Fridge/freezer efficiency - should you keep them well stocked?
I’ve had a request from a reader to look into what could turn out to be a popular misconception. The question that they put to me was whether it is really true, as some people believe, that fridges and freezers are ‘more energy efficient’ when full?
I guess, in part, this comes down to what is meant by ‘energy efficient’, but the crux of the issue is whether it’s true that a full fridge or freezer uses more electricity than one pretty much empty. If so, this would seem to be counter intuitive (which is probably why the notion has spread, it has the ’surprising’ factor). After all, you’d reason, it takes longer to boil a full kettle than a one-cup-full kettle, and longer to freeze a big joint of meat than a tray of small ice cubes.
I can see some possible truth in the idea that a full freezer will, once cooled to the temperature on the thermostat, be less prone to suddenly warming up when you open the door than an empty freezer. But this is surely simply because opening a freezer door exposes it to a certain amount of heat, and with a full freezer this heat will be spread out amongst a bigger surface area of frozen foods. It must still have taken more energy in the first place to freezer the larger amount of food.
If my science is completely off, let me know.
Energy efficiency is a complicated issue (probably), and whilst I want to keep this blog on-topic to the discussion of popular misconceptions, it’d be good to get to the bottom of this question.
A related topic that I’ll return to in the future, which I also suspect could be a popular misconception, is the amount of electricity that appliances uses when on standby. Because I’ve heard people claim that it can be as high as 90% of the amount used when on full power, which can’t possibly be true, surely?
on May 24th, 2007 at 1:34 pm
It seems this theory is repeated in almost every home efficiency website, including official ones. A typical example is biggreenswitch.co.uk, who state: “a half full fridge can use 20% more energy than a full one”, others suggest filling an empty fridge with jugs of water or newspaper.
Of course, repetition of a ‘fact’ does not make it become true, At best, the freezer full theory is an exaggeration of the truth, but It would be better called a myth, for its not really helpful advice and could in fact lead to using more energy, not less.
Quoting vesma.com:
“It’s sometimes said that a full freezer uses less energy - not true. When the door is closed and the load is at the required temperature, all the refrigeration circuit is doing is removing heat that leaks into the compartment through the insulated walls. The freezer could be full or empty - the rate of heat transfer will be no different. It will only change if the room temperature changes. You can detect that it must be dubious advice because of what people suggest for filling the space: bread (high mass and volume); empty ice-cream cartons (low mass, high volume); and crumpled newspaper (low mass, low volume). The only thing they have in common is they impede air circulation (of which more later).
Some people say that by keeping the freezer full, you reduce the amount of chilled air that can spill out. Maybe, but door openings account for only 1 - 2% of annual running costs so the effect is small… In fact one kilowatt-hour of electricity would rechill all the air in a 100-litre freezer about a thousand times.
Much more important in causing high running costs are the following:
Having damaged door seals
Allowing the condenser coil on the back to get dirty
Obstructing air flow over the condenser coil
Not cooling produce before putting it in
Allowing ice to build up inside
Siting the freezer in a warm place
Freezing stuff that then gets out of date
Opening the door too frequently
Keeping the door open too long
This last one is the killer: that’s exactly what happens if you keep the freezer full; you need to keep rummaging around to find what you are looking for. But there is an even more compelling technical reason why efficiency falls and running-costs rise if the freezer is chock-full. In a full freezer, air cannot circulate properly, which impedes the thermostat’s operation and causes inefficient operation because the refigeration compressor runs on after it should have stopped cooling.
This myth is almost universal and is promulgated by organisations that should know better. So where did it come from? It probably started with the perfectly true observation that a larger freezer uses more power, so it is unwise to buy one that is bigger than you need. Related to this is the idea that the running costs per pound stored are lower for a freezer that is fuller; but this “operational” efficiency has then been confused with thermodynamic efficiency, which is affected by a lot of things, but not by the loading.
Conclusion: store as much or as little as you need; but leave room for air to circulate. ”
I will end with one more comment. What does it mean to ‘use up energy’ in this context anyway? A freezer is a heat pump, basically chucking out heat in the freezer to the space outside. Running the heat pump also requires its own energy power, most of which is also ‘lost’ as heat. But don’t our houses need heating most of the time anyway!