Calculating Power Consumption

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There are a number of approaches that can be taken when trying to determine the power consumption of applicances around the home. Unfortunately there is no single approach that is suitable in all situations.

Note : Mains voltages fluctuate with location and time. In most locations in Australia, electricity is said to be 240 volts. However, to use my house as an example, I've recorded variations of 236 volts to 254 volts over a 12 hour period. These fluctuations will cause some variations in readings. eg A light globe rated at 100 Watts at 240 volts will draw 97 Watts at 236 volts or 112 Watts at 254 volts.

Terminology used in these pages

To differentiate between instantaneous power measured and measurements over a period of time, I'm using two of my own symbols:

Pi: (Instantaneous Power): Energy used at a point in time, measured in Watts (W)
Pt: Power used over a period of time, measured in Watt-hours (Wh) or Kilowatt-hours (kWh).

Hard Wired - Appliances that are not attached to a normal 240V power outlet, but are 'hard-wired' to the electricity meter board. eg Electric Stove, Fixed lighting, Ceiling Fans, Electric Hot Water Service, larger air-conditioning units etc.

Constant Load - Those appliances which draw about the same amount of power all the time (eg Lights, TV's, Stereo's, some heaters etc).

Note: (1) Some of these appliances vary slightly in load. eg A Stereo will consume a little more power when the volume is turned up high than if set low, but the difference is relatively insignificant. A toaster or other heating device will draw a little less power as it heats up.
  (2) I'm ignoring the fact that some appliances draw a large or peak load for a very short time when they are first switched on. This also applies to some Varying Loads (see below) when they begin their 'on' cycle.

Varying Load - Those appliances which perform some sort of cyclic functions. The two most common types are:
(1) Appliances that perform a heating or cooling function, and are controlled by a thermostat. For example, fridges, irons, electric ovens, some heaters and air-conditioners. Once the correct temperature is reached, these devices will virtually shut down until the temperature has reached a trigger level, at which time the appliance will start heating or cooling again.
(2) Some motorized appliances. Examples would be washing machines and dishwashers.
Fans and indicator lights may continue running even when these appliances are in their 'off' state.

Also see the definitions of W, kW, Wh & kWh.

 

1 Use the Power Consumption Tables

I've measured the consumption of a number of appliances around the home using the methods outlined below, and presented the figures in tables. See the Power Consumption Tables page.

2 Using the label attached to the appliance as a guide to Pi.

Most appliances have markings on them to indicate peak power consumption. There are several ways this figure can be marked.

Note : Some appliances like stereo's and microwaves may also quote output power. Because appliances are never 100% effecient, the input power will be higher than the output power. Also be cautious of the wild claims made by some manufacturers of audio equipment as to the claimed output power. There are various ways that they make this number seem a lot higher than it really is..
Advantages :
  • This method can be a quick way to calculate consumption of appliances with a constant load.
Disadvantages :
  • Difficult to accurately calculate consumption of appliances which have a varying load.
  • On appliances with multiple settings, the rating will be for the highest setting (eg An oscillating fan with 3 speed positions may be rated at 70 Watts, but may only consume this power on the highest setting. If you normally use it on the lowest setting, it may only consume 40 Watts).
  • Even taking the above point into consideration, some manufacturers seem to overstate the consumption of their appliances.

 

 

3a Calculate Pt for constant loads
Example:
  A 1000 Watt toaster takes 3 minutes to cook toast. The electricity consumed will be 1000 Watts * (3 minutes / 60 minutes) = 50 Wh or 0.05 kWh.

 

 

3b Estimate Pt for varying loads
Example:
  A fridge draws 150 Watts when the compresssor is on. The compressor can be heard operating for 10 minutes every half an hour. Assuming this same pattern is maintained for 24 hours, calculate consumption over 24 hours. Assume the fridge draws 0 Watts if the compressor is off.

If the compressor was on for 24 hours, consumption would be 150*24 = 3600 Wh. Because compressor is on for 10 min every 30 min or 1/3 of the time, total consumption would be 3600 * 1/3 = 1200 Wh.

 

 

4 Using a standard hosehold electricity meter

See Seperate page on Electricity Meters

 

 

5 Using a portable electricity meter

As above, but a seperate rotating-disc meter is purchased and fitted with normal 240V plugs and sockets, so individual appliances can be monitored. Cost for one of these meters is probably around $100. The advantage with this approach is the meter can be left attached to one appliance, without having to turn off everything else on the circuit.

 

6 Measuring Current

Note : This method can be quite dangerous, as it brings the user into close proximity to potentially lethal AC electricity. Do not attempt this unless you have the correct equipment and know exactly what you're doing!!! You often don't get a second chance with AC.

A meter capable of measuring AC current of the magnitude expected is substituted temporarily for the appropriate fuse cartridge in the meter box. Once the current is known, Pi and Pt (assuming a constant load) can be calculated.

 

7 Using a Power Meter

Electronic power meters keep a running total of kWh consumed, as well as indicating instantaneous consumption and several other parameters. They are placed in-line between the power outlet and the appliance being tested.

The unit which I purchased several years ago is no longer imported into Australia. However, there are two other units which I know of that are now available. These units measure W & KW, but don't measure VA or kVA directly.

Sparometer
The 'Sparometer'

Power-Mate
The 'Power-Mate',
made in Australia
by CCI Pty Ltd

Both units are available from the ATA (Alternative Technology Association). Try this direct link

Advantages :
Disadvantages :
  • Expensive to buy
  • Portable units can not be used for appliances that are hard-wired.

 

 

8 Calculating the amount of electricity consumed to heat water

The following formula can be used to calculate the amount of power required to heat a quantity of water. It assumes 100% effeciency, with no losses.

Pt = (4.2 * L * T ) / 3600 where Pt is the power used in kWh
    L is the number of litres of water heated
    T is the Temperature difference between the hot water ended up with and the cold water started with in C

 

Example 1:
  Calculate the amount of electricity required to boil 1.5 litres of water in an electric jug if the starting temperature of the water is 20C.

P = (4.2 * 1.5 * (100-20)) / 3600 = 140 Wh.

If we add ~10% to overcome inefficiencies then the figure becomes 154 Wh.

 

Example 2:
  Calculate the amount of electricity consumed by an electric hot water service to provide 45 litres of water at 50C for a shower, assuming the water started off at 20C.

P = (4.2 * 45 * (50-20)) / 3600 = 1575 Wh or 1.575 kWh.

This figure isn't allowing for any heat losses in the storage tank which can be quite significant (40% or more) if the water is heated and then stored for later use. See the Tank Insulation paragraph of Getting into Hot Water for more info.

It is also possible to work out how much time it should take to heat water:

Example:
  After coming back from holidays, the 250 Litre hot water system was turned on after being off for a few weeks. How long would you have to wait for the water to heat to say, 50C so you could enjoy a hot shower? The starting temperature of the water is 20C, and the heater element is rated at 3.6 kW (15 Amps).

First calculate the kWh required to heat the water from the formula above:

Pt = (4.2 * 250 * (50-20)) / 3600 = 8.75 kWh.

To work out the time taken, divide Pt (kWh) by the element rating (in kW)

Heating_Time = 8.75 kWh / 3.6 kW = 2.43 hours

Again, this figure is assuming 100% effeciency. A more realistic guess might be around 2.8 hours.


Last Modified: 24 May, 2004.