Power (kW) VS Energy Consumption (kWh)

by Doris T May 24, 2020

Power (kW) Is NOT The Same As Energy Consumption (kWh)

Very often, we see sellers of electrical products (eg. Fridges, heaters, humidifiers, etc) using a lower power rating (kW) to tout the product’s “energy-efficiency” or energy-saving properties. However, this may not be the case. A lower power rating (kW) does not necessarily mean lower energy consumption (kWh). Now, I’m certainly not an expert in this field, but seeing how my clients tend to get confused in this aspect, I will try to share the information that I have, in as simple terms as possible, so that everyone can benefit. The terms used have been simplified to facilitate understanding.

BASICALLY…

"kW" reflects the rate of electricity usage (“power”), while "kWh" reflects the total amount of electricity used in an hour (“energy consumption”) https://www.artisenergy.com/utilities/difference-between-kw-kwh/)
1kW = 1,000W

Examples:

If you switched on a 100 watt (0.1kW) light bulb, it would "consume" 0.1kWh of energy in an hour
A 2,000 watt (2kW) appliance would "consume" 1 kWh of energy in half an hour.

BUT….

But not all appliances require their full-rated power for the entire time. This is especially so for refrigeration products. In a refrigerator, the compressor, in most cases, is the single largest energy-consumer among all other components.

Does this mean that a 1kW compressor "consumes" 1kWh of energy/electricity in an hour? Not necessarily.

Compressors may not run all the time for the entire time that the refrigerator is operating.
Typically, the thermostat in a refrigerator starts and stops the compressor whenever the desired temperature is achieved. Therefore, the compressor may just be "running" 30%-40% of a typical hour.
This also means that the sooner the desired temperature is achieved, the sooner the compressor stops and the lower the energy consumed.
This comes down to multiple factors such as insulation quality, power of compressor, efficiency of the cooling system, etc. It's not just about the "power rating".

Of course, for compressors, there are still other factors such as “starting” and “operating” power, which I will not go into this time.

SO… WHAT DOES THIS MEAN FOR ME?

Whatever the “power rating” for an appliance is, we all want to pay less for electricity as long as the job gets done, don’t we? Since most people pay for their electricity in kWh, it makes sense to compare “energy consumption” (kWh) rather than just “power rating” (kW).

✔️Compare energy consumption(kWh) rather than power rating(kW)

✔️ A lower "kW" device may not be more energy-efficient than a higher "kW" one. In a refrigerator, a lower "kW" model may have to run for a longer time to achieve the desired temperature as compare with a higher "kW" one.