Solar Diverter for an Immersion Heater: Is It Worth It?

How a solar diverter works

A diverter watches the electricity flowing at your meter, usually with a small clamp around the incoming supply cable. When your panels are generating more than the house is using, that surplus would normally be exported. The diverter sees it and sends that amount, and only that amount, to the immersion heater instead.

It does this by varying the power to the element rather than switching it fully on. If you have 800 watts spare, the element gets roughly 800 watts. If a cloud passes or someone puts the kettle on, the diverter backs off within a second or two so the house is not drawing from the grid to heat water.

When the water reaches the immersion thermostat setting, the element stops taking power, the diverter has nowhere to send the surplus, and export resumes as normal. The thermostat is still in charge of the temperature. The diverter only decides where spare power goes.

Is a solar diverter worth it for your home?

Four things decide it, and it is worth working through them before buying anything.

First, what the exported power is worth to you. A diverter uses power you would otherwise have sold. If your export rate is low, turning that power into hot water is good value. If you are on an export tariff that pays close to what you buy electricity for, the diverter gains little, and exporting may even be better.

Second, what currently heats your water. If the immersion is your only heat source, as in many London flats without gas, every unit diverted is a unit of day rate electricity you did not buy, and the case is at its strongest. If a gas boiler heats the cylinder, the diverter is replacing gas, which is cheaper per unit of heat, so the saving is smaller.

Third, how much surplus you really have. In late spring and summer a typical domestic array often produces more than a house uses in the middle of the day, and a diverter can do most of the water heating for weeks at a time. From November to February there is little spare, and the boiler or the timed immersion does the work as before.

Fourth, whether something else is already using the surplus. A home battery, an electric car on a daytime charger or a heat pump all compete for the same spare power. Most diverters can be set to give the battery priority, but if the battery takes everything, the diverter rarely runs.

• Strong case: electric only hot water, low export rate, no battery, decent summer surplus
• Middle case: gas boiler heats the cylinder, low export rate, no battery
• Weak case: high export tariff, or a battery or car that already uses the surplus
• No case: no cylinder, or a combi boiler with no stored hot water

What your cylinder needs before a diverter goes in

The diverter is only as good as the immersion it feeds. We see diverters installed on elements that were already struggling, and the owner concludes the diverter does not work when the element is the problem.

The element needs to be sound and not heavily scaled. In hard London water an element that has run for years can be crusted so badly that it takes far longer to heat the tank and eventually burns out. A diverter feeding low power into a scaled element for hours a day makes that slower still.

The thermostat needs to switch off at the right temperature. If it is stuck, the diverter will keep pushing power into water that is already too hot, and on an unvented cylinder the thermal cut-out then trips. If the diverter came with the immersion and the water suddenly got scalding, that is the first thing to check, and our guide on the hot water cylinder overheating covers what to do.

If the cylinder has two elements, it helps to know which one the diverter is wired to. Feeding the lower element heats the whole tank, which suits most homes. A dual immersion heater explainer covers how to tell which element is which.

On an unvented cylinder the immersion is part of the safety design, with a thermal cut-out that must stay in circuit. A diverter controls the supply to the immersion and nothing more. It must never be wired to bypass the thermostat or the cut-out, and replacement elements should be the manufacturer's own part for that model.

Keep the timer and the boiler

A diverter does not replace your other controls. In winter it will barely run, so the boiler programmer, or an immersion heater timer on an electric only cylinder, still has to make sure there is hot water when you need it.

Temperature matters too. On a dull week the diverter may warm the tank only part of the way. Water that sits lukewarm for days is exactly what Legionella likes, so the stored water should still be brought up to 60 degrees regularly by the boiler or a timed boost. Most diverters have a boost setting for this. Our article on what temperature should a hot water cylinder be explains why the number matters.

Size also plays a part. A small cylinder fills with heat quickly and then the diverter has nowhere to send the rest of the afternoon. A larger cylinder stores more of the surplus. If you are replacing a cylinder anyway and have panels, it is worth thinking about capacity with the diverter in mind, and our guide on how to choose a hot water cylinder covers the other factors.

Who fits it

The diverter itself is electrical work: the clamp at the meter, the unit, and its connection to the immersion circuit. That is usually done by an electrician or the solar installer.

The hot water side is where we come in. Before a diverter is fitted, or when one has been fitted and the water is not getting hot, we test the element, the thermostat and the cut-out, and tell you plainly whether the cylinder is in a fit state to use it. If the element or cylinder is the weak link, fixing that first saves you paying for a diverter that never delivers.

Call 020 4652 5111 if your diverter shows it is sending power but the water stays cold, or if you want the immersion checked before one goes in. Our rates are on the pricing page and agreed before we attend. An immersion heater repair visit covers the element, thermostat and cut-out on a meter.