Immersion heater and hot water tank: from cost to income

An immersion heater is a simple heating element in a hot water tank or buffer tank — and precisely that simplicity makes it one of a home’s most underrated devices when it comes to flexibility. The water in the tank holds its heat for hours, so it doesn’t matter exactly when the heater switches on. It can therefore run when power is cheapest and pause briefly when the grid needs it — and Energinet pays for exactly that kind of pause.

Why is the immersion heater so well suited to flexibility?

Three properties work together:

• The water is storage. A heated tank delivers hot water long after the power is cut. A short pause simply draws on the heat that is already there — nobody in the house notices.
• It responds instantly. An immersion heater can switch on and off immediately, with no wear concerns — unlike a heat pump’s compressor, which must be handled more gently.
• It is easy to control. The heater is controlled through its power supply, so there is no need to integrate with a manufacturer’s electronics.

The comparison with heat pumps and batteries is in the article on which device suits flexibility best — the water heater and immersion heater are the simplest and cheapest way in.

How much electricity does an immersion heater use?

That depends on the tank’s size, the household’s hot water use and whether the heater is the primary heat source or a supplement — for example to a heat pump or a solar thermal system. An immersion heater is pure resistance heating: it delivers one kilowatt-hour of heat per kilowatt-hour of electricity. That is exactly why timing matters so much: with an hourly-priced contract there is a big difference between heating the water in the cheap night hours and at the expensive evening peak. See your own price hour by hour on the electricity prices page.

How the immersion heater becomes income

The grid must be kept at 50 Hz at all times, and Energinet buys reserves such as FCR-D in daily auctions — one for every hour. An immersion heater can deliver exactly that product: a short pause of seconds to minutes frees up power for the grid, while the tank’s hot water covers in the meantime. A single household is too small to participate alone, but in a virtual power plant it counts — read the background in the article on grid balancing for households.

In practice it happens with HeatSync if you have a Metro Therm hot water tank made after 2010: a licensed electrician fits the control unit in about 20 minutes, and it needs 2.4 GHz wifi. If you already own a compatible Shelly device, signing up is free — there is no hardware cost to NM1.

Activations come in bursts — some days 5–6 of them, then perhaps ten days without, on average a few per week — and revenue is shared with you, with quarterly payout as an ordinary bank transfer — see what your heater can earn under calculate your earnings. FCR-D prices vary from auction to auction; past earnings are no guarantee of future earnings. In the web app you follow your devices, the activations and your payouts.

Get started in minutes

1. Check your tank. If you have an electric water heater or immersion heater, you are a candidate — the capacity is typically already in the house.
2. Have HeatSync fitted (or sign up your existing Shelly) — an electrician does it in about 20 minutes.
3. Let it work. NM1 bids the flexibility into Energinet’s auctions, and you follow along in the web app.

Start on the get started page — and see how earnings are calculated on the earnings page.