You can calculate kilowatt-hour from kilowatts and hours, but you can't convert kilowatt to kilowatt-hour, since kilowatt and kilowatt-hour units represent different quantities.
The energy E in kilowatt-hour (kWh) is equal to the power P in kilowatts (kW), times the time period t in hours (hr):
E(kWh) = P(kW) × t(hr)
kilowatt-hour = kilowatt × hour
kWh = kW × hr
What is the energy consumption in watt-hour when the power consumption is 5 kilowatts for time duration of 3 hours?
E = 5kW × 3h = 15 kWh