How a solar diverter works
Without a diverter, surplus solar electricity flows out to the national grid, earning you a Smart Export Guarantee payment of roughly 4–15p per kWh depending on your tariff. With a diverter, that surplus instead powers your immersion heater — effectively heating water at a cost of zero pence per kWh, replacing gas or electricity that would otherwise have been purchased at 3–12p/kWh (gas) or 24–30p/kWh (electricity).
The diverter connects to the household's electricity supply and monitors the flow of power at the consumer unit (fuse board). When it detects that generation is exceeding consumption — i.e. that surplus is about to be exported — it activates the immersion element in the hot water cylinder and diverts exactly as much power as is surplus, continuously modulating to track the available excess. This is called proportional or triac control, as opposed to on/off switching.
A correctly set-up diverter does not consume any more solar generation than you would otherwise export. It only diverts what would have left the building.
Popular diverter products in the UK
The Immersun and the Marlec Solar iBoost are among the most established UK diverter products. Both use current transformers (CTs) clipped to the household supply cable to monitor power flow without interrupting the circuit.
The Eddi from myenergi is a more recent and more capable product that can also communicate with myenergi's Zappi EV charger and Libbi battery, allowing a single management system across multiple diversion loads.
Most diverters require a standard copper hot water cylinder with a compatible immersion heater (most UK cylinders have one). They are not compatible with combination (combi) boilers, which do not have a storage cylinder.
The financial case
A typical 4 kWp system in the UK generates 3,000–3,400 kWh per year. A household that self-consumes 50% and exports the remainder exports roughly 1,500–1,700 kWh annually. Without a diverter and with gas-heated water, that export earns perhaps £150–£200 at mid-range SEG rates.
With a diverter, a significant proportion of that exported electricity heats water instead. A typical UK household uses 1,500–2,500 kWh per year heating water. If the diverter offsets 1,000 kWh of gas-heated water at 10p/kWh, the annual saving is £100 in avoided gas costs — plus avoided grid export at SEG rates is foregone, but the net position is typically similar or better depending on gas/SEG rate differential.
The strongest financial case for a diverter is for households on electricity-heated hot water (an immersion heater as primary water heating). Replacing 30p/kWh grid electricity with 0p/kWh solar electricity for water heating creates a significant annual saving. For gas-heated households the benefit is smaller but still meaningful — and the hardware cost (£200–£400 installed) typically pays back in two to four years.
Diverters vs batteries — which is right?
Diverters and batteries solve different problems. A diverter redirects surplus to a single fixed load (hot water) and is simple, cheap, and maintenance-free. A battery stores surplus for use at any time — including powering lights, appliances, EV charging, and anything else — but costs £3,000–£8,000 installed.
For most households without an existing battery, a diverter is the highest-return first step for increasing self-consumption. It costs less than a tenth of a battery installation and addresses the most predictable surplus load (hot water) efficiently.
Some households add a diverter first and a battery later. A well-managed system can use both — the battery prioritised for evening consumption, and the diverter as a secondary destination for any remaining surplus that would otherwise be exported.
If you already have solar panels and a hot water cylinder and are looking for the simplest way to reduce your energy bills, a diverter is worth investigating before committing to a battery.