Hot water cylinder?

Tamzin from Auckland has been trying to save power by switching off their hot water cylinder at night. “I am very keen to find out the real deal re hot water savings. I too can check daily usage online -BUT after alternating off/on for the hot water cylinder am now using MORE power. This does not make sense to me or my provider. I have tried on at 6pm and off at 9am, but there seems to be a meter reading surge once the power is switched back on the cylinder. Anyone know what’s going on? I need to get this sorted to stop wasting power when not needed.”

Oily rag ed replies: Good question. The answer seems to come down to a trade off between the daily cost of keeping the water in the cylinder up to temperature, and the one-off cost to bring the cylinder back up to temperature when it is turned on again. A factor is the type of cylinder you have. Modern hot water cylinders have a higher R-value and cool down at a slower rate than those with a lower R-value. In other words, the lower the R-value, the more benefit to be gained from turning off your cylinder because the greater the cost of maintaining the water temperature when it is on (but a greater benefit would be gained by wrapping an old cylinder).

One could get pretty technical about this and quote Newton’s Law of Cooling but all said and done, the Oily Rag Rule of Thumb is that if you have a modern cylinder its not likely to be worth your while unless you are heading away for about a week.

The best way to save money on hot water is to wrap cylinders that have a low R-value, set the thermostat temperature below 60 degrees, fix leaky taps and faulty valves, and use low flow shower heads.


  • MEC says:

    We made a huge difference to our power bill by turning our hot water cylinder off during the DAY! It gets turned off in the morning and back on at bedtime, and it has saved us about 20 – 25% per month.

    It does help that the cylinder is well-insulated and set to 55C (the recommended maximum), and measuring the temperature at the end of the day shows it does not get below about 45C — not a lot of reheating needed.

    It’s worth a crack.

  • says:

    Please turn your cylinder up to 60C. Your cylinder should at least reach 60C everyday to kill the harmful respiratory bacteria Legionella which causes pneumonia. This bacteria can happily grow at 45C, the temperature of 55C mentioned is the temperature the water should be at the mixing tap, NOT you cylinder.

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