How Much Power are Your Appliances Consuming in Reality?

Air Conditioner Condensing Units. Image obtained with thanks from chooyutshing on Flickr: http://www.flickr.com/photos/25802865@N08/

On this page I will explain the power consumption patterns of various types of appliances, how to determine how much appliances consume in standby mode, and other little things that you should know. I will add more appliances/devices soon.

1. Basics (Toaster Oven Example)
2. Air Conditioners
3. Heaters
4. Water Heaters
5. Refrigerators
6. Washing Machines
7. Speakers

Before you continue reading, you should read this and know that the duration of time that you use your appliances is equally important as their wattage. If an appliance was 10,000 watts but your turned it on for only 1 second, the power consumption would only be 2.7 Wh (watt-hours), which is about as much as a flashlight consumes hourly (next to nothing). You can know that their importance is equal because power consumption = wattage x time used.

There are appliances that are a high wattage and are not operated long, such as toaster ovens, and most other appliances are of the type that are a low wattage and are operated for long periods of time. Air conditioners and heaters are the worst of both because they are a high wattage and they operated for a long time.

People often point to high wattage appliances such as toaster ovens, water heaters, and more and think that they account for most of the electric bill, but they don’t.

Basics (Toaster Oven Example)

Read this section of the Understanding Power Consumption page first

A toaster oven is usually 1,200 watts, but remember that is the hourly rate of electricity consumption. So it consumes 1,200 Wh after an hour. Due to the fact that it is operated only for a few minutes at a time, the consumption per use is not as much as you would expect. To determine the minutely consumption, divide 1,200 by 60 because there are 60 minutes in an hour.

The result is a power consumption rate of 20 watts for every minute that it is on. This means that it consumes 20 Wh for every minute that it is left on. If the oven was on for 5 minutes, then multiply 20 by 5 and the result is a total power consumption of 100 Wh per use. If your electricity rate is $0.10 USD/kWh then the cost of that is $0.10. 100 Wh = 0.1 kWh. I calculated the cost by multiplying the 0.1 kWh by the 0.10 per kWh electricity rate.

Toaster ovens are operated a few times per day, and there are an average of 30 days in each month. If your oven is operated 4 times per day, then it would cost $0.024 USD per day which totals $0.72 USD per month.

Appliances usually have their power consumption in watts printed on the back of them, but it actually varies in many cases due to the fact that people change settings, such as air conditioner or heater thermostat temperature, monitor brightness, and more.

Computers also have a variety of settings that can affect power consumption, including a performance mode (this uses the most electricity), balanced mode (this tries to balance energy efficiency while still offering decent performance), and power saving mode which is a low performance mode that saves the most electricity.

There are certain appliances of which the power consumption cannot be determined by simply checking their labels including:

Air conditioners

Air conditioners do have a rated power consumption, but it is their maximum, and that is not usually what they are consuming most of the time. If you are using an underpowered unit such as a 5,000 BTU one which is too small to cool your large room, then it is likely to operate on the highest setting and stay on. In such a case the power consumption printed on the label would be correct, but this is not usually the case. Air conditioners are one of the few types of appliances that, by themselves, can have a significant impact on your electric bill.

People often utilize higher performance 9,000, 12,000 and 24,000 BTU units that increase or decrease the speed of their compressors as the weather gets hotter or cooler respectively. The non-inverter units operate at the same speed all the time, and when the room reaches the desired temperature, they shut off until it becomes too hot again, then they turn themselves back on to cool the room back off.

Most modern air conditioners are of the inverter type. This means that a device called an inverter controls the speed of the air conditioner’s compressor based on the thermostat temperature that you, the user set it to. The compressor is what mechanically powers the process that actually cools the air.

If you turn the temperature on the thermostat up, the compressor turns down because you are basically telling it that you don’t need it to cool as much, and the result is that it consumes less power because the compressor has less work to do. The more work it has to do, the more energy it will need to do that work.

DOE – Average Annual Power Consumption of Air Conditioners (start at page 7-21)

Heaters

Space heaters/room heaters/central heating systems can have either multiple temperature settings such as low, medium, or high, or you can set the thermostat to the exact temperature that you want. The more you turn up the thermostat temperature, the more heat the heater has to generate to achieve that temperature.

Water heaters

Typical tank water heaters  may be equipped with a setting that gives you some control over how quickly you heat your water, but unfortunately the heater is not most efficient at the highest setting, so it is up to you to decide whether you want your water heated quickly, or if you are willing to wait longer and save money as well as electricity.

Keep in mind that water heaters don’t need to operate as long at the higher setting, so their power consumption would normally be the same no matter what if it wasn’t for the issue mentioned above. Remember what I emphasized at the top of this page about the fact that wattage and time used are equally important.

Refrigerators

Refrigerators cool themselves by removing heat from the cabin (which is where you put your food) and when it is cool enough, they shut themselves off to save energy. The wattage of your refrigerator alone does not tell you the overall power consumption and it is actually nowhere near what it actually consumes. You would need an energy use monitor to determine the cumulative power consumption by plugging the refrigerator in it and leave it for a week. Multiply the result by four.

You cannot obtain an accurate average power consumption figure after metering it for only a day because sometimes it does not cycle off for most of the day, I have seen refrigerators on all day sometimes, and it may actually do the opposite the next day.

Washing Machines

The rule that the wattage printed on appliances is not adequate information applies to washing machines too. Washing machines consume widely varying amounts of power from less than 20 watts, to more than 500 depending on what it is doing. It is filling that requires less than 20 watts, washing requires a few hundred watts on average because it has to rotate a large heavy barrel full of clothes and soap water, rinsing does as well, this applies to spinning as well because it literally has to spin a large heavy barrel full of clothes and soap water. More power is required to move heavier items.

The only way to determine the actual energy usage of your washing machine is to leave it plugged into an energy use meter for a week. A day alone will yield an inaccurate result because people sometimes wash their clothes at random times, and load sizes also vary. The meter will tell you the cumulative power consumption in kWh after that week, then you multiply that by 4. If you have people come over to do laundry less frequently than weekly, then leave the machine plugged into a meter for a month.

Speakers and Amplifiers

First: Speakers draw electric current from amplifiers, and then amplifiers draw electric current from your power outlet in turn to facilitate that.

The electricity usage of your speakers is absolutely nowhere near what is printed on the label. This is because their power consumption is almost fully determined by two things:

The volume that you set the speaker system to, and the amount of bass in the song that you listen to because bass frequencies require more power to produce than the rest of the higher frequencies (yes, I verified this with an energy use meter, the power consumption spiked every time bass lines were played).

Volume affects power consumption so much that electricity usage can double if you turn the volume up considerably.  This is just for the sake of knowing, though. Speaker systems are for your enjoyment, they aren’t meant to fulfill a practical purpose, so please comfortably listen to them at whichever volume you like.

Don’t worry about how much they are using when you turn up the volume, because they actually use very little power.

Manufacturers grossly exaggerate the power of their speakers (this cannot exceed the power consumption, by the way, so please look at the power consumption printed on the back label, and you can be sure you won’t get anywhere that much power out of your stereo, due to the “law of conservation of energy”, which is a law of physics you should know about).

A very tiny amount of power is required to produce big sound.

Two Examples of Exaggeration I have Seen

1. JVC “520 watt” bookshelf stereo system. The average power consumption at half volume was 52 watts. 520 watts is a peak rating which you can’t actually get from it. Most stereos have this unrealistic label on them, and it is in bold to get your attention. It is a highly successful gimmick. The power consumption printed on the back is 245 watts, but, that is the maximum, and at volumes which you shouldn’t listen to it at. This stereo is very loud in a 100-200 square foot room.
2. Klip Extreme 40 watt speaker system. The average power consumption is 13 watts, and, due to other physics laws, the power I get out of it has to be less than the power consumption rating.

More appliance information is on it’s way…

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