A timer is cheaper. A 200 litre cylinder loses roughly 0.8–2 kWh a day standing, and every one of those kilowatt-hours is paid for again if the tank is held hot around the clock. Heating once or twice a day, timed just before use, typically cuts hot water energy by 10–20% without ever running out.
Is it cheaper to leave the hot water on or use a timer?
Use a timer. The physics is not in dispute: a hot cylinder loses heat continuously to the room, and the loss is proportional to how hot it is kept and how long it is kept there. Hold a tank at 60 °C for 24 hours and you pay for a full day of standing loss. Let it cool between draw-offs and you pay for less, because a cooler tank loses heat more slowly.
The persistent myth — that reheating from cold costs more than topping up — confuses energy with power. Reheating uses a burst of power, but the total energy needed to raise a given volume by a given number of degrees is fixed. Water heating is the second-largest energy use in European homes after space heating, at roughly 15% of household energy consumption (Eurostat), so a 10–20% cut is worth having.
How much does a tank lose just standing there?
More than most people expect. EU Ecodesign rules cap the standing loss S of a hot water storage tank at 16.66 + 8.33 × V^0.4 watts, where V is the volume in litres. That is the legal ceiling, not a target — good cylinders come in well under it.
| Cylinder volume | Ecodesign ceiling | Energy per day | Well-insulated tank | Energy per day |
|---|---|---|---|---|
| 150 L | 78 W | 1.9 kWh | ~31 W | 0.74 kWh |
| 200 L | 86 W | 2.1 kWh | ~34 W | 0.82 kWh |
| 250 L | 93 W | 2.2 kWh | ~37 W | 0.89 kWh |
| 300 L | 98 W | 2.4 kWh | ~40 W | 0.96 kWh |
An older cylinder with a loose jacket or bare pipe tails can lose 3–4 kWh a day. Over a year that is well over 1,000 kWh — comparable to the entire hot water demand of a careful two-person household.
When is leaving it on actually the right answer?
Three genuine exceptions, and they all have a reason behind them:
- Heat pumps. Do not run a heat pump cylinder in short bursts. One longer charge a day, timed for the warmest part of the afternoon or the cheapest tariff window, gives a better COP than several small reheats — but that is still a timed schedule, not permanent operation.
- Solar thermal. Deliberately let the tank cool overnight and do not reheat it in the morning. A cool tank at sunrise gives the collector somewhere to put its harvest and raises the solar fraction; a tank already at 60 °C at 8 a.m. wastes the whole day's sun.
- Off-peak tariffs and PV surplus. Charge on a cheap night rate or on your own solar surplus even if some of that heat is later lost. Cheap kilowatt-hours beat efficient expensive ones. A SG Ready input lets the tank respond to tariff or surplus signals automatically.
How should I set the timer?
Work backwards from when hot water is actually used:
- Two heating windows suit most households — one finishing just before the morning peak, one before the evening.
- Finish, don't start, before use. A cylinder that reaches temperature at 06:30 for a 07:00 shower loses almost nothing; one that starts at 05:00 loses two hours of standing heat.
- Shorten summer windows. Cold feed water is warmer, so the same tank needs less energy — and with solar thermal, the collector should be doing the work.
- Run the anti-legionella cycle weekly, not daily. One 60 °C pasteurisation a week is the standard recommendation for a domestic cylinder; a daily 60 °C charge is expensive and unnecessary.
- Do not fight the thermostat. Setting 70 °C to "make it last longer" simply increases standing loss and scaling.
What cuts the losses further?
Insulation and pipework, in that order. Modern cylinders — including the Solimpeks enamel-coated and hygienic ranges — come with factory-applied insulation — 50 mm polyurethane foam on most models — so an add-on jacket brings little; older tanks with a soft jacket often can. The bigger unclaimed saving is usually the first metre of pipe off the top of the cylinder, which sets up a thermosiphon loop that quietly drains heat all day unless it is insulated and fitted with a heat trap.
Beyond that, size the tank correctly in the first place — an oversized cylinder loses heat around the clock whether or not the extra volume is ever used. Sizing figures are in what size hot water tank do I need, and the cooling behaviour is set out in how long a hot water tank stays hot.
Frequently asked questions
Does it cost more to reheat a tank from cold?
No. The energy needed to raise a given volume by a given number of degrees is fixed regardless of how it is delivered. Reheating from cooler costs less overall, because a cooler tank loses less heat to the room in the meantime.
How many hours a day should the hot water be on?
Most households need two windows of one to two hours, timed to finish just before morning and evening use. A well-insulated cylinder holds temperature comfortably between them, so continuous operation buys nothing but standing loss.
Should I leave my heat pump hot water on all the time?
No, but do not run it in short bursts either. Schedule one longer charge a day in the warmest afternoon hours or a cheap tariff window; that gives the best COP while still avoiding 24-hour standing losses.
How much can a hot water timer save?
Typically 10–20% of hot water energy for a household that previously ran the cylinder continuously — roughly 150–400 kWh a year on a 200 litre tank. The saving is larger on older, poorly insulated cylinders.