kWh – Cost of Electric Energy

Sep 172022

Home / Instruments & Measurements /

What is kWh, kilowatt-hour?

The unit of measurement used in the electricity bill is the kWh, however in the electrical or electronic products that we buy in stores, the unit of electricity consumption is given in watts or kilowatts. This seems confusing, but we’ll clear it up below.

A kWh is the amount of power that was demanded in that 1 hour, but Energy is power x time, and energy has units in joules. In other words, the company that supplies us with electricity charges us for the joules we consume.

So energy = power x time

What is a Joule of energy?

A definition of joules from an electrical point of view is: 1 joule is the energy dissipated as heat when an electric current of 1 amp passes through a 1 ohm resistor in 1 second. This definition doesn’t look the same as the joules we discussed earlier, but they are the same.

Watts and Kilowatts

The wattage shown on the packaging of electrical or electronic products tells how many kilowatts or watts the item draws per second.

1 Watt = 1 joule/sec.
1 kW = 1000 joules/sec.
1 kWh = 1000 joules/sec. x 3600 sec.
1 kWh = 3,600,000 joules.

Kristoferb at English Wikipedia, CC BY-SA 3.0, via Wikimedia Commons

Examples of calculation of energy consumed in kWh

1 – We want to calculate how many kilowatt-hours were consumed by a 1.5 kW toaster for 2 hours.

We know that energy is equal to power x time, so: 1.5 kW x 2 h = 3 kWh.

2 – What happens if a 1 kilowatt heater only works for 20 seconds?

First, we convert seconds to hours:
20 sec x (1 min/60 sec) x (1 hr /60 min) = 0.0056 hr.

Now we calculate the energy
Since energy = power x time: 1 kW x 0.0056 h = 0.0056 kWh.

3 – If we have a 100 watt light bulb that is working for 8 hours, its consumption would be:

First, it is necessary to convert from watts to kilowatts:
(100 watts) x (1kW/1000 watts) = 0.1 kW.

Using the formula energy = power x time: 0.1kW x 8 h = 0.8 kWh.

The cost of electricity

If we now multiply this consumption in kilowatt-hours (kWh) by the price of each kWh, we obtain the amount that must be paid to the company that provides us with the electricity.