Avoiding "Just this"

Wrong Track: That's hottest, so it costs the most to run

Wrong Track: That's the brightest light, so it costs the most to run

Wrong Track: The freezer is on for longest, so it costs the most to run

Right Lines: Find out how much water in the bath and how hot the water is to work out the cost of a bath

Right Lines: You need to find out how much power the device demands when it's turned on and how long it's turned on for to find out how much it costs to run.

Judge carefully, including all facets

Latching onto one salient property as a reliable guide. From Piaget's classic investigations on conservation of volume, research has identified a tendency to fix on one sensible feature of a situation to reason about a quantity. Children in Piagets investigations latched onto the height of the fluid in a container, using this as a proxy for volume.

Similar salient features of situations or processes are common in powering a home. Careful thinking — often reimagining am the situation or process — will be needed to avoid this tripwire.