sec/min) x (6 minutes) = 306,000 joules
