How to calculate kilowatts, kilowatt-hours and electricity consumption?

Heating Trends, last updated on November 14, 2022

A kilowatt-hour (kWh) is a unit of measurement used by companies that supply users with electricity to express the amount of electricity consumed during a certain period.


How to calculate kilowatts, kilowatt-hours and electricity consumption?


Before we start with how to calculate electricity consumption, lets first explain some basic terminology. The power of an electric device is expressed in watts (W) which is a unit of power, corresponding to the rate of consumption of energy in an electric circuit. Power gives us an indication of the rate at which the device will consume energy. For example, the power of an LED TV ranges from 50 to 200 watts.

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Watts vs. Kilowatts

The basic unit of power is the watt (W), and 1000 watts is equivalent to 1 kilowatt (kW). 1 kW (kilowatt) is often used as a unit of measurement because most electrical appliances consume 1000 W or more. The same thing goes for some other metric units, it's much easier to express weight in kilograms than in grams, and distance in kilometers than in meters.

1 kW = 1000 W


The next important factor in the method of calculating electricity consumption is working hours. Operating hours are the number of operating hours of the device. A kilowatt-hour (kWh) is a unit of energy a certain electrical device needs to run for one hour, and it is used by companies that supply users with electricity to express the amount of electricity consumed during a certain period.

The following formula can be universally used for electricity consumption:

Power of electricity (kW) x consumption time = final consumption of electricity (kWh)

This is the unit we want to use to calculate what the electricity supplier charges us for every kilowatt hour of electricity consumed by the household.

How to calculate electricity consumption?

To calculate the energy consumption of any device, we need to multiply its power by the number of hours it is used (operating hours).

For example, a 1000 watt electric iron that runs for one hour will use 1 kilowatt-hour (kWh) of electricity. Similarly, to calculate monthly energy consumption simply multiply daily energy consumption by 30 days, and for annual energy consumption multiply daily energy consumption by 365 days.

1 kW (1000 W) consumed during 10 hours without interruption leads to a daily consumption of 10 kWh. In monthly terms, this would mean a consumption of 300 kWh. This would be equivalent to keeping 10 100 watt light bulbs on for 10 hours a day.

Here's an example of electricity consumption for 4 LED light bulbs used 8 hours a day:

4 10 W LED light Bulbs x 8 hours / day = 0,04 kW * 8 h = 0,32 kWh / day

Every electrical device we use has a technical specification on the bottom or on the back, on that label you will find the power of the device.

For a better illustration of the above, lets take a look at a table showing average energy consumption by some common household appliances.

Appliance Power (kW) Daily usage (h) Monthly usage (h) Total energy consumed
Electric stove 1,5 kW 3 h 90 h 135 kWh
Light bulb 0,06 kW 8 h 240 h 14,4 kWh
Electric oven 2,4 kW 1 h 30 h 72 kWh
Microwave 1,3 kW 2 h 60 h 78 kWh
Refrigerator 0,7 kW 24 h 720 h 504 kWh
Washing machine 3 kW 1 h 30 h 90 kWh
Dishwasher 3 kW 1 h 30 h 90 kWh
TV 0,01 kW 8 h 240 h 2,4 kWh
Total 985,8 kWh

In the example shown in the table, the total electricity consumption is the sum of all individual consumptions per household appliance. In the case of using more than one light bulb in the household (which is almost always the case), we need to multiply the total consumed power of a single lightbulbs with the number of all used light bulbs.

Calculating energy consumption of a space heater

Suppose that your space heater consumes exactly as much as it says on the declaration - 2 kW (2000 W). If you leave a 2 kW heater running at full intensity continuously for an hour, it would consume exactly 2 kWh, but in reality it consumes roughly 1 kWh. This is due to the fact that space heaters turn off once the set temperature is achieved or they turn off to prevent overheating.

This means that one space heater consumes 1 kW of energy per hour, or 1 kWh.

Household appliances energy consumption

Some appliances that are constantly on (for example, a refrigerator) don't necessarily lead to the highest consumption, because they effectively operate just a few hours a day. For instance, most average refrigerators run for about 8 hours per day because the compressor is turned on when a thermostat registers a temperature change.

However, some devices lead to high consumption (for example, a washing machine), and they do not have to be in constant use. In this case, you can save money by using a lower daily rate. This will not reduce consumption in quantity, but given that companies define daily periods in which the unit price of electricity per kilowatt hour is lower, it is a very convenient way of saving money.