What a diverter does
A solar diverter — sold under names like Immersun, iBoost, and myenergi Eddi — monitors the flow of electricity at your meter. When it detects that your panels are generating more than your home is consuming (i.e., you are about to export to the grid), it diverts that surplus to your immersion heater instead, heating your hot water cylinder for free.
Diverters are simple, reliable devices with no moving parts and no battery chemistry to degrade. Installed cost is typically £200–£400 including the immersion element if needed. They work with any existing solar installation and any immersion cylinder. Payback is usually 1–3 years.
The limitation is narrow: a diverter can only redirect surplus to resistive heating loads — primarily immersion heaters. It cannot power your kettle, TV, or EV charger from stored electricity, and it provides no backup power. If you do not have a hot water cylinder (common in homes with combi boilers), a diverter has nowhere to send the surplus.
What a battery does
A solar battery stores surplus generation as electricity, making it available for any use later in the day — lighting, appliances, EV charging, anything. It is a fundamentally more flexible device than a diverter, but at 10–25 times the cost.
A battery also enables time-of-use tariff optimisation: charging from cheap overnight grid electricity (5–10p/kWh on Octopus Go or similar) and discharging during expensive peak periods. This additional use case can significantly improve the financial return beyond solar self-consumption alone.
The payback on a battery without time-of-use tariff optimisation is typically 12–16 years — close to or beyond the warranty period. With an EV and a smart overnight tariff, payback can fall to 7–10 years, which is within warranty.
The right order of decisions
If you have a hot water cylinder, start with a diverter. It is cheap, effective, and pays back quickly. You can add a battery later without removing the diverter — they work well together, with the diverter handling the hot water load and the battery handling the rest.
If you have a combi boiler and no hot water cylinder, a diverter is not an option. In this case a battery is the primary way to increase self-consumption, and the financial case should be modelled carefully before committing.
Neither product is the right first step if your panels are dirty. A soiled array exporting 1,500 kWh per year that could be generating 1,800 kWh with a clean costs you money on both the generation and self-consumption sides. A professional clean should always precede a decision to add storage or diversion equipment.
Summary comparison
Diverter: £200–£400 installed · payback 1–3 years · redirects surplus to hot water only · requires hot water cylinder · no battery degradation · compatible with any solar installation.
Battery: £4,000–£9,000 installed · payback 7–16 years depending on tariff/EV · stores electricity for any use · enables time-of-use arbitrage · battery degrades over time · warranty typically 10–15 years.
For most homes with a hot water cylinder, the diverter first and battery later approach is financially optimal. For homes fully committing to electrification (heat pump + EV), a battery from the outset makes more sense.