Summary
Solar PV systems generate their most power in the middle of the day, when most households use the least electricity — occupants are at work, appliances are idle, and the kettle isn't on. Without intervention, that surplus is exported to the grid and paid for at the household's Smart Export Guarantee (SEG) rate, which is typically far lower than the price of the electricity the household would otherwise have bought to heat water. A solar diverter closes that gap by using the surplus to heat the cylinder instead of selling it back to the grid at a loss.
The concept is simple, but the electronics doing it are more sophisticated than a basic on/off relay. A diverter needs to react within a fraction of a second to changing surplus (a cloud passing over, a kettle switching on) and deliver precisely the right amount of power to the immersion element — not just full power or nothing — so it doesn't accidentally start importing expensive grid electricity to "top up" the immersion heater. This is done with phase-angle or burst-fire control, the same principle used in dimmer switches, applied to a resistive heating element instead of a lamp.
Diverters are a genuinely useful retrofit for existing solar households, especially since SEG rates dropped well below the retail price of electricity — self-consuming surplus power to heat water is usually worth more to the household than exporting it. But they are not a substitute for a correctly maintained hot water system: the cylinder's thermostat, safety cut-out and periodic full-temperature (legionella-safe) heating cycle must all keep working exactly as they would without a diverter fitted.
Key Facts
- CT clamp measurement point — the diverter's current transformer clamp is fitted to the incoming supply tails (or generation meter tails), measuring real-time import/export, not the PV output directly.
- Phase-angle / burst-fire control — the diverter chops or phase-controls the mains waveform delivered to the immersion element so the power sent matches the available surplus almost continuously, rather than switching the element fully on or off.
- Response time — good diverters react within roughly one second to changes in surplus, minimising both wasted export and accidental grid import.
- No DNO notification for the diverter itself — Engineering Recommendations G98/G99 govern the PV or battery generation system connecting to the grid, not a diverter added to redirect self-generated surplus.
- No standalone MCS requirement — MCS certification applies to PV/battery generation systems for Smart Export Guarantee eligibility; a diverter retrofit to an already-certified system doesn't need its own MCS sign-off.
- Element targeted — diverters are usually connected to the lower immersion element on a twin-element cylinder, since it heats the larger volume of water first and is the element normally used for off-peak/economy tariff heating too.
- Boost/manual override modes — most diverters allow a manual "boost" to heat the cylinder from the grid on demand (e.g. before a shower on a cloudy day), separate from their automatic solar-diversion mode.
- Single-phase vs three-phase installs — single-phase is the default for most UK homes; three-phase properties need either a three-phase-capable diverter or one unit per phase, since surplus can occur on one phase while another imports.
- Compatibility with battery storage — where a home battery is also fitted, the diverter and battery inverter both want to use the same surplus; priority order (battery charge first, then immersion, or vice versa) must be configured so they don't work against each other.
- App monitoring — most diverters report live diversion data (kWh sent to the immersion, estimated cost/SEG value saved) via Wi-Fi to a smartphone app, which is useful for customer-facing energy reporting but not essential to the diverter's function.
- Cannot replace the cylinder thermostat — a diverter supplements the immersion circuit; it does not remove the requirement for the cylinder's own thermostat and safety cut-out to function, and the water must still reach a periodic full-temperature (legionella-safe, typically 60°C) heating cycle regardless of solar input.
- Winter performance is limited — solar surplus is smallest exactly when heating demand is highest, so a diverter's contribution to hot water costs is heavily weighted towards spring, summer and autumn.
- Not a substitute for export limitation — where a DNO has imposed an export limit on a PV system, a diverter reduces the surplus available to export (helpful for staying under the limit) but is not itself an export-limiting device recognised for G99 compliance purposes.
Quick Reference Table
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Try squote free →| Feature | Typical specification |
|---|---|
| CT clamp measurement point | Incoming supply/generation meter tails (import/export) |
| Control method | Phase-angle or burst-fire modulation of immersion element power |
| Typical response time | Under ~1 second to changing surplus |
| Target element | Lower immersion element on twin-element cylinder |
| DNO notification | Not required for the diverter itself |
| MCS certification | Not required for the diverter in isolation |
| Manual boost mode | Yes, on most units — grid-powered override |
| Three-phase support | Requires three-phase-rated unit or one unit per phase |
| Battery storage interaction | Priority order must be configured (battery vs immersion) |
| Legionella-safe cycle | Must be maintained independently via cylinder thermostat, not solar-dependent |
| Best performance season | Spring–autumn; limited in winter (low surplus, high demand) |
| Typical Part P status | Domestic electrical work — competent person self-certification |
Detailed Guidance
How the diversion decision is made moment to moment
The CT clamp continuously reports the net flow of current at the property's supply — positive for import, negative for export. When the reading shows export (more solar generation than the house is currently using), the diverter's control electronics calculate how much of that surplus can be sent to the immersion element without tipping the house back into import, and phase-control the element's power supply to match. As household demand changes — a kettle switching on, a load finishing — the diverter continuously recalculates and adjusts within roughly a second, so in practice the immersion element output tracks the available surplus almost like a dimmer being turned up and down automatically.
Why phase-angle control matters over simple on/off switching
A simple relay-based system could only turn the immersion fully on or fully off, meaning it would either waste surplus that's below the element's full rated power, or import expensive grid electricity to make up the shortfall when switched fully on. Phase-angle (or burst-fire) control avoids both problems by delivering exactly the available surplus, whether that's 200W or 2.8kW, without ever intentionally drawing from the grid.
Interaction with battery storage systems
Where a property has both a diverter and a home battery, the two systems are effectively competing for the same surplus power unless configured with a clear priority. Most installers set the battery to charge first (since stored electricity can be used for anything in the home, not just hot water) and the immersion diverter to activate only once the battery is fully charged or its charge rate is capped — though the opposite priority can make sense for households who value guaranteed hot water over battery autonomy. This is a configuration decision to make explicitly with the customer, not a default to leave unset.
Safety: what a diverter does not change
A diverter adds an alternative power source to the immersion circuit; it does not replace or bypass the cylinder's own thermostat and high-limit safety cut-out, both of which must continue to function exactly as designed. Because solar-diverted heating is opportunistic and low-power for much of the year, the household's normal scheduled full-temperature heating cycle (usually to around 60°C, for legionella control) must still run independently of solar availability — a diverter should never be relied upon as the sole means of reaching pasteurisation temperature.
Export limitation and grid impact
Where a DNO has capped the export capacity of a PV installation (a condition sometimes attached to G99 approval on constrained parts of the network), a diverter genuinely helps by keeping more of the generated surplus in the property rather than sent to the grid — reducing the amount that needs limiting. However, a diverter is not itself a recognised export-limiting device for G99 compliance purposes; where a hard export limit has been imposed by the DNO, dedicated export limitation equipment is still required alongside, not instead of, a diverter.
Frequently Asked Questions
Does a solar diverter reduce my electricity bill in winter?
Only marginally. Diverters work by redirecting surplus solar generation, and UK solar generation in winter months is small — often barely covering daytime household loads before any surplus exists to divert. The financial benefit of a diverter is concentrated in spring, summer and autumn, when generation regularly exceeds daytime household demand.
Can a solar diverter be added to an existing PV system, or does it need to be part of the original design?
It can be retrofitted to virtually any existing grid-connected PV system without needing to modify the PV installation itself or trigger a new DNO application — the diverter simply monitors import/export at the property's supply and controls the immersion circuit independently of the PV inverter.
Will a solar diverter stop my cylinder reaching a safe temperature for legionella control?
No, provided the cylinder's existing thermostat and heating schedule are left in place and functioning. The diverter is a supplementary, opportunistic heat source — it should never be the only thing heating the cylinder, and the normal scheduled full-temperature heating cycle (independent of solar availability) must continue to run to maintain legionella-safe temperatures.
Does fitting a diverter affect my Smart Export Guarantee payments?
It reduces the amount of surplus electricity available to export (since more of it is being used on-site for hot water), which in turn reduces SEG income — but this is usually a net financial gain overall, because self-consuming that electricity as hot water is worth more than the SEG export rate paid for it. It does not affect SEG eligibility or MCS certification status.
Regulations & Standards
BS 7671:2018+A4:2026 — UK wiring regulations governing the immersion and consumer unit connection work.
Engineering Recommendation G98 — DNO notification for PV/generation systems up to 16A per phase; governs the underlying PV system, not the diverter.
Engineering Recommendation G99 — DNO approval/notification for larger PV/generation systems, including any export limitation conditions.
MCS (Microgeneration Certification Scheme) — certification for the PV/battery generation system, gateway to Smart Export Guarantee eligibility.
Smart Export Guarantee (SEG) — statutory payment scheme for exported renewable electricity that makes self-consumption via a diverter financially attractive.
Part P, Building Regulations (England & Wales) — competent person self-certification for the electrical connection work.
Notifying DNOs | MCS — official guidance on G98/G99 scope
Iboost+ Eddi etc, solar diverters why? What am I missing? | DIYnot Forums — practical homeowner/installer discussion of diverter function
The DNO Guide, G98, G99 and G100 applications for UK solar — DNO notification process reference
A Guide to Solar Diverter Manuals | ITS Technologies — installation and operating principle reference
solar diverter installation pricing guide — labour, materials and margin for quoting this job
solar pv — PV system fundamentals, DNO notification and MCS certification
solar battery systems — battery storage systems and their interaction with diverters
immersion heater — immersion heater fault-finding, relevant to diverter-connected circuits