How do I run a dishwasher on a cheap electricity tariff?

The saving here is real but modest, and the arithmetic matters more than the habit. A dishwasher uses most of its electricity in two heating bursts — the main wash and the final rinse — and both fall in the second half of a long eco cycle. A cycle that starts in the cheap window but finishes outside it may draw most of its power at the expensive rate, which is the mistake that makes people conclude the tariff does not work.

Safety

Step by step

1. Find out what your cheap window actually is.Economy 7 gives seven off-peak hours, typically somewhere between 11pm and 8am, and the exact times vary by region and supplier. Modern time-of-use tariffs may have a cheap overnight block, a cheap midday block, and an expensive peak between about 4pm and 7pm. Check the tariff terms rather than assuming.
2. Check the peak rate too, not just the off-peak one.Economy 7 buys cheap nights by charging more during the day. If you only shift the dishwasher and nothing else, you may be worse off overall. The tariff has to suit the whole household load, not one appliance.
3. Time your actual eco cycle once.The display estimate is often optimistic and sensor programmes extend themselves. Run it once and note the real duration. Three and a half to four hours is common.
4. Set delay start so the cycle finishes inside the window.Most machines let you delay by a number of hours before starting. Work backwards from the end of the cheap window by the cycle length. Finishing at the end of the window rather than starting at its beginning is the correct way round.
5. Use eco, not a short programme.Eco is the cheapest programme on the machine and its length is an advantage rather than a problem when the machine is running while you sleep. There is no reason to use a fast cycle on a delay timer.
6. Set the machine to finish before you go to bed if you would rather not run it while asleep.This is the honest safety compromise. Starting a four-hour eco cycle at 7pm means it finishes at 11pm, still inside the evening, and captures a cheap late-evening block on many time-of-use tariffs even if it misses the deepest overnight rate.
7. Never run it while the house is empty.The saving is a few pence. A leak that runs unnoticed for eight hours under a fitted kitchen is not.
8. Confirm what you are actually saving before rearranging your evenings.Take the cycle energy in kWh from the energy label, multiply by the difference between your peak and off-peak rates in pence, and divide by 100 for pounds. For a 0.8kWh cycle and a 15p per kWh gap, that is 12p a cycle — about £40 a year if you run one every day. Worth doing, and worth knowing the size of.

Tips

• A smart plug with an energy monitor gives you the real consumption of the programmes you actually use, which the eco-only energy label does not.
• If you have solar panels and no battery, the cheap window is the middle of a sunny day, not the night. Delay start is just as useful pointed at lunchtime.
• Batch the shifting: dishwasher, washing machine and any immersion heater all moving to the same window is what makes a time-of-use tariff pay. One appliance rarely does.

Common mistakes

• Setting a delay so the cycle starts at the beginning of the cheap window — A four-hour cycle started at the start of a seven-hour window is fine; started at hour six it does most of its heating at the peak rate. Work backwards from the end of the window instead.
• Switching to Economy 7 for the dishwasher alone — The higher daytime rate applies to everything else in the house. The tariff only makes sense if a substantial share of your total consumption moves with it.

Questions people ask