Saving Power at Home & Work

Medium Fridge

fridgeprofileThe graph above shows the way a normal fridge uses electricity. The fridge thermostat was set to position 4 out of 8 (midway between the coldest and warmest settings) during the test. You can see a number of key characteristics:

Fridges cycle between two different states continually; one in which they draw very little power (about 30 Watts in this case) when the compressor isnt running and no cooling is taking place, and the other in which the compressor is running.
When the compressor is off, the fridge is a completely resistive load, meaning that current and voltage are completely in phase and thus Watts consumed are exactly the same as the Volt-Amps (VA) consumed. In this test the compressor switched off for about 15 minutes at a time, before switching back on again.
The compressor is a highly reactive load because when its running the VA shoots up to more than twice the value of the watts consumed. The relationship between real power (Watts) and apparant power (VA), is defined by the ratio between them called the power factor (which reached a minimum of 42% in this test). Unfortunately this concept is fairly technical. The significance of the much larger value of VA for the average home owner is that if you are selecting an alternative source of power you need to check that it can provide sufficient VA over and above its power rating in Watts or kilo Watts (in this case the maximum values recorded were 145W vs 295VA).
The effect of a hot afternoon combined with some opening and closing of the fridge door can be seen in the length of time the compressor ran before switching off during elapsed hours 0 – 12. Once the fridge had cooled down at about midnight, one can see that the compressor only needed to run for a much shorter interval each cycle before switching off.

The summary figures for the test are:

Max. Power drawn 145W
Min. Power drawn 30W
Average Power drawn 102.6W
Max. VA 295VA
Min VA 29VA
Average VA 220.5VA
Min. Power Factor 41.7%
Average Power Factor 55.8%
Total energy consumed in 24h cycle 2.47kWh

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