The cheapest battery in your home is the boiler

A 100–150L hot-water tank stores 4–8 kWh as heat — about the same usable capacity as a small home battery. A €50 smart relay on the existing tank, scheduled to heat only during cheap spot hours, pays itself back within the first heating season. The cheapest battery in your home isn't a battery; it's the boiler you already own.

The boiler in most Estonian homes is, structurally, a battery. Nobody calls it that, partly because nobody thinks of hot water as energy storage. But that is exactly what it is — a tank of warm water with a heating element, holding a few kilowatt-hours of energy at a time, ready to give it back when the tap opens. The fact that hardly anyone treats it that way makes it the lowest-effort flexibility upgrade in the average house.

Hot water is the second-largest electricity load in most Estonian homes, behind space heating. Depending on family size and habits, it accounts for 10 to 25 percent of the annual bill. The boiler also has a useful property that radiators and EV chargers do not: it does not care when you heat it, only that the water is warm by the time someone turns the tap.

That decoupling between when you pay and when you use is the entire game on the day-ahead spot market. A typical winter day in Estonia has a 4-6x ratio between the cheapest night hour and the most expensive morning hour. A boiler running on a dumb thermostat reheats whenever the temperature drops, which on average means it draws power during the day, when prices are highest. A smart boiler heats overnight (usually between 02:00 and 06:00), coasts through the morning peak on stored heat, and tops up again in the cheap afternoon trough if needed.

The retrofit is almost embarrassingly simple

You do not need a new tank. You do not need a plumber. A WiFi smart relay rated for the boiler's current — a Shelly Pro or a Sonoff POW, typically 30 to 80 euros — wires inline with the existing power feed. From there, software decides when the relay closes. The tank, the heating element, the thermostat all stay exactly as they are.

Two heating windows of about 90 minutes each, placed at the two cheapest hours of the day, will beat a 24/7 thermostat by a wide margin without anyone in the house noticing. The control horizon you need is 24 hours; tomorrow's prices are published by Nord Pool every afternoon, which is plenty of warning to plan tonight's heating.

Compare the economics to a real battery. A 5 kWh lithium home battery in 2025 costs roughly 600 to 1100 euros per kWh installed. The thermal equivalent already exists in your boiler; the marginal cost to make it dispatchable is the price of one smart relay. Round-trip efficiency is lower than a lithium pack — call it 80-85 percent including standby losses — but the kWh you store comes back to you as hot water at retail price, not as electricity sold to the grid at wholesale. A 50-euro relay that saves 150 to 250 euros a year is, in our opinion, the highest-IRR thing you can do to a house in 2025. Nothing else comes close.

There are caveats. The tank has to be reasonably well insulated; an old, uninsulated cylinder in a cold garage will lose enough heat overnight to eat into the savings. Legionella risk means the water has to reach 60-65 °C at least once a week, which any serious controller handles automatically. And households with very irregular hot-water demand — guests, shift work — benefit from a controller that learns usage patterns rather than running a fixed schedule.

Volton Home runs this on the boiler the same way you would run it yourself if you had the patience to do it manually. A customer plugs a Shelly relay onto the tank, the app discovers it, and tomorrow's heating windows get scheduled against published spot prices each evening. Of every device the platform controls, the boiler retrofit pays itself back fastest — usually inside the first heating season.