What is a kWh? (part 2)

In part 1, we went over the very basics of the kWh, but in part 2 we'll dig down a little deeper.

It's hard for many people to understand why you need to multiply Watts (instantaneous value of power) by a unit of time (in our case our unit of time is the hour. The minute or second are other units of time) to get total energy. The technical answer is that one way to represent Energy is by the Joule, just like one way to measure Distance is by the Meter. Taking this a step further, one way to represent Power is the Watt. A Watt is equivalent to a Joule/Second. If you need to get total Joules (which is energy). You need to multiply the Watt (Joule/Second) by total number of Seconds an appliance is in use, which gives you total Joules!

So why is your power bill in kWh? Well, it could just as easily be in Joules, or Wh, or kWs (kilowatt seconds), but kWh is the convention power companies adopted. Just like car companies (in the US at least) adopted miles per hour for the speedometer. They could have just as easily adopted feet per second for your speedometer. So the speed limit on the interstate could be 110 Feet/Second instead of 75 Miles/Hr!

If you have a kill-a-watt, you'd be able to see that your flat panel TV probably consumes somewhere between 213 Watts (LCD) and 339 Watt (plasma). If you want to figure out how much energy your plasma TV will consume if you left it on for 30 minutes you'd have to do a few simple calculations.

The average plasma TV consumes 339 Watts, but we'll uses 340. If the TV is on for 30 minutes, that equals one-half of an hour (.5 hrs.). Therefore, the total amount of energy you have consumed is:

Total Energy (kWh) = (340 Watts)*(.5 hrs) = 170 Wh = .170 kWh

Homer at Work

If you live in the South, you probably pay somewhere around 9 cents/kWh. To figure out how much 30 minutes of "The Simpsons" costs you, just multiply the total energy the TV consumed (.170 kWh) by your electric rate ($.09/kWh) to get $ .0153. So about 1.5 cents! I'd say laughter is worth 1.5 cents! The TV is a relatively small consumer of power compared to your Heating/AC, water heating, washer/dryer, or your total lighting energy consumption. Now that you are armed with knowledge about the kWh, look at your big consumers of power, and figure out how to use them less (lowering the time part of the "Total Energy" equation)!

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Comments

Anonymous (not verified) Wed, 09/23/2009 - 19:45

I'm in the process of building a wood gas generator to supply fuel to a V6 engine which I plan on driving multiple 7200W belt driven generators. I have access to an overabundance of storm felled tree's including mountains of wood chips over 50ft high. I also plan on capturing the extra heat generated to heat water for bathing and radiant floor heat.

Anonymous (not verified) Tue, 09/22/2009 - 20:19

My electric bill averages 1100kWh's a month, if I could generate 5 kW for 220 hours a month and had the set up to sell my power back to utility would I have zero usage?

Anonymous (not verified) Tue, 09/22/2009 - 20:56

Rick, You are exactly right. You could also generate 10 kw for 110 hours. The only thing is that would be pretty difficult to do. If you had a solar array, it would probably only average 5 hours of fully rated production a day. i.e. a 5 kW array would average 25 kWh a day (5 kw * 5 hrs). A wind turbine would need to be in a very windy spot. So your thinking is right, implementation would be difficult. Do you have a specific application?

Anonymous (not verified) Sun, 07/05/2009 - 20:33

Im looking into buying solar panels and it just says like "224watts" and I don't know what that means. could u give me a ratio easy to understand. Thanks a lot for your help.

Anonymous (not verified) Mon, 07/06/2009 - 04:44

Christian, Have you read this blog of mine? http://mapawatt.com/2009/05/08/calculating-solar-energy-output/ How many of the 224 Watt panels do you plan on installing? In other words, what is your total system size?

Anonymous (not verified) Mon, 05/04/2009 - 12:16

Hey thanks for the comment on Energy Circle; just came by to check out your kw/kwh analogies to see if you one-upped my "what is a watt?" explanation and I think the Shrek characters take the cake. You're right though that it's a tough issue for people to get - I've seen some people who should have a pretty good grasp on this stuff say kilowatt when they meant kilowatt-hour, etc., and I do think understanding is a pretty necessary first step towards conservation. Good work, good site, keep it up.

Anonymous (not verified) Mon, 05/04/2009 - 12:23

I get just as frustrated when people say "kW" when they mean "kWh"! I think most of the time is that even they dont realize the difference. I think the only thing more frustrating is when people think Oil price fluctuations affect electricity prices (they might a tiny little bit, but most people don't realize we only generate about 2% of the electricity in this country from oil)!

Anonymous (not verified) Wed, 03/25/2009 - 08:46

As much as I enjoy knowing how much a device uses in electricity, I don't think it's integral to saving electricity. The saving opportunity is triggered by the amount of time something draws power. What you want to change is the time. Unplug it, turn it off, put it on a Smart Strip, a timer etc. There is very little opportunity to reduce the power used while it's on except for lightbulb changes and even then you need to focus on lights that are on for more than just minutes. Time is the key to savings...

Anonymous (not verified) Wed, 03/25/2009 - 15:12

Mark, essentially you are right. The time something is on usually has the most impact on energy because for the most part, the devices we have in our house are not changing. However, there are times when we may have inefficient appliances that could be replaced with more efficient ones. You mentioned light bulb, but another one that comes to mind is the refrigerator. With your fridge, you really can't influence the time it stays on (because it always is), but you can make sure you have an efficient one. This brings about the difference in Efficiency and Conservation. Conservation is NOT using something (turning it off), while Efficiency is buying devices that do the same amount of work with less electricity (CFLs, more efficient fridge). Thanks for the comment!

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