Of Watts and Watt-Hours

This post has nothing to do with gardening – but it does have a great deal to do with sustainability and environmental awareness. It follows up to Erica’s experimentation with the Kill-A-Watt with a (brief) engineering lesson. Please, don’t be scared. I studied engineering for three years before realizing I hated it switching to English – so pour yourself a homebrew and take a moment to contemplate the wonderful, confusing, and absolutely critical world of energy and power.

Join me on a trek into the world of Watts and Kilowatts, of Watt-hours and Kilowatt-hours. These terms are easy terms to confuse, since they are usually bandied about without explanation, but they aren’t just semantics or industrial jingoism. They are very different things – and confusing them puts you at risk for brown outs of your off-grid micro-hydro system. No micro-hydro? Well then, this is still good stuff to know so you can understand why your electricity bill reads the way it does.

Grab that drink and let’s go.

Watts are a measure of power. Watt-hours are a measure of energy. The easy analogy for power vs. energy (Watts vs. Watt-hours) is to compare them with speed vs. distance (miles-per-hour vs. miles). Speed is an immediate measure – how fast are you going at a given instant. Distance is the cumulative effect of speed acting over a period of time.
Similarly, Watts measure the immediate rate, or speed, at which energy is converted from one form into another. We call this rate power. Watt-hours measure the cumulative effort of power acting over time. We call this cumulative effort energy.
Just like we know that a 60 mph speed maintained for 1 hour will give us a distance traveled of 60 miles, we know that a 60 watt power draw maintained for 1 hour will give us an energy use of 60 Watt-hours.
Let’s get into a bit more detail:

Watts (Power)

Power, measured in Watts, is an instantaneous thing. Your car produces power when it converts the potential chemical energy in gasoline (or biodiesel for you biodiesel types, or batteries for you Leaf types) into the rotary motion of the engine’s driveshaft.

Your toaster consumes power when it converts the kinetic energy of electrons wiggling back and forth in a copper wire into heat to warm your gluten-free breakfast bagel.

Notice that I spoke of the car “producing” energy and the toaster of “consuming” energy. These are misnomers. The car consumes energy (potential chemical energy in fuel) but also produces energy (forward motion as well as wasted heat and sound). The toaster consumes energy from the grid but produces it in the new form of heat to toast that tasty bagel. It all depends on which side of the equation you are looking at – and just remember that power is the rate at which energy is converted from one form into another.

We tend to speak of “consume” or “demand” or “draw” in electrical applications because the supply system (wall socket, micro-hydro plant, or exercise bicycle attached to converted washing machine motor) is a limiting factor. Ask too much of your supply and it’ll either fail (blow a breaker, overheat, etc.) or just max out and stop giving you more than it can (brownout). Similarly we tend to speak of “produce” or “deliver” in applications where we are interested in the amount of work something can do. Cars, Cuisinarts, and stereo amplifiers are all rated in their power output (horsepower is also a measure of power).

Watt-Hours (Energy)

Energy, measured in Watt-hours, is a stockpile. That’s the key right there. To be less glib, energy is the cumulative effort of power acting over time. That’s why we express it in Watt-hours or Wh. A Watt-hour is one Watt of power acting for one hour. 60 Watt-hours could be a 60 Watt draw (e.g. an incandescent light bulb) running for one hour or a 1 Watt zombie load remaining on for 60 hours.

Energy can also be stored as the potential to deliver power later on. Anything that “holds” energy has its capacity measured in Watt-hours as well. One gallon of gasoline contains about 35,000 Wh (35 Kilowatt Hours or kWh). The battery in my iPad contains 25 Wh. (It gets tricky with batteries because chemical inefficiencies mean their effective energy capacity is lower at high discharge rates.) The battery in a Prius holds 1.3 kWh at a nominal discharge rate.

Those of us interested in conserving energy are primarily interested in Watt-hours or Kilowatt-hours. Sometimes we do this by reducing the power consumed by the tools we use: replacing 60 Watt incandescent light bulbs with 12 Watt compact fluorescents would save 48 Watt-hours per light for every hour we leave those lights on. Replacing a 17″ CRT computer monitor (80 Watts when in use) with an LCD (25 Watts) saves 55 Watt-hours for every hour we are working – and switching the whole thing to a laptop would save even more.

We can also conserve energy by reducing the number of hours over which we draw power. This reduces our total Watt-hours. The simplest energy-reduction trick is possibly the oldest: turn things off. Switching the lights off when you leave the room reduces their power draw to zero. This works great in infrequently inhabited but well lit rooms like kitchens and bathrooms. My bathroom is lit by a gothic array of eight 60 Watt bulbs (we have not yet converted to CF’s in this room because of the ugliness problem that their spectral content causes…but that’s another topic). Switching them off for ten minutes would save 80 Watt-hours.

By the way, my strange combination of anal-retentive English student and anal-retentive engineer makes me get very twitchy when people speak of saving “power.” You are not saving power. You might reduce the amount of power you use, but the end result is to save energy!

So there you have it – a quick engineering vocabulary lesson. Now there’s no reason to confuse Watts and Watt-hours. I’ve kept this simple, by the way, ignoring issues of reactive power and a few other complications, so please don’t actually wire an off-grid power system based on this post and then get angry when your refrigerator turns on and blows out the inverter. AC power has a lot of subtlety (and a surprising number of sines and cosines) that can trip up the unwary.

A Sense of Scale

So how do Watts and Watt-hours play into our lives?  Well, the average American home consumes something like 10,000 kWh (Kilowatt-hours) per year. That’s 10,000,000 Watt-hours or the equivalent of leaving a solitary 60 watt bulb on for 166,666 hours. Since there are only 8760 hours in a year, that’s quite a trick. How do we manage to consume so much energy?

I’ve done something unusual here – I’ve normalized a bunch of typical household appliances against a 60 Watt incandescent light bulb running for one hour.  In other words, this list compares how long you can run each load to consume the same amount of energy as running a 60W light for one hour.

One 60W light bulb run for 1 hour consumes the same energy as …

A 3W iPad run for 20 hours
A10W WiFi base station & router run for 6 hours
A 12W compact florescent run for 5 hours
A 15W laptop computer run for 4 hours
A 30W HD TV recorder (e.g. TiVo) run for 2 hours
A 60W 32″ LCD TV run for 1 hour
A 120W 55″ LED TV run for 30 minutes
A 190W XBox 360 run for 19 minutes
A 250W desktop computer run for 14 1/2 minutes

A 350W freezer run for 10 minutes
A 360W 50″ plasma TV run for 10 minutes
A 450W refrigerator run for 8 minutes
A 600W vacuum cleaner (about .8 horsepower) run for 6 minutes
A 1200W microwave run for 3 minutes
A 1500W blow drier run for 2 1/2 minutes

A 3600W electric oven run for 1 minute

Note that there are error bars here – your mileage may vary.  Some of the values are worst case while others are more typical usage.  For things-with-switches, this value reflects the device in use.  Some many have a small “zombie” load (usually 1-5W) for clocks and controllers, but many will have none.

And some devices (e.g. refrigerator) operate on a duty cycle and will have a very small draw when plugged in (1-5W based on my research) and only hit their peak use when the compressor(s) and fan(s) are running.

Putting this list together taught me some interesting things.  First off, I tend to freak out when I see lights left on. I do a big song and dance routine as I go through my morning evolutions, switching lights on and off as I need them.  But I don’t hesitate to bust out the Dyson at the end of the day.  20 minutes of vacuuming consumes the same energy as leaving three light bulbs on for an hour.

Also, anything heat producing is a huge energy draw – though often this is mitigated by some duty cycle factors. Most counter top appliances are in the 600-1200W range.  Don’t leave your waffle iron on.

FYI, the power consumption figures are based on my own experience (using the Kill-A-Watt and looking at battery lifespans on my electronics) and some Internet research, primarily from these sites:



  1. Anonymous says

    good post ! ! i am an engineer (sin exceptional english skills), with a growing interest in living as much off-grid as i can – yet in a comfortable manner. i found your blog by way of blog-to-blog jumping . . . i'm not even sure i could recreate the steps. your engineering training definitely shows – not just by way of your math, but your choice of terms, too (i.e. – normalized, error bars). i bookmarked your site and will check back from time to time. cheers, tim

  2. says

    Wow, being a business major I never knew that. Thanks! I love dumbed down explanations of complex concepts. Now I know to be more mad at myself to leave the oven on (even though it's gas) than leaving a CFL on all day. Good to know.

  3. says

    So, if I read this right, I should quit feeling guilty about my dog-fur covered, dusty carpet because I'm saving so much energy by not vacuuming it regularly. Woohoo! Dirty houses rule!

    The bathroom fixtures in this house hold four 60W globes. We unscrewed all but one in each bathroom and have been fine without having the lighting needed for a star's dressing room. If and when those bulbs ever burn out, I've got replacement lower watt bulbs to put in.

    Oh, and before you point it out, I do know to dust off the lightbulbs regularly since dust absorbs light. ;-)

  4. elena says

    very helpful, especially for the practical clarification, thanks!

    would you provide also some clarification on why the iPad has a 25 Wh battery and an iPhone a 1,4 Ah one? which is the relationship among these units of measurement? thanks in advance

    • Just Nick says

      Thanks, Elena, I’m glad it was helpful. Now on to your question (which is a really excellent question, by the way).

      Watt-hours and Amp-hours are very much related to each other. To get the former from the latter, just multiply by the average voltage that the battery is providing during discharge. A bit of online research shows that the iPhone uses a 3.7 Volt battery. So 1.4 Ah * 3.7 V gives about 5.2 Wh (which squares with some other numbers I’d found online).

      As for why one is specified one way and the other the other…who knows! Probably an artifact of Apple’s marketing copywriters choosing one for one product sheet and another for another product sheet.

      Honestly, though, discussing the capacity of batteries is a pretty fraught business – their total storage capacity is highly dependent on the rate of discharge. You can see this if you look at specification sheets for big batteries like boats or off-grid power systems use. A Trojan 8D-AGM (just for comparison, this is a 180 lb battery) has a capacity of 254 Ah if discharged nice and slowly over 100 hours but only 179 Ah if discharged in 5 hours. There are also issues of deciding what means that the battery is “empty” – because you might still have potential power stored in the battery but insufficient voltage delivered to do anything useful. Fortunately there are industry standards that are (usually) followed in specifying this stuff.

      Anyway, thanks for the question – I hope I didn’t just overload you!

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