Surplus can go to the cylinder or to the battery, and they are not equivalent

Two ways of catching the same surplus

When an array produces more than the house is drawing, the default is that the surplus leaves through the meter. Two pieces of equipment exist to prevent that, and they do it in completely different currencies. A diverter senses the export and sends it to an immersion heater in a hot water cylinder, storing it as heat. A battery stores it as electricity, to be given back later as electricity.

The comparison is usually framed as cheap-and-limited against expensive-and-flexible, which is broadly right and skips the interesting part. What actually separates them is what the household does with the energy afterwards, and whether the house has a cylinder at all.

A house with a combination boiler and no stored hot water cannot use a diverter at all without installing a cylinder, at which point the comparison changes completely. That single fact settles the question in a lot of homes before any arithmetic starts.

The diverter has almost no losses to speak of

An immersion heater is a resistive element, and resistive heating converts electricity to heat at essentially unity. There is no conversion penalty, no inverter stage, no chemistry. What goes in as a unit of electricity arrives in the water as a unit of heat, and the only losses afterwards are through the cylinder insulation over the following hours.

A battery cannot match that on efficiency. Every unit stored passes through conversion in and conversion out, and round-trip efficiency takes its share on the way. That penalty is unavoidable and it applies to every single unit.

So on pure energy-in to energy-out, the diverter wins comfortably. The catch is what the energy has become. Heat in a cylinder can only do one job, and once the water is hot, the diverter has nowhere to put the next kilowatt. The battery can supply the kettle, the lights, the freezer and the car, and that flexibility is what its cost buys.

Displacement is what determines value

The saving from either device is not the energy stored. It is the cost of whatever that energy displaced, and this is where households differ enormously. A diverter saves you the fuel that would otherwise have heated the water. If the cylinder is heated by gas, the diverter displaces gas, which in most markets is cheaper per unit than electricity, so the saving per stored unit is modest.

If the cylinder is heated by an immersion on the standard supply, the diverter displaces electricity at the full import price, and the same stored unit is worth considerably more. If it is heated by a heat pump, the diverter displaces a fuel that arrived with a multiplier attached, since the heat pump delivered several units of heat per unit of electricity, and the diverter delivers one for one. That case is genuinely poor.

A battery, by contrast, almost always displaces imported electricity at the evening price. Whether that beats the diverter depends on the household’s water use, its heating fuel and its tariff structure, and the answer is not the same in two houses on the same street.

Volume is the diverter’s hard limit

A cylinder holds a fixed quantity of water and raising it from cold to a useful temperature takes a fixed amount of energy. Once that is done, the device stops mattering for the rest of the day, and on a long clear summer day the water is usually hot by early afternoon.

That ceiling is low relative to what a decent array produces in midsummer. It is also seasonally backwards, because the ceiling is reached most easily in the months when generation is highest, and it is barely approached in winter when there is little surplus to divert.

Cylinder losses matter too. Heat stored at midday leaks away steadily, and by the following morning a good deal of it is gone into the airing cupboard. Storing heat overnight is a leakier business than storing electricity overnight, though a well-insulated modern cylinder is far better at it than an old one.

Which to choose, and when to choose neither

For a house with a cylinder and a meaningful daytime surplus, a diverter is one of the least regrettable purchases in this field. It is inexpensive relative to storage, it has few components, and its saving is small but reliable. It does not transform a bill.

A battery is a larger commitment and it earns its place through the tariff rather than through the hardware, which is a subject in its own right. Where it clearly beats the diverter is in a household with high evening electrical demand, a wide gap between import and export prices, and no gas.

And there is a case for neither, which deserves stating plainly. A household that is home during the day, consumes most of what it generates as it generates it, and has a well-matched array may have very little surplus to capture. Buying equipment to catch a surplus that barely exists is spending to solve a problem you don’t have, and the honest first step is to look at how many units are actually being exported before buying anything to stop them.