# 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**: Brian Cryer
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.