Making Sense of Watts and Watt-Hours

One of the hardest concepts to grasp related to energy savings is what is the difference between a watt (W) and a watt-hour (Wh).  If you are going to try to understand your electricity consumption and savings, it is very important to get this key concept.  Unfortunately, we can’t see electricity flowing through a wire and a lot of people misuse these units making things even more confusing.

What's a Watt demo at Museum of Science

My son showing us what a watt of electricity is equivalent to at the Museum of Science

In technical terms, watt (W) is the measure of power, or the rate at which you do work measured in J/s (that is Joules per second) and watt-hour (Wh) is the measure of energy, or the potential to do work.  Don’t worry if this didn’t help, because I am going to make it more tangible in a moment.

The light bulb example

Light bulbs are usually referred to by their wattage.  For example, a 60W light bulb when on will consume 60 watts of electricity.  The rate at which the light bulb consumes the electricity is the power.  If you leave that light bulb on for 2 hours, then the light bulb will have consumed 120 watt-hours (2 hours X 60 watts).  That is the measure of the energy consumed to do the work of lighting up the light bulb.

You are probably most familiar with electrical energy in the form of kilowatt hours (kWh) which is the unit you pay for on your electricity bill.  A kilowatt hour is simply one thousand watt hours.  The average home in the US consumes 958 kWh per month.

Now lets look at something similar that you can see and feel – water.

Understanding electricity through water

Let’s draw a parallel between electricity use and water that you can feel and see.  Below is a fun demonstration I saw this weekend with my son at the Museum of Science in Boston, but let me set the stage.

The amount of water contained in a bucket (tube) in gallons in this example can be through of the amount of electrical energy or watt-hours available.   The speed (rate) at which the waters flows out of the tube is the power or rate of electricity consumption.  In this example we consider the tube when full as having 10 Wh of energy.  We have two loads to experiment with: an electric mixer rated at 250 W and a hair dryer rated at 1,000 W.

When we turn on the hair dryer, notice how fast the water drains out of the tube.  That is the power (or flow of energy) in this case.  Doing a little math, we can calculate that at 1,000 W the hair dryer will use up 10 Wh of energy in 26 seconds.

10 Wh = 3,600 sec/hr X 10 Wh = 36,000 Ws
36,000 Ws / 1,000W = 36 sec

If we use the electric mixer instead, it uses one quarter the electricity as the hair dryer and would take over 2 minutes to drain the 10 Wh from the tube.  Now enjoy the short video and feel free to ask any further questions you have in the comments.

If you enjoyed that, you’ll should check out our previous post What Impact Does 1 Watt Have?

Happy Greening!

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