How to measure your electricity consumption

Household electricity meter

In the UK, the main electricity meter has an aluminium disk which rotates at a rate proportional to the instantaneous power consumption. Each revolution of the disk represents a particular amount of electrical energy and the amount is probably marked on the face of the meter somewhere.
In my case the amount was 1/300 kilowatthours per revolution.

Timing one revolution of the disk

The number of revolutions is counted by gears and dials and these will give a direct reading of a number of kilowatthours but these are rather large units and it is much quicker to simply use a stopwatch to time the time for one revolution of the disk. The disk has a black mark on the circumference and you can start the stopwatch when the black mark is at a particular point and then stop the stopwatch after one revolution when the black mark is again at the same reference point.

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 a reference baseload

You should temporarily switch off all devices which have an intermittent operation (often controlled by thermostats) such as fridges, freezers, immersion heaters, electric space heaters, irons, central heating pumps. If you do not switch them off then you might instead ensure they are at least in their "off" state when you take the measurements by comparing measured instantaneous power with expected power and waiting and repeating the measurement until the reading is consistent with expected.

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.

Example baseload and device measurements

In my house which includes an annex, the minimum convenient load was 2 security alarm systems with sensors, a doorbell transformer, and 3 external security lamps (lamps not on because it is daylight but movement sensors presumably still powered).

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.

Conversion from instantaneous power to an annual operating cost


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)))

Powers and annual costs of my household devices


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


Minimising annual costs

Devices that consume excessive power on standby might be routed via one of the plug-in timers with manual override switch which should consume less than 1 watt and can power off devices during time of definite no use. Ideally, the timer should consume no electricity itself and retain time of day and settings when it loses power (unfortunately not the case with the common mechanical timers).

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.

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