Is a dishwasher cheaper than washing up by hand?

The comparison is usually rigged in one direction or the other. Dishwasher manufacturers compare against a running tap; hand-washing advocates compare a full dishwasher against one plate. The variables that actually decide it are the number of litres of hot water you use, how hot they are, and whether the machine runs full. All three are things you can measure in your own kitchen in an evening.

Step by step

1. Get your dishwasher's real figures off the energy label, not off the internet.The label gives water in litres per cycle and energy in kWh per 100 cycles on the eco programme. Divide the kWh figure by 100 for one cycle. Both numbers are for eco — if you run a different programme, the machine's manual usually tabulates all of them.
2. Measure your own hand-washing water.Wash up as you normally would, but fill the bowl from a measuring jug and count the jugs. Do it for a rinse bowl too if you use one. Most people are surprised: a washing-up bowl is about 8-10 litres and two changes is 20 litres.
3. Measure a running tap if that is how you rinse.Time how long a 2-litre jug takes to fill from your hot tap, then time how long you actually run the tap during a wash-up. A kitchen tap delivers roughly 8-12 litres a minute. Four minutes of running hot tap is 40 litres and it is the figure that decides this comparison.
4. Convert hot water into energy.Heating one litre of water from about 10°C to 50°C takes roughly 0.047kWh. So 20 litres of hot washing-up water is around 0.9kWh at the point of use, before boiler or cylinder losses. That is the arithmetic — apply it to your own measured litres.
5. Apply your own tariff to both.Electricity in pence per kWh from your bill for the dishwasher. For hand washing, use your gas rate per kWh if you have a gas boiler, and add a real allowance for boiler efficiency — dividing the gas figure by about 0.9 for a modern condensing boiler is reasonable. Then add water and sewerage cost per cubic metre if you are metered.
6. Compare like with like: one full dishwasher load against the same crockery by hand.A full 14-place machine is genuinely a lot of washing up. Comparing it against one bowl of plates flatters the hand washing enormously.
7. Include the things the arithmetic misses.The dishwasher's final rinse is far hotter than hands can tolerate, which matters after raw meat. Against that, the machine costs several hundred pounds, occupies a cupboard, needs consumables and eventually needs replacing. Neither of those belongs in a per-cycle cost but both belong in the decision.
8. Do the cheap things regardless of which way you land.Run the machine only when full and on eco. If you wash by hand, use a bowl rather than the sink, do not leave the tap running, and wash in order from cleanest to dirtiest so one bowl lasts.

Tips

• If you are on a water meter, the water and sewerage charge is a real part of this and it is billed per cubic metre — check both rates, because sewerage is often charged at close to the same rate as supply.
• A plug-in energy monitor on the dishwasher socket for a week gives you the true figure for the programmes you actually use, which no label can.
• A dishwasher that is habitually run half full loses the argument immediately. If you are one or two people, a slimline machine run full beats a full-size machine run half empty.

Common mistakes

• Comparing the eco label figure against a real-world hand wash — The label measures the longest, gentlest programme. If you run a one-hour programme daily, your machine's consumption can be roughly double the label figure and the comparison shifts.
• Forgetting the standing cost of heating the water — Hot water drawn from a cylinder or a combi has losses on the way, and a cylinder reheats after every draw-off. The energy that arrives in your bowl is less than the energy the boiler used.

Questions people ask