kilowatt hour and kilowatt per hour

Electric energy is sold by the kilowatt hour (kWh). A kilowatt is a unit of power. A kilowatt hour is a unit of energy. It's the sum of power consumption over a given period in hours.

As in example, saying a motor consumes 100 kW and runs for 20 hours or 2000 kWh

A kilowatt per hour (kW/h) I think it gives the average power.

For the same above example it would be 5 kW/h.

What do you think?

• No, because like you said, if a motor consumes 2000 kWh in 20 hours it must average to 100 kW consumption averaged over the 20 hour period. What is the question, other than what we think - I think the units make no sense. A load that consumes 1 kW in an hour meas 1 kWh, it could consume 2 kW for half an hour and nothing for another half an hour. Commented Aug 15, 2021 at 7:48
• power is energy per time, or the rate at which energy is changed. power per time is the rate that power is changed. Commented Aug 15, 2021 at 14:27
• Kw/h is comparable to knots per hour. Commented Aug 16, 2021 at 0:55

Power is the rate at which you consume energy.

It can be regarded as analogous to physical speed (metres per second).

Physical speed is the rate at which you "consume" distance.

If you wanted to work out what speed per second is, that equates to acceleration so, in electrical terms, watts per second is how quickly the power consumption increases (or decreases) over time and, can be meaningful in some analyses.

It's not a precise analogy but, it works for me.

• Makes LOT of sense
– OMAR
Commented Aug 15, 2021 at 11:51

1 kilowatt = 1 kilojoule/second (1 kW = 1 kJ/s). The dimension of time is already taken into account in the definition of the watt.

1 kWh = 1 kW × 3600 s = 1 kJ/s × 3600 s = 3600 kJ. (The 'second' units have cancelled out.)

"kW/h", therefore, can only refer to a rate of change of power. The only sensible use I can think of would be something like, "The load was 100 kW initially but decreased at 5 kW/h."

Figure 1. Energy is power by time. The average power is measured in units of power (W), not watts/hour.

Average power is just total energy consumed divided by time and is measured in W or kW.

It doesn't matter what we think. Just use the definitions and the units properly.

If you want average power, then that's energy per unit time. It would therefore be kWh/h, and cancelling h above and below gives kW.

Power, instantaneous power, average power, all have the same dimensions, power, kW.

No ... the average power is simply 100kW in your example.

If the motor ran for every second hour for 10 hours total in that 20 hour period, it would use 1000 kWh, and the average power would be 50kW.

The only sane use of kW/hour is to measure the capacity of a solar cell manufacturing plant. If, in an 8 hour shift, it can manufacture enough solar cells to generate 40kW, then its capacity is 5 kW/h.

As Andy says, this is like an acceleration, as the size of the solar farm gets bigger every hour and it generates more power.

• When there's a fault in a power grid - like when a power plant shuts down - the change of available power is a sane use of W/h (typically MW/s) ;) Commented Aug 15, 2021 at 18:42
• @SimSon Also true.
– user16324
Commented Aug 15, 2021 at 19:16