Exascale Supercomputers Power Consumption
MW (megawatts) is not a unit related to time, it is the power usage in a moment in time. 

According to the formula P = V * I, P (in watts) is the product of voltage times current. I doubt the computer uses standard 120V of 220V but let's assume it does, it means the computer uses 20MW = 110V * I  => I = about 180 kA (kiloamperes).

To translate this in time, the unit kWh is used (kilowatt-hour), meaning the average number of kilowatts used over one hour. Thus if this computer would be on full power for one hour, it would consume 20 MWh.