It's not an unreasonable assumption that 800 watts on the box means just that, and many online sellers do not list electrical input power versus microwave power. It seems that domestic magnetrons just aren't that efficient when compared to electrical resistive heating in percentage terms, but...
Threads like this sadden me deeply. Before I descend in to a rant about everything that's wrong with the current situation which makes me so sad, I'll just offer up a quick caveat about calculations.
An 800 watt microwave consumes more than 800 watts of electricity. A lot more in some cases...