You can calculate kilowatt-hour from watts and hours. You can't convert watt to kilowatt-hour unit since watt and kilowatt-hour unit represent different quantities.
The energy E in kilowatt-hour (kWh) is equal to the power P in watts (W), times the time period t in hours (hr) divided by 1000:
E(kWh) = P(W) × t(hr) / 1000
kilowatt-hour = watt × hour / 1000
kWh = W × hr / 1000
What is the energy consumption in watt-hour when the power consumption is 5000 watts for time duration of 3 hours?
E = 5000W × 3h / 1000 = 15 kWh