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.