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Updated : 16/10/2016

Energy Usage

Total Energy Usage

The first steps to reducing energy consumption are to understand how much energy is being used and for what purpose. The obvious primary source of this information comes from the electricity meter and the monthly municipal account. Neither the meter nor the municipal accounts are easy to use for this purpose for the following reasons:-
  • Many of the meters still in use are the older mechanical dial type which, with their alternating direction of rotation, are notoriously difficult to read even by professional meter readers as evidenced by the numerous errors encountered in the monthly accounts.
  • Not many people have the time or inclination to read their meter on a regular basis.
  • Municipal accounts are notoriously erratic because of the intermittent readings resulting in numerous 'interim' consumption values.
However, a review of past municipal accounts can provide a useful insight to consumption trends. The figure below has been compiled from my municipal accounts from 2006 to 2013 using only those accounts for which actual meter readings were given i.e. ignoring all 'interim values'. The data has been converted to a daily consumption reading by dividing the change in meter reading by the number of days between the dates of the readings. Historical Electrical Power Usage
From this data it can be seen that the daily consumption has been reduced from 55kWh per day in 2006 to 22kWh per day in 2013 an overall reduction of 60 per cent. In cost terms the daily cost of power has moved from R25 per day in 2006 to R29.70 in 2013, however without the reduced consumption the cost per day in 2013 would be R74.25. The reduced consumption is therefore providing a saving of R44.55 per day or R16 260 per annum.

Detail Daily Power Usage

Using a combination of a standard digital multi-meter (reading rms Volts and rms Amps and ignoring any power factor corrections) and an Efergy single point energy monitor wherever possible, the power and standby power of each item of electrical equipment in the houisehold was estimated. The normal 'on-time' for each item was measured, as was the duty cycle (ie ratio of on-time to off-time). The data collected was collated into the following table and ordered in descending order of kWh used per day.

For further information on energy measurement methods please refer to Energy Measurement

Item Current Standby kW Power kW Duty cycle hrs/day 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
Oven large Grill 12.174 0 2.800 0.250 0.000 0.000
Microwave oven 10.000 0 2.300 0.200 0.000 0.000
Oven large 9.565 0 2.200 0.250 0.000 0.000
Hob Large Ring (2) 9.130 0 2.100 0.250 0.000 0.000
Oven Small Grill 8.696 0 2.000 1.000 0.000 0.000
Sandwich Maker 7.826 0 1.800 1.000 0.000 0.000
Oven small 7.826 0 1.800 0.200 0.000 0.000
Microwave 7.174 0 1.650 0.200 0.000 0.000
Microwave Grill 6.957 0 1.600 0.200 0.000 0.000
Oil heater (1) 6.522 0 1.500 0.200 0.000 0.000
Toaster 5.500 0 1.265 1.000 0.000 0.000
Hob Small Ring (2) 5.217 0 1.200 0.250 0.000 0.000
Panel Heaters (2) 1.739 0 0.400 1.000 0.000 0.000
Old Freezer 1.400 0 0.322 1.000 0.000 0.000
ADSL 0.000 0 0.000 1.000 24.000 0.000
Telephone chargers (4) 0.000 0 0.000 1.000 24.000 0.000
Printer (HP 960C) 0.000 0 0.000 1.000 0.000 0.000
Printer (HP PSC 1400) 0.000 0 0.000 1.000 0.000 0.000
Total 20.245

The data in the above table is for a typical middle class family house comprised of four double bedrooms, dressing area, two bathrooms, lounge, dining room, study, kitchen, family room and laundry room. Additionally there are three garages a storeroom and a large (100 000 litre) swimming pool. The house is occupied by two people. The house is a single story constructed of brick with a tiled roof. There is no air-conditioning or electrical space heating and whilst there is standard (150 mm aerolite ) roof insulation there is no wall insulation or double glazing.
(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

Whilst the compilation of the above table involved numerous estimates of duty cycles it is worth noting that the final daily power consumption of 20.245 kWh is within 8 per cent of the average daily power consumption of 22 kWh as invoiced by the power supplier. The significance of this is that the above table in a spreadsheet programme enables a user to identify the items that consume the most power and look at various 'what if' scenarios to see the effect on the total daily power power usage and the possible savings on your power bill.

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