In my case the amount was 1/300 kilowatthours per revolution.

The relationship between time t in seconds and the instantaneous power P in kilowatts when the energy per revolution is R kilowatthours is given by the formula:-

P = 3600*R/t

In my case, since R=1/300 this is P=3600/300/t = 12/t

Measure the time t for one revolution and convert to a power. Later you can repeat this with various household devices either added or subtracted from the load and you can subtract the baseload reading in order to find the additional power consumed by the device deliberately added or removed from the system.

t = 9 minutes and 13 seconds = 553 seconds.

P = 12/t = 12/553 = 0.02169 kWatts = 21.69 Watts.

Adding a garage circuit with 2 security lamps changed the reading:-

t= 7 min 37 seconds = 457 seconds.

P=12/t= 12/457 = 0.02625 kWatts = 26.25 Watts.

Subtracting this from the baseload gives the additional power of 2 security lamps = 26.25-21.69 = 4.56 Watts.

Each security lamp then must take 2.28 Watts.

Taking this as the new baseload, and adding on 2 central heating boiler controllers (not active since it was a hot day) gave a reading:-

t = 5 minutes 33 seconds = 333 seconds.

P = 12/t = 12/333 = 0.03603 kWatts = 36.03 Watts.

Subtract from the changed baseload gives the additional power of 2 boiler controllers = 36.03-26.25 = 9.78 Watts.

Each boiler then takes 4.89 Watts.

Continue with all your devices, such as your video/satellite/TV/hifi stack on standby.

I did not bother measuring incandescent lamps since the power consumption is usually reliably marked on the bulbs.

Remember to reconnect the fridge and freezer as soon as possible. It is likely that they will have warmed up slightly and so the motors will start operating when you connect them and you can take a power reading.

For my combination of 1 fridge-freezer + 1 fridge, the power was 750-444 = 306 Watts.

Each motor was then probably 153 Watts.

The number of hours in a year is 24*365 = 8760.

If your electrical energy tariff is T units per kilowatthour then a device of P kilowatts continuously powered would cost:-

C = 8760*P*T units.

In my case, T=0.08 Great Britain Pounds.

My Satellite/TV/Video stack which is usually on standby at a suprising 43.73 Watts would then cost

C=8760*0.04373*0.08 = £30.64 per year.

I now make sure I switch this stack off at the wall socket when not in use. The video still retains the correct time and record settings.

The average powers of intermittent devices such as fridges will a function of the on/off ratio and the power when on.

If a fridge is on for L seconds and off for M seconds then the average on-time fraction is L/(L+M) and the average off-time fraction is M/(L+M) = 1-(L/(L+M)).

Where my room temperature is at 19.5 Celcius and my fridge thermostat was set to a medium coolness I measured the following times:-

The fridge turned off at 10:01 am. Later it turned on at 10:41 am and then turned off at 10:54 am.

L=54-41 minutes= 13 minutes

M= 41-01 minutes = 40 minutes

Therefore L/(L+M) = 13/(13+40) = 0.245

The annual cost will then be this fraction of the annual cost if the device was continuously on (Assume that when device is off it consumes no power).

C=8760*P*T*L/(L+M)

Alternatively we can just measure L and know that L+M is the total of on plus off.

E.g a computer used 8 hours per day then L+M must be 24 hours so L/(L+M) = 8/24 = 0.333.

If the device still consumes power little-p in the off-state, you can estimate the additional annual off-state power cost c with,

c=8760*p*T*M/(L+M) = 8760*p*T*(1-(L/(L+M)))

Device |
Power in Watts |
average ratio of time in this
state |
Annual cost in Pounds |

Ceiling fan max speed |
43.20 |
1 |
30.27 |

2xAlarms+2xSecurity
lights+doorbell |
21.69 |
1 |
15.20 |

2xSecurity lights |
4.56 |
1 |
3.19 |

2xboiler controller |
9.78 |
1 |
6.85 |

Radio clock |
2.43 |
1 |
1.70 |

Video/Satellite/TV on standby |
43.73 |
1 |
30.64 |

Mobile home fridge/freezer |
361.81 |
1 |
253.55 (thermostat will 1/4 this) |

1 fridge+1fridge-freezer |
306.00 |
1 |
214.44 (thermostat will 1/4 this) |

1 fridge |
153.00 |
0.245 |
29.27 (this includes measured thermostat on-off ratio) |

Computer with LCD screen |
157.81 |
1 |
110.59 |

Computer with LCD screen |
157.81 |
0.333 |
36.82 |

100W incandescent light bulb |
100 |
1 |
70.08 |

Freezers might be super-insulated with additional panels. Be sure not to insulate the heat rejection devices (black motor or pipework or anything that feels warm when operating) and be sure not to allow air circulation between the new insulation and the original device walls (these will now be colder and may then condense water vapor in the air).

Check the energy efficiency ratings of your devices. A class A rating might use half the electricity of a class C for the same performance.

A condensing gas boiler will generate 10% more heat per unit of fuel than a non-condensing gas boiler (the difference comes from the latent heat of vaporisation for water which is 500 calories per gram). This could save £1200 over 20 years.

In the UK, much electricity is generated from carbon fuels at only 33% efficiency so saving 1 Joule of electricity save 3 Joules of fuel.

Support Cycom, saving the planet with energy research, perhaps by subscribing to Club Cycom.