Your Water Heater Is the Cheapest Home Battery You Can Buy
Home batteries promise to buy electricity when it is cheap and use it when it is dear. A hot-water tank does exactly that for the energy you spend on hot water anyway, with no conversion losses, no permits, and a controller that costs about as much as a pizza. Here is how to turn the tank you already own into storage, and what it is worth.
A tank of hot water is a battery
Heating 200 litres of water from 20 °C to 70 °C takes about 11.6 kWh. That is the same order of energy as a mid-size home battery, and the tank holds it for hours with only slow standby losses. If you heat the water in the cheapest hours of the day and let the thermostat hold it, you have bought your hot water at the night price and used it at the evening price. Nothing about your shower changes.
The difference is what it costs to get there. A battery has to be bought, installed by an electrician and, in many countries, registered with the grid operator. A water heater only needs a switch that knows the electricity price. If you want more storage, a second or larger tank is still a fraction of the price of a battery, and it never wears out from cycling.
Energy in a 200 L tank
11.6 kWh
heated from 20 °C to 70 °C
Controller cost
≈ 25 €
Shelly smart plug with metering
Conversion loss
0%
heat goes straight into water you would heat anyway
Battery vs water heater, side by side
Both shift energy from cheap hours to expensive ones. The table shows where a battery is better and where a tank simply wins.
| Criterion | Home battery | Water heater tank |
|---|---|---|
| Upfront cost | Several thousand euros installed for 5–10 kWh | You already own it; ~25 € smart plug or ~35 € relay |
| Usable storage | 5–10 kWh typical | 6–12 kWh for 100–200 L; add a tank for more |
| Round-trip losses | 8–15% lost charging and discharging, plus inverter idle draw | No conversion loss; 0.5–1.5 kWh/day standby loss that exists with or without a schedule |
| Lifespan | 10–15 years, capacity fades with cycles | 10–20 years, no cycle wear; element and thermostat are cheap parts |
| Installation | Electrician, often a permit or grid registration | Plug in, or one relay fitted by an electrician for hardwired heaters |
| What it can power | Anything in the house | Hot water only, but that is 15–25% of a household's electricity |
| Payback | Typically 8–15 years on price arbitrage alone | Weeks to a few months |
- Home battery
- Several thousand euros installed for 5–10 kWh
- Water heater tank
- You already own it; ~25 € smart plug or ~35 € relay
- Home battery
- 5–10 kWh typical
- Water heater tank
- 6–12 kWh for 100–200 L; add a tank for more
- Home battery
- 8–15% lost charging and discharging, plus inverter idle draw
- Water heater tank
- No conversion loss; 0.5–1.5 kWh/day standby loss that exists with or without a schedule
- Home battery
- 10–15 years, capacity fades with cycles
- Water heater tank
- 10–20 years, no cycle wear; element and thermostat are cheap parts
- Home battery
- Electrician, often a permit or grid registration
- Water heater tank
- Plug in, or one relay fitted by an electrician for hardwired heaters
- Home battery
- Anything in the house
- Water heater tank
- Hot water only, but that is 15–25% of a household's electricity
- Home battery
- Typically 8–15 years on price arbitrage alone
- Water heater tank
- Weeks to a few months
A battery is the right tool if you need to run the whole house on stored power or ride out outages. If the goal is simply to stop paying peak prices, start with the tank: it is the cheapest kilowatt-hour of storage in the building.
How the tank works as storage
Left to itself, a water heater reheats whenever the water cools below the thermostat setting, which means it also fires during the most expensive evening hours. With a price-aware switch in front of it, the heater is only powered in the cheapest window of the day. It heats to the thermostat's maximum in that window, then holds the heat until the next cheap window.
The thermostat stays fully in charge of temperature and safety; the switch only decides when power is available. Because the tank is well insulated, a full heat in the night still means a hot shower in the evening. Households that use a lot of hot water can add a second cheap window in the early afternoon, when solar pushes prices down in many markets.
Want more storage than one tank offers? Fit a larger tank, or a second tank in series, and set both to heat only in cheap hours. The extra capacity costs a few hundred euros, not a few thousand, and lets you skip the expensive hours entirely even on heavy-use days.
Set it up in five steps
From an ordinary heater to price-aware storage in about 15 minutes, plus an electrician visit if the heater is hardwired.
Pick the right switch for your heater
Plug-in heaters up to 2.5 kW (12 A) run through a Shelly Plug S Gen3: plug it in between heater and socket, done. Most tank heaters above 2 kW are hardwired; those get a Shelly Plus 1PM or Pro 1PM in the circuit, fitted by a licensed electrician. Both models meter energy, which is what lets Elewatt show real savings later.
Add your contract and a price filter in Elewatt
In Elewatt, add your electricity contract under Contracts so prices include your grid tariff and taxes. Then create a price filter under Filters: for a water heater, the usual choice is the N cheapest hours of the day, with N set to the time your tank needs for a full heat, typically 2–4 hours.
Connect Shelly to Elewatt
Under Integrations, open Shelly and connect your Shelly Cloud account. Your plug or relay appears in the device list within a few seconds. Elewatt reads its power and energy so the savings you see later are measured, not modelled.
Link the heater to the filter
Select your heater and assign the price filter you created. From now on, Elewatt rebuilds the schedule every afternoon when tomorrow's prices are published, and switches the heater on only in the chosen hours.
Per day
280.45 Ft
Per month
8413.50 Ft
Per year
102502.50 Ft
Set the thermostat and watch the savings
Set the heater's thermostat to 60–70 °C so the tank stores as much heat as possible in each cheap window, and fit a thermostatic mixing valve at the outlet if the tap water would otherwise be too hot. Then let it run; the calculator below shows what a typical tank saves, and the Elewatt dashboard shows what yours actually did.
Figures come from the calculator below; adjust it to your tank.
Safety and hygiene: three rules
Hardwired means electrician
Any heater above 2.5 kW, or one without a plug, must be switched through a relay installed by a licensed electrician. Never bypass the heater's own thermostat, pressure relief valve or overheat cut-out.
Keep Legionella out
Water stored between 25 and 45 °C is where Legionella grows. Heat the full tank to at least 60 °C regularly, ideally daily and at minimum once a week. Scheduling to the cheapest hours does not change this: the tank still reaches the thermostat setting each cycle.
Mind the tap temperature
Storing at 65–70 °C is excellent for capacity but scalding at the tap. A thermostatic mixing valve on the outlet gives 45–50 °C water from a hot tank, which is how most new installations are done anyway.
What does a scheduled tank save?
Enter your tank volume and temperatures. The calculator uses your country's real hourly prices from the last twelve months, your grid tariff and VAT, and compares heating in the cheapest hours against the most expensive ones.
Energia · Idő
11.6 kWh · ~6 h
Nord Pool átlag (2025. márc. – 2026. márc.) · c/kWh
Elewatttal
13.2 c/kWh
605.66 Ft
Elewatt nélkül
26.4 c/kWh
1211.81 Ft
Megtakarításod
Naponta
606.15 Ft
Havonta
18184.53 Ft
Évente
221245.17 Ft
Éves átlag Nord Pool árak, Észtország. 1 ciklus/nap, 2 kW melegítő feltételezve. Hálózati díjak: Elektrilevi.
Why Elewatt insists on metered energy
A tank heater only draws power while its own thermostat calls for heat, so relay-on time tells you nothing about consumption. When we compared 91 days of real meter readings from two water heaters against the usual nominal-power estimate, the estimate overstated consumption by 63%, and the two identical heaters differed from each other by 3.2×. That is why Elewatt only reports savings from devices with a meter, and why the Shelly models above all have one.
Turn your tank into storage today
Connect a Shelly plug or relay, pick a cheap-hours filter, and Elewatt heats your water when electricity is cheapest, every day, with metered savings you can check.
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