Sun, 12/02/2012 - 22:21 -- Chris

This article answers the question "What is a kilowatt hour (kWh)?"

The kilowatt hour (or kWh or kilowatt-hour) is a unit of measure for energy and is one of the most important concepts to understanding the basics of energy. The kWh is the unit of energy used by electric utilities to measure how much electrical energy your home consumes. This is measured by the electric meter on the outside of your home. The main focus in explaining what a kWh is will be on the necessity to multiply power (watts) by time (hours) in the residential setting to get total amount of energy consumed, which is expressed in kilowatt-hours (kWh). The article only addresses kWh in a purely resistive circuit.

If you just want to see an analogy of how electricity and cumulative electrical energy is like a water flow rate and cumulative amount of water just skip to the bottom of the post for the analogy.

The kWh is made up of two components:

The utility charges you based on how much electrical energy the electronic devices in your home consume. This is determined by multiplying the real power an electronic device uses in Watts, W, by the amount of time that device is on, in hours, h. The total electrical energy is then given in watt-hours, Wh:

But the utility records your electrical consumption in kilowatt-hours (kWh), not watt-hours (Wh). Remember that 1,000 watts (W) = 1 kilowatt (kW). Another way of saying this is that there are 1,000 watts per 1 kilowatt. The equation below shows how you calculate kilowatt-hours. Notice how you divide by 1,000 to convert the answer from watt-hours to kilowatt-hours:

To recap, the watt (Power) is an instantaneous measure of electricity. The kWh (Energy) is the cumulative amount of power, or the sum total, of electricity consumption over time. To get a cumulative total of energy (kWh) from an instantaneous value (watt) you need to multiply it by time (hour) and divide by 1,000. It is necessary to divide the product of power and time by 1,000 in order to convert your answer from watt-hour to kiloWatt-hour. The utility chooses the kiloWatt-hour instead of the watt-hour simply because the size of the number looks more manageable and easy to understand. For example, 340.7 kWh looks easier to work with than 340, 695 Wh!

For some, the most confusing aspect of the kWh is the combination of power with time. In order to calculate energy (kWh), the power an appliance uses has to be known and the duration of time the appliance is on has to be provided for the time period that you want to find the cumulative energy for. That is confusing, so let's look at an example.

Let's say you have a 100-watt light bulb. This means that when you flip the light switch, that light bulb will start using 100 watts. It uses 100 watts if it is on for 1 second, 1 minute, 5 minutes, 7 hours, etc. The wattage is an instantaneous value, so whenever that bulb is on, it will be using 100 watts (unless it is on a dimmer, then it will use lower watts if you dim the the light). But this instantaneous measure of power tells you nothing about the cumulative amount of energy (kWh) the light bulb uses! You know that the light bulb is going to use more energy if it is on for 1 day than if it was on for 1 minute. So to calculate this total amount of energy (kWh) you have to take the power (watts) and multiply it by the time the light bulb is using that power (a.k.a. light turned on). So if you want to know how much energy the light consumes in 1 minute:

That isn't that much energy as it relates to your utility bill. If you paid 10 cents per kWh, you wouldn't even be close to a penny! But let's say you wanted to calculate how much energy (kWh) the bulb consumes in 1 day (if left on for the whole day):

If you paid 10 cents per kWh, then leaving a 100 watt bulb on for 1 day would cost you $0.24. (10 cents/kWh * 2.4 kWh) Almost a quarter. This doesn't sound like a lot, but think about how many lights and other electrical devices are in your home!


Analogy: When water flows out of a pipe the flow rate is usually given (in the U.S.) as gallons per minute, or GPM. The flow rate is how much water is flowing past a point per a measure of time. The amount of water is given in gallons, and the measure of time is given in minutes. Water flowing through a pipe is often used as an analogy for electricity flowing in a wire. Just like GPM is a flow rate for water, watts can be thought of as the flow rate of electrons (the watt is equal to a joule per second). If you want to know how much water flows through a pipe in a certain amount of time, you would multiply the flow rate (Gallons per minute - GPM) by the amount of time you care about (in minutes). This would give you the total amount of water (in Gallons). Likewise, if you want to know how much energy flows through a wire in a certain amount of time, you would multiply the energy rate - a.k.a. power - (watts) by the amount of time you care about (in hours). This would give you the total amount of energy in watt-hours (you then divide by 1000 to get kWh).

This page expands on our original post, "What is a kilowatt hour (kwh)"


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