A fridge-freezer is modest moment to moment, but it is the one appliance that never switches off, so it quietly runs up a year-round bill. Small improvements to how it works repay themselves every single day.
Get the temperatures right
Colder than necessary just wastes energy. A fridge sits happily around three to five degrees and a freezer around minus eighteen; many people run them colder than that out of habit. Nudging an over-cold fridge up a degree or two saves energy without any risk to the food, so check the dial against a cheap fridge thermometer.
Defrost and clear the coils
Frost is an insulator in the wrong place: a freezer caked in ice has to work harder to stay cold, so defrost it before the build-up gets thick. At the back or underneath, the condenser coils gather dust that stops them shedding heat efficiently; a gentle vacuum once or twice a year keeps them clear and the motor running less.
Seals and placement
A perished door seal lets cold leak out and the motor run constantly. Test it by closing the door on a sheet of paper: if it slides out easily, the seal needs replacing. Placement matters too, since an appliance jammed next to the oven or in a sunny spot has to fight the heat. Give it a few centimetres of air around the back and keep it out of direct warmth.
Full, but not stuffed
A reasonably full freezer holds its cold better than an empty one, as the frozen mass acts as a buffer, so it cycles on less. A fridge, on the other hand, wants air to circulate, so do not cram it. And when it comes to replacing a very old unit, a fridge-freezer from twenty years ago can use several times the electricity of a modern efficient one, so the running-cost saving alone can justify the swap.