Blog Post

What is a Watt, Anyway? Understanding Energy and Power Metrics

April 20, 2011

It's easy to get confused about the difference between energy and power, between watts and watt-hours.  But if you can master inches and pounds, you can master this.

How many mystery writers does it take to change a 60-watt lightbulb?

Two--one to screw the bulb almost all the way in, and one to provide a surprising twist at the end.

How many Energy Solutions columnists does it take to change a 60-watt lightbulb?

One--all he does is tell you what a watt is and he doesn't even change the lightbulb.

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If you are still with me, dear reader, you have chosen the columnist over the mystery writer. You are brave. (Or if you are just into lightbulb jokes, see my green building lightbulb jokes here.)

Chances are, you may be worried about higher energy prices, global warming, energy insecurity, or all of the above and more. You may want to become more aware of your energy use, and become more efficient. A few weeks ago I wrote about radiation terminology; today I'm going to focus on energy terminology. Knowing is half the battle.

Watts are like miles-per-hour

Let's start with that 60-watt lightbulb. Power is a measure of the rate at which energy flows, and in electrical systems it is measured in watts (W). Watts are basically the miles-per-hour measurement of the electrical world--they tell you how fast the electrons are speeding down the highway. For those who are keeping track, one watt is equivalent to electricity flowing at a rate of one joule per second in the metric system, which is also equivalent to 3.4 Btus per hour.

A 60-watt lightbulb will consume electricity at a rate of 60 watts. A laborer working through the day will put out 75 watts of power. A medium-sized car might consume 100,000 watts. (One horsepower is equivalent to 750 watts, so that's a 286-hp car.) A small gasoline generator puts out 2,000 watts; the Vermont Yankee nuclear power plant puts out 650 megawatts, or 650,000,000 watts. Many other pieces of equipment come with power ratings to describe the rate at which they use energy.

Watts measure power--kilowatt-hours measure energy

When you get your utility bill, the electricity you've used is measured in kilowatt -hours (kWh). While a watt is a measure of power, a kWh is a measure of energy. Energy is defined as the capacity to do work, such as creating heat, light, or motion. If you run a 60-watt lightbulb for one hour, you've used 60 watt-hours, or 0.06 kilowatt-hours, since a kWh is 1,000 watt-hours. In other words, 0.06 kWh is the amount of energy you need to run a lightbulb for an hour.

Homes are typically charged only for the electricity they use, measured in kWh. But commercial and industrial facilities also pay "demand charges," which are calculated based on their peak power draw (usually measured in megawatts, or MW), which compensates the electric utility for ensuring that it has enough power available to meet that demand.

Appliances are rated based on power

Boilers and furnaces are also sized based on their heating power, in Btus per hour in the U.S., and in kilowatts elsewhere. A typical residential unit puts out 100,000 Btu/hr (29 kW) while commercial units tend to be much more powerful. A 100,000 Btu/hr boiler burning at full power for a day will produce 2.4 million Btus of heat (700 kWh).

How much energy does your entire home or workplace use? Overall energy consumption of buildings, including both electricity and other fuels, is typically counted in million Btus per year (which is usually abbreviated as MMBtu--don't ask).

To get a single whole-building MMBtu number, we have to convert all fuel sources into that unit, and then add them up. You can find conversion ratios for doing this online, which include all fuel sources you might use, such as propane, cordwood, natural gas, coal, and others. The Home Energy Yardstick from the federal Energy Star program makes this really easy--Google it.

Comparing from one building to another

In order to compare energy use from one building to another, we typically normalize it by the building's floor area, giving us energy numbers in thousand Btus per square foot per year (kBtu/ft2·yr). The average onsite energy use for office buildings in the U.S. is 76.3 kBtu/ft2·yr. The average for single-family detached homes is 43.8 kBtu/ft2·yr. (For multi-family homes of five-plus units, it's 49.5; for mobile homes it's 73.4 kBtu/ft2·yr. If these numbers look surprisingly high compared with single-family homes, keep in mind that we're talking per-square-foot, not per-home.

If all this feels intimidating, remember that (if you grew up in the U.S.) you've managed to master the arbitrary system of inches, feet, and yards. These energy metrics are much simpler!

Getting to know metrics hands-on

I got to know some of these metrics hands-on after I installed our solar photovoltaic power system. The system includes a charge controller which takes in power from the solar panels and feeds it to the batteries for storage. The controller includes a digital readout telling me exactly what the system is producing at any given time. I have 1,050 watts of panels, which means that under optimal conditions they are rated to produce that much power.

In reality, the power on the readout is constantly changing as the sun and temperature conditions change. Solar panels like it cool, so on a nice cool, sunny April day, I might get a peak reading of 1,365 watts. Right now, with some clouds dancing across the sky, it's reading 723 watts.

As you know, watts measure rate. When you see a cop on the Interstate and take your foot off the gas, you might drop from 70 mph to 65 mph in a few seconds. The same thing with the solar panels and sun. How far you have driven at the end of the day is determined by your average speed, and how long you drove. As I write this, my solar array has been online for 6:01, and has produced 2.13 kWh in that time. That's about enough power to have done a load of dishes in my dishwasher.

Is replacing windows a waste of money?

Dear Energy Solutions, I have heard that replacing windows is a waste of money. Is that really true? – Paula

Dear Paula, A lot of people have the idea that replacing old windows is one of the first things they should do in an energy renovation of an old building. There are a lot of good reasons to replace old windows, and not all of them are about energy. These include aesthetics, maintenance issues, and comfort. (Yes, I consider comfort and energy to be different considerations. It can be very uncomfortable to sit next to an older single- or double-pane window in the winter, but in absolute terms, that window may not be costing you a ton of energy.) Any of these factors, along with overall energy use, may point you toward window replacement.

I would slow down and look at other options, however. Analysis of "payback" is tricky, but there are credible calculations showing payback, based on energy saved, for window replacement, of 10–40 years. For most people, that's a long time. There may be other measures, such as sealing up air leaks, that will improve your comfort and finances much faster. Rehabbing older windows, or simply adding storm windows, can also be very cost-effective, with paybacks of under 10 or even five years.

What are your thoughts, questions, or comments on energy metrics, operating a solar array, and replacing windows? Please discuss below.

Tristan Roberts is Editorial Director at BuildingGreen, Inc., in Brattleboro, Vermont, which publishes information on green building solutions. Read more Energy Solutions columns, including columns by Alex Wilson, for whom Tristan is filling in, on the Energy Solutions homepage. You can also keep up with Alex's adventures on sabbatical at ATWilson.com.

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Comments

April 25, 2011 - 10:50 am

Thanks for the catch! I have changed it to kBtu in the text.

April 25, 2011 - 10:39 am

"The average onsite energy use for office buildings in the U.S. is 76.3 kBtu/ft2·yr. The average for single-family detached homes is 43.8 kWh/ft2·yr. (For multi-family homes of five-plus units, it's 49.5; for mobile homes it's 73.4 kWh/ft2·yr."

Should these all be in kBtu/ft2·y, or...?