Wednesday, 6 June 2012

Energy star labels for computers

I have sometimes wondered why computers don't have energy labels like washing machines and refrigerators. However, it turns out there are lots of different environmental labeling systems and some of them do apply to computer equipment. Directgov has an overview of various labeling systems here. Two that apply to computer equipment are the EU Energy Star system, which is tied to the US Energy Star system, and the Energy Saving Trust Recommended Products list (EST). It turns out my computer (a rather obsolete Mac Mini) qualifies but my monitor just misses. (I didn't choose it, it was a present.) I had a good look at the Energy Star criteria for certification and was surprised that
  • For computers the active power consumption doesn't matter, they are expected to spend most of their time idle, asleep or off.
  • Games consoles are excluded, so they can't be Energy Star at all.
  • For monitors the active power allowed depends on size and for screens over 50" 300W or more is accepted, allowing even some plasmas to qualify (click here for a list).
The EST criteria are calculated and set differently. For computers EST is considerably more strict but for small displays the EST criteria are much less exacting.

 Criteria for computers

Here are the Energy Star criteria for computers  (pdf download) and here are the EST criteria

The Energy Star system has several different categories for notebooks, desktops and workstations (high end graphics machines for professionals). I have listed the categories and Typical Energy Consumption (TEC) criteria for desktops in the table below. These levels were agreed in 2009.  The Energy Saving Trust (EST) Recommended products list uses the same categories but sets much more stringent levels for the TEC. It has only 21 qualifying computers listed compared to thousands for the Energy Star rating. However, that is probably because the EST standard is less well known rather than that most products do not qualify. For example there are no Apple products listed though I am sure they would qualify.

 The TEC for desktops assumes the computer spends
  • 55% of the time turned off (like a TV on standby, you need to manually turn it on)
  • 5% in sleep mode (will wake automatically if something happens)
  • 40% idle - which means it isn't actually doing anything but it hasn't been idle long enough to automatically go to sleep
 TEC levels allowed for desktop computers
CategoryDefinitionEnergy Star* kWh/year (mean W)EST kWh/year (mean W)
ANot B, C or D148 (17)60 (7)
B2 cores and at least 2 Gb memory175 (20)126 (14)
CMore than 2 cores and at least 2 Gb memory or a separate GPU209 (24)152 (17)
D4 or more cores and at least 4Gb memory or a bigger separate GPU234 (27)152 (17)
* These figures are the base TEC - more energy is allowed for additional memory, graphics cards and internal storage

My 5 year old Mac Mini comes into category B so the EST allows it 126 kWh/year or an average of 14W. Given that it uses about 1W when switched off and 21W when idle it easily qualifies. However,  it does use considerably more when it is busy - up to 35W when flat out running a spreadsheet or if I am watching a video - and frankly this is not enough; my machine is under powered for my current needs and I intend to upgrade it soon.

The spec page for the latest 2.3 GHz Mac Mini says it can use up to 85W but it is Energy Star rated. Since it has a separate graphics card I think it must be category C which  means it can have an average power consumption of 24W but since for this calculation the computer is off half the time the idle power could be up to 50W. The spec page does not give an idle power consumption but the environment page does; it says the idle power is only 13W.  This is considerably better than my current 21W and I am impressed.

For workstations, which are defined as high end professional equipment, the TEC still depends only on off, sleep and idle power consumption but the ratios for off, sleep and idle are different (only 30% of the time off) and the level allowed increases depending on the maximum power rating for the computer. High powered machines are allowed more idle power as well.

Criteria for computer displays

Here are the Energy star criteria summary for displays and the EST criteria for displays

For computer displays there is a maximum power consumption for the on mode as well as sleep and off modes.  The EST levels are more stringent than the Energy Star in some areas. There are no computer displays listed as recommended by the EST.

Sleep/Off power consumption for computer displays
Energy Star (W) EST (W)
less than 30"2/10.8/0.7
more than 30"2/11.8/0.7

For the active mode, the power allowed depends on the size of the viewable screen area in square inches (A) and the resolution of the display in mega-pixels (MP). The EST also sets a overall maximum of 150 W.

Active power consumption for computer displays
Energy Star (W) EST (W)
less than 30", less than 1.1 MP6xMP + 0.05xA + 3 1.51xA (max 150)
less than 30", more than 1.1 MP9xMP + 0.05xA + 3 1.51xA (max 150)
more than 30"0.27xA + 8150

The EST rating turns out to be much less stringent than Energy Star for ordinary sized displays but more so for large ones. For example:

24" high definition display
  • Resolution is 1920 x 1080 which is 2.1 mega pixels.
  • Viewable screen area is about 21" by 12" which is  252 square inches.
  • Energy Star allows 9x2.1 + 0.05x252 = 18.9 + 12.6 + 3  = 34.5 W
  • EST allows 1.51 x 252 with a maximum of 150 = 150 W
For example the  Acer S240HLBid which is an LCD display with an LED backlight easily qualifies with an active power consumption of 26.1 W

40" high definition display
  • Resolution is 1920 x 1080 which is 2.1 mega pixels.
  • Viewable screen area is about 35" by 20" which is 700 square inches.
  • Energy Star allows 0.27 x 700 + 8 = 196 W
  • EST allows 1.51 x 700 with a maximum of 150 = 150 W
For displays, brightness can make a big difference. My 22" LCD display doesn't quite qualify at normal brightness but when I turned it down to about half I reduced the power consumption from 35 W to 28 W. Unfortunately that was just a bit too dark and now I have settled on 32 W. Some displays have automatic brightness control depending on ambient light levels.

Criteria for printers

Here are the  Energy Star criteria for printers (pdf download)   EST criteria for printers (inkjet only)

The EST seems to keep its criteria for laser printers secret but for inkjet printers the requirements are:
  • < 1 W standby power consumption
  • < 1.7 W -3.6 W in sleep mode depending the nature of its network or pc connection, storage and scanning capability
The Energy Star specification is much more comprehensive. It applies to all office imaging equipment including dye sublimation printers as well as laser and inket and also copiers and mailing machines too. It allows for 2 ways to qualify, based on TEC which involves actual printing, not just sleep mode consumption and OM which is similar to the EST criteria based on just standby and sleep mode consumption. The OM criteria also includes limits for the default setting for delay to sleep time. Presumably they expect most users not to change this.

The calculation of the TEC for printers is complex.The number of jobs per day and the number of pages per job is more for faster printers but the TEC allowed also increases. For example a printer capable of 10 pages per minute  (ppm) is expected to do 50 pages per day and allowed 3.8 kWh/week but a printer capable of 32 pages/per minute, 3 times as fast is expected to do 10 times as many pages and allowed only a little more energy: up to 6 kWh/week.


I suspect the reason so few products are recommended by the EST is simply that manufacturers are not under pressure to get that certification. On the other hand the Energy Star criteria are usually a good deal less severe - except for normal sized displays where the EST is ludicrously lax.  Some points to note:
  • For computers, the energy rating criteria assumes that your computer does nothing all the time. However, this is not necessarily the case especially if you watch a lot of video.  Checking the maximum power consumption is probably useful too, but that won't be typical either if your computer is over-powered for your needs. Setting genuine typical energy criteria is probably impossible because computers are used in so many different ways.
  • The Energy Star criteria for computers could do with tightening up; if the Mac Mini can exceed the standard by 75% it doesn't give a good guide for comparison between different products.
  • There are no energy standards for games consoles.
  • For printers, Energy Star rating would be a lot more useful if you knew whether the product qualified by standby power consumption or typical energy consumption.
  • For monitors, size matters. Just as with TVs and fridges and washing machines, the allowed energy consumption increases rapidly with size so if you care about energy consumption don't buy a bigger display than you need, even if it is Energy Star rated 


  1. I understand the Japanese 'Top Runner' program is highly regarded and about the most effective national legislation to decrease energy use.

  2. I downloaded the top runner standard for computers from It sets a target power consumption based on the average of low power/idle power consumption divided by processor speed in giga-calculations. Can someone tell me what a giga-calculation is and how I find out how many I have?

    Top runner has standards for disk drives too, but only solid rotating drives, taking into account storage capacity, physical size and rotating speed.

    In any case, I can't find any mention of products claiming to meet these specifications. The Top Runner standard seems to apply to a manufacturer's whole product line rather than individual products, weighted by the number of units sold. This is akin to the EU fuel economy standards for 'average' new car. It is admirable but not helpful to the prospective buyer.

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  6. hi..Im student from Informatics engineering, this article is very informative, thanks for sharing :)


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