How much energy will be spent in 15 minutes by a 5KW electric heater?

We have, 
Time taken by the electric heater, t = 15min. 
Power of heater, P = 5 KW.

As we need answers in joules so we need to convert the units of power and time in Joules/sec and second respectively.
$\mathrm{t} = 15 \min = 15 \times 60 \sec = 900 \sec$ 
$\mathrm{P}=5\mathrm{KW}=5\times 1000\mathrm{J}/\mathrm{s}=5000\mathrm{J}/\mathrm{s}$ 
Now we have the relationship between Energy, Power, and time:
Energy = Power × time 
$\mathrm{Energy}=5000\times 900$ 
$\mathrm{Energy}=4.5\times {10}^6\mathrm{J}$ 
Hence, energy spent is $4.5\times {10}^6\mathrm{J}$.