Question

an electrical heater is rated 1200w. how much energy does it use in 10 hours?

Answer 1

1200 watts means it consumes 1200 watts of power  in 1 hour so
1200 x 10 hours = 12,000 watts or 12 kilo watts

Answer 2

energy = power x time
------------------------
power = 1.2  kw
time = 10 hr

energy = 1.2 x 10

12 kwh