Energy and Power and Confusion and Consternation

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energy power units james watt

I like James Watt as much as the next person. He was one of the people whose inventions helped advance the Industrial Revolution with his refining of the steam engine. He invented an early copying machine. And he was Scottish. (Now you'll have to read this whole article in the mellifluous accent of Fiona Ritchie.) I feel closer to my Irish roots than the Scots-Irish, but a man named Allison can never escape his Scottish heritage.

Why should you care about James Watt? Because his name is now used for arguably the most confusing unit in our field, that's why. Let's work our way up to that, though, and start with energy. That's something that seems fairly obvious, right? It's what makes everything go. It's the stuff that makes things happen. It's in the gasoline we put in our cars, the food we eat, the natural gas that heats our homes, and the electricity that's allowing me to type this article in Tucker, Georgia so you can read it in Cincinnati or Santa Fe or Cork.

Actually, energy is a pretty abstract idea, and it took physicists a long time to figure it out. It's not like motion, which we can see, or force, which we can feel. And the fact that it appears in such different forms—chemical, mechanical, electrical, gravitational, and nuclear—made it all the more difficult to understand. (James Clerk Maxwell, another Scot, showed that electricity, magnetism, and light were all related and has a set of four famous equations named after him as a result.)

Now that we do understand it, though, we still have the problem of physicists. Yes, we physicists have a knack for making the ideas of physics more difficult than they need to be, and we've certainly done that with energy, the quantity, and power, the rate at which we use energy.

When you get your electric bill each month, you're not paying for power, even if some people call it a 'power bill.' You're paying for the amount of energy you used. Power is how fast your meter is spinning right now, how much energy you're using per second or per hour. It's the difference between a quantity (energy) and a rate (power).

Some units we use for this make perfect sense. For energy, we have: British Thermal Unit (Btu), calorie, and joule. For power, all we have to do is turn those energy units into rates by dividing by a time unit: Btu/hour, calories/minute, or joules/second.

All was right in the world of energy and power units until 1889, when the Second Congress of the British Association for the Advancement of Science adopted the watt as a unit for power. Not energy, mind you, where it would make more sense, but power, where it disguises itself by hiding the time unit. A watt equals a joule per second.

The result is that the energy unit has time in it:

Rate = Quantity / Time

Power = Energy / Time

Rearranging a bit:

Energy = Power x Time

So, if you have a 24 watt CFL (the replacement for the 100 watt incandescent light bulb), it uses 24 joules of energy for each second that it's turned on. You don't get billed for the rate at which you use energy, so you have to factor in the time. If you leave that light on for one hour, which is 3600 seconds, you'll pay for:

Energy = 24 joules/second x 3600 seconds = 86,400 joules

That's NOT 24 watts per hour, as some people like to say. It's 24 watt-hours, a quantity of energy used. Because the watt has time hidden inside itself, the time units can cancel out, leaving only energy. But the electric utilities decided to keep the confusion going and bill us for energy used in kilowatt-hours.

Energy = 24 watts x 1 hour = 24 watt-hours = 0.024 kilowatt-hours (kWh)


Tread carefully when you're dealing with watts. They're not watt they seem.



Portrait of James Watt from the Wikimedia Commons, freely licensed media file repository.


Paul McGovern
Jan 23 2012 - 8:58am

Well, I guess that explains that! 

Bob Seaton
Jan 23 2012 - 9:16am

Thanks so much for this article, this is a good deed and there'll never be world peace as long as power and energy continue to be confused.

M. Johnson
Jan 23 2012 - 9:30am

I worked for an electric utility for years, and it took several for me to understand and appreciate the difference between demand and energy. 
But it's worth it for understanding prices and cost. I tell people peak demand dictates how big the wires and machinery have to be, and energy pretty much tracks how much fuel we burn. A typical house in my region has monthly peak demand maybe 7,000 watts, with average demand maybe 2,000. The difference is of crucial importance to anyone wanting to live off the grid, or who wants a backup generator of appropriate size. And it represents a savings for people who generate collectively (usually via a utility) rather than individually. The reason is "diversity", the fact that people vary in their timing of peak demand, and therefore the utility can serve needs with far smaller generation than the sum of individual needs. People who want to be self sufficient often miss this point, initially. 
But if a group of people wanted to be collective but otherwise off the larger grid, they could attain most of those same savings. We could conceive of neighborhood generating utilities if we wanted to.

Allison Bailes
Jan 23 2012 - 9:49am

Paul M.: Exactly! 
Bob S.: Glad you liked it.  
M. Johnson: Connecting power with the size of the wires and energy as how much flows through them is a great way to illustrate the difference. Thanks! 

Jan 23 2012 - 9:59am

This is why utilities are wanting to move to Time Of Use rates. A TOU rate structure is excellent for reducing PEAK demand which is what costs the utility the most money. Buying power off the grid during peak times costs 5x what off peak power costs. 
The average consumer doesn't understand because the rates have been "averaged" for them for years. Now that smart meters are affordable and are being implemented times are changing...

Jan 23 2012 - 10:15am

I can't tell you how frustrated I get when I read an article on energy and they confuse kW with kWh! I tried to explain Watts in one of my earliest blog posts, and I really dont know if I did a good job or not (but I think my animation should have won a Webby) 
What is a Watt? 
Let me know what you think.

steven davis
Jan 23 2012 - 10:16am

comment by SoCal Edison to electricity consumer during the summer, "Watt's up Doc!"

Rob George
Jan 23 2012 - 11:59am

This is great! I understand this very well but there's something I have never really seen a good explanation of that a layman can understand. That is the concepts of kW and kWh as they relate to utility energy efficiency programs. For example, it has been determined that an AC tune-up program will result in a .6 kW savings and a 596 kWh savings (per system) that the utility can claim. Please explain the difference between the two values in this context and what they mean relative to 'energy savings'. 
Thank you very much! 

John Poole
Jan 23 2012 - 12:01pm

The real problem, Allison, is that the guys running the utilities are influenced by evil Maxwellian demons, who are known tricksters and reprobates! :-)

Jan 23 2012 - 1:40pm

Rob, presumably that means the tuneup will save 0.6kW (600W) of power while the AC is running (which seems a little odd, but ok) - and that power savings over time (which must be about 1000 hours (596/0.6)) will save 596 kWh of energy. 
Even my utility is confused. Their last blog post spoke of a 330 volt solar panel. They can't even keep voltage and power straight. ;)

David Butler
Jan 23 2012 - 3:16pm

Rob, utility sponsored AC tune-up programs are mostly interested in peak kW savings. This is because kW (demand) determines how much (inefficient) standby generation must be brought online to satisfy peak loads, which are often driven by AC. 
From the homeowner's perspective, saving 0.6 kW is meaningless since very few residential customers pay a demand charge. That's why kWh savings estimates must be reported. The utility could care less about reducing kWh, were it not for the utility commission requirements. 
Load diversity comes into play when considering the system-wide impact of a tune-up program. Interestingly, adding freon to an undercharged AC will actually increase its kW draw, but capacity will increase by a higher %, thus reducing run times (assuming the system isn't undersized). However, if you end up adding freon to 1,000 systems, the diversified load (kW) will drop because fewer systems will be running at any given time. 
The most effective way to retire inefficient generation capacity and postpone new capacity is to reduce peak kW, not reduce kWh. Efficiency improvements do both, which means the utility loses revenue. In the residential sector, the utility's and customer's objectives are somewhat at odds. If the goal is to reduce kW, then different rate structures and energy management strategies are warranted. Secretary Chou has signaled that the time has come for residential tariffs to better reflect a utility's fixed and variable costs. This means more time-of-use-billing, the addition of kW charges, and lower off-peak kWh charges. When (not if) that happens, traditional approaches to energy efficiency must be supplemented by advanced control strategies and energy/thermal storage technologies.

M. Johnson
Jan 23 2012 - 3:32pm

Although utilities are interested in KW and not KWH, it is not really easy to reduce the former and not the latter. I submit that by and large, KW and KWH track each other. This is 2nd hand opinion from a really smart guy who worked in the regulatory department of an electric utility... I don't know it firsthand but take it on faith when this guy believes it.

M. Johnson
Jan 23 2012 - 3:45pm

Also I must submit that TOU (time of use) rates are pretty unpopular when residential customers have the ability to choose. Is there any place in the union where they are welcomed by customers? Practically the only cases I have learned of, the rates were either forced upon the customers, or an experiment. Merely getting "smart meters" is not enough, somebody needs to get a PUC order to implement this -- if it will come about. 
Residential demand charges? Technically has some appeal but more or less unheard of in the USA. At one time Houston Lighting & Power was gung-ho to introduce KVA meters for residential (like KW but more technically correct) but their PUC disallowed the project. That was somewhat embarassing since the utility had already bought the first batch of meters, they had to eat the expense. 
I agree that TOU rates, demand meters, and the like are technically very interesting. I have no fear of such change personally, but objectively this is the last thing great majority of residential customers wants.

Jan 23 2012 - 3:57pm

TOU rates are are becoming the default for new accounts. If you don't specify fixed rates you get TOU automatically. We did TOU last summer and it worked well for us. This summer we're going even more aggressive to variable peak pricing where the peak rate varies from day to day depending on anticipated demand.

David Butler
Jan 23 2012 - 4:09pm

Mark, while I agree with your smart guys' assessment that load shaping is not easy, that's a short-sighted perspective. The technology exists, it's just a mater of price signals and economies of scale.  
Many C&I; facilities already practice load management since demand charges may represent as much as half of their overall electric bill.  
Optional real-time and demand-based tariffs for residential have predictably been unsuccessful. The problem is that participants must opt-in. This means most participants who self-select will already have favorable a load profile, so further changes under the tariff will be minimal. This will change when residential tariffs are brought in line with a utility's fixed and variable costs. Today, politics not technology is the main impediment.

M. Johnson
Jan 23 2012 - 4:35pm

I am encouraged by the OGE presentation of TOU rates. There is an element of trying to be attractive to the customer, which is so often not seen in Texas. 
Since customer choice has been enacted in Texas, rates have gotten simpler not more complex. Since a TOU rate is inherently complex, it certainly appears to me the customer must be enticed to use it. While technology is one issue and politics is another, I certainly hope that customer wants are not steamrolled in the end.

Jan 23 2012 - 10:04pm

Can anybody get a utility to give a rate for the cost of a non-subsidized kWh? This would be a help in getting more sustainable programs a level playing field.