Posts Tagged ‘power


It seems that the two terms energy and power are often confusing for many people and as a result there could be misunderstandings regarding the energy consumed for a given period of time as well as how much energy is consumed by a device. Let us say that you have a device rated at 100W of power required for it to operate, for example a 100W light bulb that will run at this power level constantly in order to give light. But how much energy will the light bulb consume, that depends entirely on the time that you have it turned on it is turning energy to light. If you leave the light bulb running for 1 hour, then it would have consumed 100 Wh of energy and you would need to pay for that energy used. But if you leave the light bulb turned on for just 30 minutes it would have used just 50 Wh, or for 2 hours the energy used will be 200 Wh.

Do you get the difference between energy and power now? The power rating of an item describes how much power the device needs in order to operate, but the energy used by it will depend on the time that you have the device operating. Since we are normally paying per 1 kWh used energy then you can easily do the math for the energy consumption of a device when you know its power rating:

Device power rating * Time in hours the device is operating = Energy used that you need to pay for

So if we turn back to the example with the 100W light bulb we can get:

100W * 0.5 hour = 0.05 kWh (or 50 Wh)
100W * 1 hour = 0.1 kWh (or 100 Wh)
100W * 2 hours = 0.2 kWh (or 200 Wh)

Now if we multiply the result we got for the energy used by the rate for 1 kWh of energy we get what we need to pay for:

0.2 kWh * $0.15 USD = $0.03 USD

So that little example in the real world would have cost us just 3 cents if we have a rate of $0.15 USD per kilowatt hour of energy used.