Your Hot Water System Is a Battery: Thermal Storage Explained

We tend to think of batteries as lithium boxes bolted to a wall. But a hot water tank is also a store of energy. When you heat the water, you are putting energy in. When you use the hot water later, you are taking it out. In between, an insulated tank holds that energy for hours.

That makes your hot water system one of the cheapest forms of energy storage you already own. You do not buy it as a battery, you buy it as a hot water system, and the storage comes for free. The trick is to charge it up when electricity is cheap or free, and draw on it when power would otherwise be expensive.

This reframing matters because it changes when you choose to heat. If a tank is just a "hot water maker", you heat whenever the water cools. If a tank is a "thermal battery", you heat it deliberately during the cheapest part of the day and let it coast through the expensive evening peak.

Using your hot water as a battery is straightforward:

1. Charge the tank when power is cheap or free, typically the middle of the day on rooftop solar, or inside a free-power window such as the Solar Sharer Offer.
2. Store the heat in the insulated tank through the afternoon.
3. Discharge in the evening, when the household actually uses most of its hot water for showers, dishes and laundry, and when grid power is usually at its most expensive.

The practical tool for the "charge" step is a timer or smart control on the system, exactly as covered in our solar soak guide. You are simply choosing to store energy as heat during the cheapest window.

Modern tanks hold their heat well, so a tank charged at midday will still deliver hot showers that night. The main thing to get right is tank size: it needs to hold enough for a day so a single daily charge is sufficient.

When lots of households treat their hot water as a flexible, shiftable load, something useful happens at the level of the whole grid. This is called demand response: shifting electricity demand to times when power is plentiful and cheap, and away from times when it is scarce and expensive.

Hot water has been used this way for a long time. Traditional controlled load tariffs (also called off-peak) let the network switch electric hot water on overnight, when demand was low. That was demand response built around cheap night-time coal power.

The modern version flips the timing. With abundant midday solar, the cheapest and greenest time to soak up power is now the middle of the day, not the middle of the night. Programs like the Solar Sharer Offer are, in effect, encouraging households to move that flexible hot water load into the sunny part of the day. Your tank becomes a tiny piece of grid-balancing storage, and you are rewarded with cheaper (or free) power for providing it.

Calling a hot water tank a battery is a useful mental model, but it is fair to be clear about the limits so nobody is misled.

• It only gives energy back as heat. A chemical battery can power your lights, fridge and everything else. A hot water tank can only give you hot water. It shifts one specific load, it does not run your whole house.
• It loses a little heat over time. Even a well-insulated tank slowly cools, so some stored energy is lost as standing heat loss. Heating close to when you need the water minimises this.
• Its "capacity" is a day, not a week. Practically, you charge it daily. It is not designed to store energy for days on end the way you might imagine a large home battery doing.

Within those limits, though, it is a remarkably cost-effective store. You are getting real load-shifting value out of equipment you were going to buy anyway.

If your hot water tank is a battery, a heat pump is the most efficient way to charge it. Because a heat pump moves heat rather than generating it, it stores the same amount of hot water using roughly a third of the electricity of a conventional resistance element.

In battery terms, that is like getting three times the "charge" for the same power drawn from the grid or your panels. It means a smaller amount of cheap midday power fills the tank, and it keeps a full reheat comfortably within any daily free-power cap.

Pair a heat pump (efficient charging) with a timer set to your cheapest window (smart scheduling) and a tank sized for a day's use (enough capacity), and you have a genuinely cheap thermal battery working for you every day.

Want a heat pump set up to work as a thermal store? Get free quotes from local installers.