What is a kilowatt-hour?

June 14, 2006 2:12 PM

What is a kilowatt-hour? Or: why is the eco program on my dishwasher better than the normal program?

I googled, and read the Wikipedia and other sites that try to explain this, but I still do not understand it. I would appreciate a for-dummies explanation.

My new dishwasher has two programs. An environmental friendly (eco) program that heats to 45 degrees C and uses 1.05 kWh, and a normal program that heats the water to 50 C and uses 1.2 kWh. The eco program takes 165 minutes, the normal program takes 60 minutes.

Why does the eco program save energy? It seems to me that it goes on for more than twice as long for not that much less energy. What am I missing?

posted by davar to science & nature (20 comments total)

Because most of the energy goes into heating the water and not running the pump. I guess it goes for longer because the cooler water doesn't clean as well.
posted by GuyZero at 2:14 PM on June 14, 2006

A kilowatt-hour is a unit of energy, specifically it is the unit on your electric bill. GuyZero is correct.
posted by caddis at 2:17 PM on June 14, 2006

Which model of dishwasher is it? Maybe there is more to the eco mode than just saving some energy, because that's not much energy it's saving.
posted by smackfu at 2:19 PM on June 14, 2006

Sorry, I forgot... a kilowatt-hour is the amount of energy you get from (tautologically) one kilowatt for one hour. You know how bulbs say "60W" on them? That means 60 Watts. A kilowatt is 1000 watts. That indicates how much electricity is being drawn at any one moment. You're billed by the kilowatt-hour, that is, for using one kilowatt for an hour. If your house was drawing a total of 5 kW for an hour, you bill would say 5 kW-h. If it drew 5 kW for 30 min, you'd get a bill for 2.5 kW-h. (Sorry if this sounds patronizing, but you did say "for dummies").

My guess is that both modes go through a heat-wash-heat-rinse cycle. Most of the energy goes into warming your water from whatever temperature it is to 45 or 50 degrees.

Also, that seems low. My dishwasher goes to either 50 or 65, but your lower temperatures will definitely save energy.
posted by GuyZero at 2:21 PM on June 14, 2006

A kilowatt is an amount of electrical power. Power is energy per time. A 60 watt bulb uses 60 watts regardless of how long it's on.

A kilowatt hour is the amount of energy you use if you are using a kilowatt of power for a whole hour. (1 kW * 1 hour). Leaving a 60 watt bulb on for an hour uses 60 watt hours, the same as leaving a 30 watt bulb on for two hours.

You're being charged for the total energy (kilowatt-hours) you use. The eco mode uses about a third as much power (kilowatts), but for a lot longer. But you still get some savings; .15 kWh total difference.

I hope this is helpful.
posted by aubilenon at 2:21 PM on June 14, 2006

Ach, you slipped in between preview and post. Damn you GuyZero!
posted by aubilenon at 2:22 PM on June 14, 2006

You are probably paying somewhere between $.05 and $.25 per kilowatt hour, so the eco program saves you somewhere between one and three cents per load.
posted by StickyCarpet at 2:23 PM on June 14, 2006

As a completely speculative aside, the eco program may use less water. Water consumption is a bigger issue in some areas than electricity consumption. That nay also explain the longer run time, a using half as much water (or whatever fraction) probably doesn't clean as well as the regular program.
posted by GuyZero at 2:28 PM on June 14, 2006

Thanks. I do understand the why on an intellectual level. It makes sense that most of the energy goes into heating the water. I also would like to understand the numbers.

I found the following formula: energy (kWh) = power (kW) × time (h)

So, does that mean for the normal program:
1.2 = 1.2 * 1
and for the eco program:
1.05 = 0,381 * 2.75

So the normal program uses 1200 watts, whereas the eco program uses 381 watts?

I think I am confused because of the name. I thought kWh meant "kilowatts per hour", so I thought the eco program used 1.05 * 2.75 = 2.88 kW. That seemed like an awful lot.

on preview: thanks GuyZero and aubilenon! I think I understand now!
posted by davar at 2:28 PM on June 14, 2006

davar, the KW Hr figure is probably the total amount used, otherwise they would have just used watts and let you multiply by time.
posted by StickyCarpet at 2:31 PM on June 14, 2006

A little further explanation: I bought this model because it is reasonably environmentally friendly. There are actually two other programs that heat to 55 and 75 degrees, but I did not want to complicate the question. All programs use the same amount of water (13 liters).

I think I'll use the eco program when I put the dishwasher on at night, but I won't feel guilty using the normal program during the day.
posted by davar at 2:35 PM on June 14, 2006

Thanks to everyone! I now understand something I did not understand and I learned it within ten minutes of asking!
posted by davar at 2:38 PM on June 14, 2006

I think I'll use the eco program when I put the dishwasher on at night, but I won't feel guilty using the normal program during the day.

Generally, it's also good to run stuff like that at off-peak hours. Your power company (or whoever they buy from) has to have enough electricity generating capacity to meet their peak of demand; the rest of the day, much of that capacity is empty. Pushing what you can into the evening hours evens out the peak-offpeak cycle, meaning less capacity is needed. Your dishwasher is obviously just a tiny drop, but the principle is there.
posted by claxton6 at 3:01 PM on June 14, 2006

Just to get completely nerdy:

The standard unit of energy in MKS is the Joule. "Power" is constant energy generation or usage, and the standard unit is a watt, which is defined as one joule per second produced or consumed.

Units in MKS are defined so as to avoid conversion constants, but that has the effect of making some of them huge and some of them tiny. It turns out that a joule is one of the small ones, and most kinds of things that people are interested in which involved production or consumption of energy deal in preposterously huge numbers of joules. (There are notable exceptions, though.)

We buy energy, not power. Power represents the rate at which the bill accumulates, but it's energy that's critical. But it doesn't really make sense to buy joules by the millions, simply because it would terrify most people who don't understand the physics. So electric companies have adopted the kilowatt-hour as a reasonable-sized unit of energy.

By definition, a kilowatt-hour is the amount of energy produced or consumed over the course of one hour at a rate of 1000 watts. In other words, it is 1000 * 3600 == 3.6 million joules.

The power company charges you the same for a kilowatt-hour of energy whether you consume it in 10 minutes or an hour or a week, so the length of time your dishwasher takes to run each cycle isn't important in terms of what your electrical bill reads at the end of each month. What's important is the amount of total energy that's involved.

The "Eco" cycle takes longer, but runs at a much lower average power level, and so the overall energy usage is smaller.

As to whether that's "better", well, it depends on what you want. If you're interested in keeping your bill down, it's better. If you're in a hurry, it isn't.
posted by Steven C. Den Beste at 3:22 PM on June 14, 2006

Pushing what you can into the evening hours evens out the peak-offpeak cycle

Here is some ontario data for the weekly and monthly electricity market. The weekly reports give the most useful information (although it isn't that useful). Looks like 9am-9pm is very high usage. Peak is hit around 1pm or 2pm, with a second peak around 6pm..

I think, if you really want to try and help on the demand side, you have to wait till 9pm to start running everything. I would love to see some more detailed information on this.
posted by Chuckles at 4:31 PM on June 14, 2006

You can see exact charts of power usage for New England, New York, Long Island, Texas, and California here.
posted by Steven C. Den Beste at 6:59 PM on June 14, 2006

I could get to the currentenergy site, but every single link on that site gave me a 404 error. Maybe they are geographically filtering me, or something..
posted by Chuckles at 7:49 PM on June 14, 2006

davar: I think stickycarpet is correct when saying the 1.2 and 1.05 Kw are figures of total amoung of energy spent to do each washing cycle.

Consider that if 1.2 and 1.05 are the hourly consumption rate (amount of energy consumed in one hour) you have the following

1,05 Kwh * 2,75h = 2,887 Kw

where 2,75h is 165 minutes (165minute/60minutes)= 2,75 hour

But the normal program would be

1,2 KwH * 1h = 1,2Kw

but considering it would takes less time ( 1h < 2,75h) to achieve an higher temperature which an only marginal increase in energy consumption and considering the economic program would consume 4 times the normal program, It is reasonable to conclude that 1.05 is the total amount of energy consumed for the eco cycle and 1.2 for the normal cycle.
posted by elpapacito at 4:21 AM on June 15, 2006

Just to clarify. Aubilenon said above that a 60 Watt bulb uses 60 Watts no matter how long it's on. This is misleading. Watts are a measure of power, which is a rate. A Watt is one joule of energy per second. A 60 Watt bulb uses 60 joules of energy per second.

A kilowatt hour of energy would be the same amount of energy you would use burning a 1000 Watt bulb for one hour.
3.6 Million joules as stated above.
posted by sciencejock at 11:31 AM on June 15, 2006

I tried that site again, I was able to access graphs via the USA summary page.

In California, looks like the second peak doesn't happen till 8:30pm, and you really can't start adding load until 10pm at the earliest.

Great site!
posted by Chuckles at 12:38 PM on June 15, 2006