FRIDGE ENERGY USAGE

Until fairly recently, makers of electric fridges indicated how much energy their products typically used per day. This information was only of marginal value however as there was, and as far as I am aware still is, no Australian standard method of measuring fridge performance. Makers’ data was thus vaguely useful for comparing different fridges made by the same manufacturer – but not for comparing brands. Now, most makers quote the current drawn whilst the fridge is running. This is more useful but it unfortunately appears to be used as a marketing tool that can mislead. What is happening is that buyers not unreasonably infer that if fridge A draws 2.5 amps, it will necessarily use less energy than fridge B that draws 3.0 amps.

Marketeers and many salespeople push this line (i.e. that the lower the current, the less the energy draw) very strongly. But the assumption that fridge A draws less current in a day than fridge B is not necessarily valid. The reason is that fridges cycle on and off continually – and the ratio of ‘on’ time compared to ‘off’ time can and does vary a great deal

An On/Off Cycle

Unlike for example a saucepan being heated over a gas ring – where the gas is adjusted to provide the heat required - almost all fridges apply full power to the cooling bits until the internal thermostat reaches the desired temperature. The power is then totally cut off until the inside temperature drops to a degree or two below the preset temperature – at which point full power is turned back on again.

The fridge’s on/off cycle varies with ambient temperature, thermostat setting etc, but also with the effectiveness of its heat insulation, door seals etc. Because of the latter two factors, a less-well heat insulated fridge that draws 2.5 amps but is ‘on’ for a total of 12 hours a day (i.e. 30 amp.hr/day) is not as energy efficient as a better insulated one that draws 3 amps, but is ‘on’ for only 8 hours a day (24 amp.hr/day).

To know daily energy draw we do need to know the energy drawn whilst it’s ‘on’. But we also need to know the typical ratio of ‘on’ to ‘off’ and to do that requires standardised operating conditions that do not appear to exist.

In practice, and with exceptions such as Auto fridge and Indel (both of which work on an inherently ultra-efficient eutectic principle), most motorhome electric fridges are very similar in efficiency. There’s heaps of campfire mythology about performance and energy usage, but when users recount their experiences, what they are mostly talking about is the effectiveness or otherwise of their respective fridge’s installation and mode of use – rather than its intrinsic performance. And by and large, most fridge installations are a country mile short of optimum.

The Auto fridge is a very good example of how one can fall into error when looking at performance data. This fridge draws about 5.0 amps but, in its most efficient mode of working, it draws current for only an hour or two each morning and night. Thus whilst it draws close to twice the current of most other fridges the same size – its daily consumption can be less than half. Generalising, chest-opening fridges use less energy than door opening fridges; big fridges use more energy than small fridges. But as surface area, and hence heat loss, becomes proportionally less as volume increases, big fridges do not use proportionally more energy than small fridges.

Few Major Differences

In practice, the energy used is more related to installation and usage than inherent differences between brands. Excluding eutectic fridges there is unlikely to be more than 10% or so difference between most of the same type and size. Differences in installation (particularly of ventilation and electric cabling) however may cause one fridge to use twice or more the daily energy of another identical unit. In extreme cases such as a very badly installed fridge used in hot climate, it may not even cycle at all – it never reaches the thermostat setting and is ‘on’ the whole time.

I have personally seen one T-rated Dometic fridge so badly installed that it stopped working at an ambient of about 28 degrees C – rather than the 43 degrees C of which it is readily capable. This fridge was totally unventilated (no air vents top or bottom). It was located directly under a skylight so the sun shone onto its top – and tightly up against a big gas oven. It was connected (for 12-volt operation whilst driving) with the 17.5 amp draw carried by cable I would hesitate to use for five amps. There was just over 11 volts at the fridge with the vehicle’s engine running (a drop of over three volts!) Sadly, the owners were bad-mouthing the fridge rather than the turkey that had installed it.

Choosing and installing fridges was covered in my two-part series published in The Wanderer in 2003. The articles are also on the CMCA website and in the ‘Articles’ section of my own website. It is also covered in my books ‘Solar That Really Works’ and current editions of ‘Motorhome Electrics’.

Biofuel Follow-up

On a different note, I was surprised by the nature of the response to the piece I ran on biofuel a few issues back. It was a summary of an article (originally published in the British ‘Guardian Weekly’) written by an acknowledged expert in the field. He postulated that, whilst biofuel is fine at low levels of usage, the huge land areas required for global usage would prejudice the growth of food in third world countries.

My first surprise was that by presenting this alternative position, many readers appear to assume that I necessarily subscribe to this position. I don’t have a position regarding this. I’m simply advising there is an alternative viewpoint backed up by data now generally accepted to be correct.

The second surprise was that virtually every respondent attacked the motives and/or the integrity of the original writer. None addressed the issue of whether his view had any merit.

Several people felt that biofuel would offer an alternative to buying from oil companies. This belief, with respect, is naive. If biofuel sales became substantial the oil industry would inevitably take it over: with their existing global distribution it would be close to impossible for newcomers to compete.