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Updated : 16/10/2016

Energy Measurement


In order to make any meaningful decision regarding how to reduce electrical energy consumption some data is required on the current energy usage, including what items in the household consume energy, for how long and at what rate. The next questions are how much data and how accurate must the data be. The answers are that any data however little and however approximate is better than nothing, and the mere fact that the data is being sought will, generally, guide you in the right direction to reduce energy consumption.


Any Internet search on "domestic electrical energy saving" will yield a daunting array of energy monitoring systems and must have instruments. Do you need any of these systems or instruments? The simple answer is no, you can analyse your energy usage with nothing more than pencil, paper and a watch. However, if in addition to reducing energy usage, you would like to have an automated monitoring and recording of energy usage then by all means purchase whatever equipment or system your wallet will allow remembering of course the folly of spending thousands of Rand to save a few cents. The following sections describe how to obtain useful energy usage data and for those, like myself, who like a continuous monitoring of their energy saving progress, how to gather and save the data automatically.

Manual Methods

Method 1

This method requires no instruments other than pen, paper and a watch.

  1. First create a list of all the electrical items in the house.
  2. For each item listed take a close look at the labelling on the item. The label may be in the form of a riveted metal label on the rear or bottom of the item or, in the case of small items, the labelling information may be embossed on the item itself. Wherever the label is, and legally it must be present on all electrical items , what you are looking for is the power rating of the item. The power rating will be a number followed by the letter W for watts or kW for kilowatts or very unusually kVA for kilo volt amps which for our purpose here can be taken as kilowatts.(e.g. for incandescent globes the information is stamped on the top of the globe itself)
  3. Make a note of the power rating in kilowatts (note 1kW = 1000W) against each item in the list.
  4. For each item make an estimate of the number of hours the item the item is typically switched on every day and note this against each item. At this stage the standby power of electronic equipment can be ignored. For items such as kettles, time how long it takes to boil the water you typically use and multiply the result by an estimate of the number of times the kettle is used each day to get the total number of hours the kettle is switched on. (Whilst doing this you may find it interesting to time how long kettle full of water takes to boil and then empty the kettle and fill it with a mug of water and again time how long it takes to boil. The difference between the two times indicates how much energy you are wasting by boiling more water than you need to make a cup of coffee). For items such as washing machines and dishwashers the user manual will indicate how long a wash cycle is, alternatively, ask whoever does the washing.
  5. For items such as heaters, fridges and freezers that switch on and off automatically determine the duty cycle by timing how long the item is on over a period of say an hour. The duty cycle is the time the item is on divided by the measurement period. For example if the freezer is on for 15 minutes over a measurement period of 1 hour then the duty cycle is 15/60 = 0.25 and any item which is on continuously has a duty cycle of 1
  6. For each item calculate the energy used per day from (hours per day )X(duty cycle)X(power)
  7. Sort the list of items with those using the most energy at the top.

This should produce a table, such as that showed below, from which you can identify the items using the most energy per day.

Item Current Standby kW Power kW Duty cycle hrs/day Energy (kWh)
Hot water (usage) 13.043 0 3.000 1.000 1.700(1) 5.100
Hot water (maintenance) 13.043 0 3.000 0.042(2) 24.000 3.002
Pool pump 6.522 0 1.500 0.067(3) 24.000 2.400
Computer (SER) 0.283 0 0.065 1.000 24.000 1.560
Lighting (evening) 1.100 0 0.253 1.000 6.000 1.518
Lighting (daytime) 0.348 0 0.080 1.000 16.000 1.280
UPS 0.152 0 0.035 1.000 24.000 0.840
DSTV Decoder 0.122 0 0.028 1.000 24.000 0.672
Kettle 8.696 0 2.000 1.000 0.333(4) 0.666
Washing Machine 5.043 0 1.160 1.000 0.500(5) 0.580
Dishwasher 4.783 0 1.100 1.000 0.500(6) 0.550
Freezer 0.389 0 0.090(7) 0.250 24.000 0.537
Fridge 0.507 0 0.117 0.164 24.000 0.459
Television 0.300 0.0184 0.069 1.000 6.000 0.414
Lighting (night) 0.130 0 0.030 1.000 8.000 0.240
Alarm System (2) 0.021 0.02 0.005 1.000 24.000 0.115
Electric Gate 0.021 0.02 0.005 1.000 24.000 0.115
Microwave stand by 0.020 0.0046 0.0046 1.000 24.000 0.110
Computer (DT) 0.191 0 0.044 1.000 1.500(8) 0.066
Backup Hard Drive 0.035 0 0.008 1.000 1.500(8) 0.012
Monitor (SER) 0.139 0 0.032 0.010 24.000 0.008
Monitor (DT) 0.152 0 0.035 0.010 1.500(8) 0.001
          Total 20.245

(1)Equivalent to 12hr per week
(2)Duty cycle to maintain water at a temperature of 55-60 degrees assuming no usage and a loss rate of 2.4kWh/day
(3)Currently set for 1.6h per day
(4)Assuming used 10 times a day at 2 minute to boil 600ml water
(5)One 30 min cycle at 40 deg. Assuming one wash per day but in fact used at 2 washes every second day
(6)standard wash at 40 degrees. used once per day
(7)Manufacturers specification. I believe the actual to be much lower than this based on current setting. Efergy confirms a power of 0.0895 and a cycle of O.25
(8)Equivalent to 10hr per week

Major energy consuming items such as ovens and hot plates are not included in the above table primarily because of the difficulty in assigning a daily use time. Each item, oven, grill, hot plate has a different power rating, a different on-off cycle, depending on the set temperature or whether or not the item is being used to boil, simmer or re-heat, consequently it is very difficult to calculate the energy contribution of individual items. An alternative approach is to measure the total energy used in the preparation of a meal which is very easy to do since we know when the preparation of the meal starts and end and so we can use the following method and read the electricity meter before preparation starts and again when the meal is ready to determine to total energy used to prepare the meal.

Method 2

This method requires easy access to the electricity meter.

  1. Turn of all the isolators (trip switches) in the consumer unit (electrical distribution box) and check that the meter shows zero consumption i.e. the rotating disc is stationaery, of the led light is not blinking.
  2. Make sure the item to be measures is off and then switch on the isolator that controls the item to be measured.
  3. Check that the meter still shows zero consumption. If the meter is showing power usage then check all the items that may be controlled by the isolator you switched on are in fact switched off. Once the meter shows a zero consumption then you will know that any power usage measured in the following steps will be for the unit of interest only and nothing else.
  4. Make a note of the meter reading and the time.
  5. Switch on the unit you wish to measure.
  6. Leave unit switched on for 15 minutes then switch it off
  7. Make a note of the meter reading and the time.
  8. The difference between the 2 meter readings will be the power in kWh used by thr item.
  9. Divide the power used in kWh by 4 (15 minutes = 1/4 hr) to find the power rating of the item in kW.

To estimate the power used in preparing a meal proceed as outlined above but extend the measuring period from the start of preparation to the completion of the preparation. The difference between the two meter readings will be the total power used in the preparation of the meal.

Using this method it does not take long to realise the energy economies of 'one-pot-meals'.Yes the energy requiremnent of a 'micro-wave dinner for one' is very low but not the purchase price so the question remains do you want to save energy to be 'green' or to save money? the choice is yours.

More information on the energy-costs associated with cooking is presented in the section on Energy Saving

Energy Monitoring

The simplest method of energy monitoring is to review the data contained in your utlility bills! Adding the data to a simple speadsheet allows you to spot trends and to review your progress in energy-saving as shown below.

Historical Electrical Power Usage

If you would like to have live information on energy usage then take a look at the section on 1-wire technolgy where a pulse counter is affixed to the electricity meter and the 1-wire system logs the number of pulses every minute to provide live feedback on energy consumption

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