Tuesday 14 February 2023

Avoiding the rated power fallacy

How much energy does an oven use? Today I have seen examples both on the BBC Food Programme and on the Uswitch website [1] where this question was answered by multiplying rated power by hours of use. This is totally wrong because ovens have thermostats. They turn the power off when they are up to heat. The power of the oven is mainly used during the initial heating phase. After that it turns down the heating and/or cycles on and off. 

Screenshot from Uswitch [1] As well as falling for the rated power fallacy Uswitch has confused watts (power) and watt-hours (energy).

I have seen the same mistake made with regard to heating systems; I can assure you a 24 kW combi boiler on for 8 hours/day does not use 192 kWh gas, currently around £20/day, regardless of the weather! Also tumble dryers and freezers ... All of these appliances use thermostats and the actual energy use will be many times less than a simplistic calculation using the rated power.

Example oven energy label.
The only way to find out how much energy these appliances are actually using is to measure it. For items that have a plug, a plug monitor is a simple approach (such as this one). Unfortunately ovens tend to be wired in so there is nowhere to put the plug.

However, ovens have energy labels. That applies to both gas and electric ovens, these days. The energy label on an oven tells you the energy use for a standard cycle. I have circled this on the example, right. For an 'A' rated oven this is typically a little less than 1 kWh, currently about 30p. Since a lot of the power is used during warm-up this is a fairly good guide to how much the cost will be in reality.

Can someone please tell me what an air fryer uses? They don't seem to have energy ratings. I do not believe the example below from Ideal Home. Once again, they assume the rated power is used all the time.

Screenshot from [2]. IdealHome has correctly used kW and kWh, which is nice, but they have fallen for the rated power fallacy.

I am more inclined to believe Julie Cook [3] because she seems to have measured the consumption. The article says she uses a smart meter for this. I find that surprising as it means turning off every other appliance in the house for the duration of the experiments. I would not do that except as a last resort - I would use a plug monitor for the air fryer. Anyway, Cook's results seem plausible and she still finds air fryers are more efficient. For example on the baked potatoes, the air fryer cuts the energy use by about half. This is certainly worth having, and a lot more realistic than other claims I have seen.

When browsing for information on energy consumption of appliances, watch out for the rated power fallacy and when you see it, ignore the results, or better still complain to the publisher.

[1] Energy Efficient cooking (Uswitch) 

Look for Calculate your oven’s hourly electrical usage

[2] How much does it cost to run an air fryer? Do they save money? (IdealHome) 

No comments:

Post a Comment

Comments on this blog are moderated. Your comment will not appear until it has been reviewed.