What does your water heater cost when you are not using it?

The subscription you did not know you had

A hot water tank has one job: to keep 200 litres of water at 65–75 degrees, around the clock, all year round. It does that job while you sleep, while you are at work and while you are on holiday. The heat seeps slowly out through the insulation, the element kicks in to compensate, and the electricity meter ticks over.

This is the standby loss: electricity you pay for without getting a single shower in return. In practice it works like a subscription, and as with most subscriptions it is worth checking what it actually costs.

A typical standby loss for a 200-litre conventional water heater is 1–2 kWh a day.The energy label for hot water tanks New, well-insulated units sit at the lower end. Older units, or units kept in a cold spot (in an uninsulated cellar or at the cabin, for example), sit higher.

The calculation

Assumption: an electricity price of 1,5 kr per kWh including grid rent and duties. Do adjust it to your own price; the direction is the same.

Note that this is the cost before you have drawn off a single litre of hot water. Heating the water you actually use comes on top, and that costs roughly the same whatever the technology: heating one litre of water from 8 to 65 degrees costs approximately 0,07 kWhCoax, own calculation. The difference between the solutions does not lie in the physics, but in everything around it: the tank sitting there leaking heat, and the pipes filled with water that goes cold.

At the cabin the sums turn silly

At home, the standby loss is at least spread across daily use. At the cabin it is not. A conventional water heater left on all year, in a cabin used for six weeks, loses 550 kWh for perhaps 40 days of actual use. That makes every day of use cost 13–14 kroner in pure standby, before anyone has had a shower.

The alternative, switching off and draining the tank between each trip, has its own calculation involving heat-up time and getting the water properly hot before a shower. We have run those numbers in the story of Per and Kari.

A flow-through heater does not have this dilemma: it uses 0 kWh when the cabin stands empty, and delivers hot water from the first minute after you arrive.

What can you do?

1. Find the figure for your own unit. The energy label states the standby loss (often as «static loss» in watts), or you can read the electricity meter over a day when the house is empty.
2. If you have a cabin or holiday home: work out what the water heater costs per day of actual use. This is often where a change pays off first.
3. If the unit is due for replacement anyway: do the sums before buying a new tank on autopilot. The consumption calculator helps you with your own usage, and the choose a model guide shows what suits your home or cabin.