Brian Cryer

 

How much Electricity does it use?

This page contains some notes and measurements on electricity consumption of various household items. I had to work out the cost of running a server for a year and was surprised at how many units of electricity were used, so I decided to start recording other household items. The items I've listed here are things which are typically left plugged in (on standby).

How much does it cost?

Electricity consumption (at least in the UK) is measured and paid for by units, where one unit is the equivalent of 1 kilowatt hour. So a 100watt light bulb left on for ten hours uses 1unit of electricity (100watts x 10hours = 1000 watt hours = 1kwh) as would a 2kw fan heater left on for half an hour.

The following table is provided as a guide and shows how many units (kwh) devices would use according to their rated power consumption:

Rating kw/hour kwh/day kwh/week kwh/4 weeks kwh/year
1 watt 0.001  0.024   0.168    0.672     8.76
2 watts 0.002  0.048   0.336    1.344    17.47
5 watts 0.005  0.12   0.84    3.36    43.8
10 watts 0.01  0.24   1.68    6.72    87.6
20 watts 0.02  0.48   3.36   13.44   175.2
50 watts 0.05  1.2   8.4   33.6   438
100 watts 0.1  2.4  16.8   67.2   876
200 watts 0.2  4.8  33.6  134.4  1752
1 kw 1 24 168  672  8760
2 kw 2 48 336 1344 17472
3 kw 3 72 504 2016 26208

So a 100 watt bulb left on 24 hours a day for a year will use 876 units of electricity, which at (say) 15p per unit would work out at £131.40. By adding a time switch to switch that same bulb on for only 8 hours a day would reduce the annual electricity cost down to £43.80 (a saving of £87.60). So if the time switch cost £20 then it would pay for itself more than four times over within a year. Contrast that with a modern high efficiency 15 watt bulb (which gives out almost as much light) that would use 131.4 units of electricity if left on 24 hours all year, or �20 - a significant saving.

If you want to see how long it takes to use one unit of electricity for a device then this is tabulated in the following table:

Rating Time to use 1 unit
1 watt  1000 hours almost 1 unit every 6 weeks
2 watts 500 hours almost 1 unit every 3 weeks
5 watts 200 hours just over 1 unit every 8 days
10 watts 100 hours just over 1 unit every 4 days
20 watts 50 hours just over 1 unit every 2 days
50 watts 20 hours a little over 1 unit every day
100 watts 10 hours almost 2.5 units a day
200 watts 5 hours almost 5 units a day
1 kw 1 hour 24 units a day
2 kw 30 minutes 48 units a day
3 kw 20 minutes 72 units a day

How much does it use?

The following table lists the power consumption figures that I have measured for a number of different devices:

Device Watts
(approx)
kwh per year (approx)
on 8 hours/day  on 24 hours/day
mechanical time switch 2 watts (or less) 6 17
laptop on 80 watts (average) 234 700
off, battery charging/charged 45 watts 131 394
video recorder on standby 4 watts 12 35
playing 17 watts 50 149
crt tv 21" on standby 3 watts 9 27
on 44 watts 128 385
cable set-top box 13 watts 38 113
20watt energy saver bulb 20 watts 58 175
100watt light bulb 100 watts 292 876
Dell 20" LCD monitor on 49 watts 143 429
standby (on but pc off) 5 watts 15 44
14" LCD monitor (unbranded) on 18 watts 53 158
standby 1 watt 3 9
2 kw fan heater 2000 watts 5840 17520
3 kw fan heater 3000 watts 8760 26280
HP Laser Jet 4 (laser) printer. printing 800 watts 2336 7008
standby 46 watts 134 403
HP Color LaserJet CP3505dn (laser) printer. printing 480 watts
(average)
1400 4205
standby 11 watts 32 96
24 port router (Linksys SR2024) 21 watts 61 184
IBM x3200M server behind APC Smart UPS 1000 120 watts 350 1051

Note:

  • All measurements are approximate and may not be typical (YMMV).
  • Power consumption often fluctuates and for many devices it is not constant. So treat these figures as a guide only.
  • Some devices (such as bulbs and fan-heaters) I have not measured but instead listed their rated value.

Working it out for yourself if it isn't listed above - how much electricity does it use?

Almost everything which uses electricity will come with or have on it an indication of how much electricity it uses. This might not be obvious, because an indication of electricity usage might take any of the following forms:

  • wattage (or watts)

    1000 watts = 1kw = 1 unit per hour.

  • amps

    Using the high school equation:

    watts = volts x amps

    you can work out the wattage provided you know the voltage. Mains voltage in the UK is notionally 230volts, in much of Europe it is 220 volts and 120 volts in America.

    So, in the UK if something draws 2.4amps then the wattage = 230 x 2.4 = 552watts.

    If this were on a power supply supplying 2.4 amps at 9volts then this would give a notional figure of 21.6 watts.

  • BTU

    BTUs are unit often used with heating or cooling. For a traditional electric fan heater you can use:

    kW = BTU / 34129

    However this equation cannot be used for air conditioning units or heaters other than fan heater types.

Treat the above as a guide, as the actual consumption figure will depend on the conditions in which a device is used and any published watts, amps or BTU typically represent the maximum and not the typical power usage.

Are there alternatives?

If you are concerned about the amount of electricity you are using then there may be alternatives. There is a modern trend towards lower power devices - although you have to balance any cost savings with the purchase cost, so sometimes it will be more economical to wait until something fails and needs replacing.



About the author: is a dedicated software developer and webmaster. For his day job he develops websites and desktop applications as well as providing IT services. He moonlights as a technical author and consultant.