How much electricity does it Use?

This page contains some notes and measurements on electricity consumption of various 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.

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 this is almost 6 weeks
2 watts 500 hours this is almost 3 weeks
5 watts 200 hours this is just over 8 days
10 watts 100 hours this is just over 4 days
20 watts 50 hours this is just over 2 days
50 watts 20 hours
100 watts 10 hours
200 watts 5 hours
1 kw 1 hour
2 kw 30 minutes
3 kw 20 minutes

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

Note:

  • All measurements are approximate and may not be typical.
  • 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.

http://www.cryer.co.uk/brian/misc/how_much_electricity.htm

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