Tag Archives: efficiency

Standby power

We are constantly being bombarded with messages to turn devices off at the mains, rather than leaving them on standby. But is it worth it? How much energy does this actually save?


According to the Lawrence Berkeley National Laboratory a television left on standby uses about one-and-a-half watts. For comparison purposes, a power shower uses between 7500 and 10800 watts. That means that for one fewer ten-minute (9 kW) shower you can leave your TV on standby for another forty-one days and still have used less electricity than you would have.

A study published in 2010 in the Proceedings of the National Academy of Science showed that people were far more likely to select curtailment activities (turning things off) rather than efficiency savings (like cutting a minute off your shower), despite efficiency savings being more likely to reduce energy consumption. Turning devices off at the mains, rather than making choices that would save far more energy, is another example of this.


Estimating energy usage and savings

There’s a fascinating paper in this week’s Proceedings of the National Academy of Sciences of the United States of America (PNAS) journal about people’s perceptions of the energy used and saved by various devices and methods.

The researchers’ conclusions are not good news, especially in the light of the energy savings that are required to reduce anthropogenic climate change:

“[P]articipants in this study exhibited relatively little knowledge regarding the comparitive energy use and potential savings related to different behaviours … [they] were also … unaware of differences for some large-scale economic activities … and everyday items.”

The researchers recruited 505 volunteers using Craigslist (which must introduce an interesting set of biases) and asked them to estimate the amount of energy used by various household devices, and to estimate the amount of energy saved by various methods.

On average the study’s participants underestimated the energy used or saved by a factor of 2.8; people estimate that a device using 1000 watts of electrical power actually only uses 350W and a method that saves 500W would be estimated to save only 180W.

Participants did understand that energy savings were possible, but underestimated the size of the saving. For example, participants knew that a laptop computer used less power than a desktop computer, but thought that the saving was less (23W) than it actually was (92W). The more energy a device/method used or saved, the less accurate participants were. Participants estimated that transporting goods by truck used about the same amount of energy as transporting by train or ship, despite the fact that trucks actually use ten times as much energy: they overestimated the use of energy by ships and trains and underestimated trucks and aeroplanes.

In this graph from the paper overestimates appear above the dashed line and underestimates below.

The activity most commonly selected in answer to a question about the single most effective thing participants could do to save energy was “turn off lights”, whereas in reality resetting the thermostat or washing clothes on a colder setting would save far, far more energy. Far more participants selected “curtailment” activities (e.g. turning off lights, not using the car) as saving more energy than “efficiency” activities (e.g. switching to compact fluorescent lightbulbs) despite the fact that the opposite is most likely correct.*

* See Gardner, G. and Stern, P. (2008) The short list: the most effective actions US households can take to curb climate change, Environment Magazine, 50, pp. 12-24. Link

The problem with the bulb ban

You might not have noticed, but the sale of standard filament lightbulbs has been banned since last September.*

Current proposals are to replace standard incandescent bulbs with more efficient compact fluorescent bulbs. Fluorescent lights work by passing a current through a tube containing mercury vapour. The excited mercury atoms emit ultraviolet photons which then collide with the phosphor coating on the inside of the tube, causing it to emit light (to “fluoresce”) in the visible part of the spectrum.

The problem with this is that some of the ultraviolet light is still emitted (in fact germicidal lamps are basically fluorescent bulbs without the phosphor coating). Many people are sensitive to ultraviolet light and cannot enter rooms lit by flourescent lamps.

People with conditions such as photodermatosis (where exposure to ultraviolet causes swelling, rashes and blistering) and people with photosensitive epilepsy (the 50Hz mains supply flicker is visible from fluroescent bulbs but not from incandescent ones) have been stockpiling bulbs since the ban was announced.

I’m not against the sale of compact fluorescent lightbulbs; in fact I’d like to see them more widely used (especially in place of the inefficient halogen spotlights that seem so popular). What I’m against is removing the consumer’s right to choose.

(If you’re desperate for lightbulbs and you’re wondering, I took the photo above outside of Rugby Electrical, in Rugby.)

* Technically, it’s only the sale of bulbs for household use that’s banned. “Industrial use” is still okay.

MPG v L/100km


Miles per gallon (MPG) is a misleading quantity.

Miles per gallon is often referred to as a measurement of fuel consumption but it isn’t. Miles per gallon is a measure of the car’s fuel efficiency: how many miles the car can extract from one tank of fuel. A more efficient car can extract more miles from a tank.

Which saves you more fuel?

  • Switching from a car that gets 20MPG to one that gets 40MPG.
  • Switching from a car that gets 40MPG to one that gets 60MPG.

For a hypothetical 400 mile trip:

  • Switching from 20MPG to 40MPG saves 10 gallons of fuel.
  • Switching from 40MPG to 60MPG saves 3.3 gallons of fuel.

As fuel efficiency increases the amount of fuel saved decreases. A 20MPG increase in the low MPG values has far more of an effect than a 20MPG increase in the high MPG values. The scale is non-linear.

Put another way: the average MPG rating for a car is about 27MPG. For our hypothetical 400 mile trip:

  • Doubling fuel efficiency to 54MPG saves a total of 7.4 gallons of fuel.
  • Tripling fuel efficiency to 81MPG saves a total of 9.9 gallons of fuel.

This dependence on MPG is (yet another) hangover from our archaic insistence on non-standard imperial units.


To measure fuel consumption, we need to measure not miles per gallon, but gallons per mile. A (volume) per (distance) measurement already exists and is already commonly used in Europe: litres per 100 kilometres.

Which saves you more fuel?

  • Switching from a car that gets 6l/100km to one that gets 4l/100km.
  • Switching from a car that gets 4l/100km to one that gets 2l/100km.

For a hypothetical 400km trip:

  • Switching from 6l/100km to 4l/100km saves 8 litres of fuel.
  • Switching from 4l/100km to 2l/100km saves 8 litres of fuel.

In this case the scale is linear, a 2l/100km saving always results in the same fuel saving.


We should use the l/100km rather than the MPG scale for measuring fuel consumption.