You can calculate watt-hour from kilowatts and hours, but you can't convert kilowatt to watt-hour unit, since kilowatt and watt-hour units represent different quantities.
The energy E in watt-hour (Wh) is equal to 1000 times the power P in kilowatts (kW), times the time period t in hours (h):
E(Wh) = 1000 × P(kW) × t(h)
watt-hour = 1000 × kilowatt × hour
Wh = 1000 × kW × h
What is the energy consumption in watt-hours when the power consumption is 5 kilowatts for time duration of 3 hours?
E = 1000 × 5kW × 3h = 15000 Wh