Energy consumed by an electric device can be obtained with the help of the expression for power,
P = W/T
Power rating of the device, P= 500 W = 0.50 kW
Time for which the device runs, T= 10 h
Work done = Energy consumed by the device
Therefore, energy consumed = Power —Time
= 0.50 — 10 = 5 kWh
Hence, the energy consumed by four equal rating devices in 10 h will be 4 — 5 kWh = 20 kWh = 20 Units.