Myenergi eddi: How diverting PV surplus compares to feed-in tariff economics

The eddi from Myenergi is designed to divert solar power into domestic hot water, electric heating, or other resistive loads before that electricity leaves the property. The British manufacturer positions the device as an alternative to exporting surplus generation at today's low feed-in tariffs, arguing that using every kilowatt-hour on site makes more economic sense. But does the math hold up for electricians advising their residential clients?

How the eddi works

The device sits between a PV array and one or more electrical loads—typically an immersion heater in a hot-water cylinder, underfloor heating, or a storage heater. By monitoring CT clamps at the meter, eddi detects surplus generation and proportionally diverts power to the connected load. That means no export flows when the heater can absorb it, and the household avoids drawing from the grid for water heating.

Myenergi's approach differs from battery storage: rather than charging a lithium-ion pack, surplus electrons become heat. Because water tanks retain thermal energy for hours, the effect is similar—energy produced during midday sunshine becomes usable in the evening. The company emphasises that eddi integrates with its existing product line, notably the zappi EV charger, allowing cascaded load management when both devices are active.

Economics: what is surplus generation worth today?

Under current GB tariff structures, residential export rates sit around 4–5 pence per kWh for Smart Export Guarantee (SEG) contracts. Grid electricity, by contrast, costs 25–35 pence per kWh depending on supplier and time of day. The spread—roughly 25 pence—is the value proposition for diverting rather than exporting.

A typical three-person household uses 3,000–4,000 kWh annually for hot water, which translates to 8–11 kWh per day. If half that demand can be shifted to coincide with PV surplus, the eddi could displace 1,500–2,000 kWh of grid consumption per year. At 30 pence per kWh, that represents £450–600 in avoided electricity costs. Against a retail price of around £350 for the eddi unit, payback lies below one year for well-matched installations.

However, that calculation assumes a sufficiently large PV system with consistent midday surplus. A 4 kWp array in southern England generates roughly 3,400 kWh annually; once household baseload (refrigeration, standby consumption, daytime appliances) is met, perhaps 1,000–1,500 kWh remains. If the hot-water cylinder already cycles during the afternoon or if the household is out during peak generation, diversion yield shrinks accordingly.

Alternatives: battery storage versus thermal diversion

Battery systems such as those from SolarEdge or Tesla offer broader flexibility, storing surplus for evening lighting, cooking, and electronics use. Lithium-ion battery storage typically costs £3,500–6,000 for a 5 kWh pack, with payback horizons of five to eight years depending on household load profile and tariff. Thermal diversion via eddi costs less but addresses only heating or hot water.

For households with electric vehicles, the load-management question becomes more complex. An EV can absorb 7 kW continuously, dwarfing hot-water demand. Myenergi's zappi charger handles PV-surplus charging directly, and when combined with eddi, the system prioritises car charging before diverting residual power to the immersion heater. This cascade logic requires careful configuration to avoid splitting available capacity inefficiently.

In practice, many installers report that households with both EV and eddi see hot-water diversion only on weekends or during holidays, when the car is fully charged or absent. That limits annual utilisation and extends payback.

Integration with smart meters and tariffs

The eddi relies on CT clamps for real-time measurement; it does not communicate with smart meters via HAN or MQTT. This means the device operates independently of time-of-use tariffs such as Octopus Agile or Intelligent Octopus Go. Households on static tariffs see straightforward savings; those on dynamic pricing may find that the eddi diverts power during periods when export rates momentarily exceed import rates—albeit rare—leading to suboptimal economics.

Future firmware updates could integrate tariff signals, but as of mid-2026 no such feature is documented on Myenergi's website. Competitors such as Schneider Electric and SolarEdge offer energy-management systems that respond to tariff APIs, giving them an edge in homes with complex load portfolios.

Installation and compatibility considerations

Electricians report straightforward installation: the eddi mounts in a consumer unit or adjacent enclosure, with one CT clamp at the grid connection point and a second optional clamp for the solar inverter. The unit requires a dedicated circuit breaker and connection to the immersion heater via a relay output. Total installation time runs one to two hours for a standard cylinder.

Compatibility issues arise with certain combination boilers that do not feature a separate immersion element, and with unvented cylinders that require G3-qualified installer sign-off. Myenergi provides wiring diagrams for most common UK cylinder configurations, but site surveys remain essential to confirm suitability.

Market positioning and wider context

Myenergi competes with legacy immersion controllers from manufacturers such as Hager UK and SolarEdge's smart-energy portfolio. The eddi's differentiator lies in its integration with the zappi charger and the hub ecosystem, which appeals to households planning multi-device deployments. For single-product installations, cheaper relay-based diverters may suffice if advanced monitoring is not required.

The broader trend toward self-consumption optimisation reflects falling feed-in tariffs across Europe. In GB, SEG rates remain voluntary and vary by supplier; in Germany, Austria, and Switzerland, export tariffs have dropped below 10 cents per kWh, accelerating interest in both battery storage and thermal diversion. The environmental payback of increased self-consumption is less clear-cut: if surplus would otherwise serve grid decarbonisation, on-site diversion may simply shift rather than reduce carbon intensity.

Installer takeaways

For households with resistive heating loads, consistent midday surplus, and no near-term EV plans, the eddi offers rapid payback and minimal complexity. Where battery storage is already installed or planned, the incremental value diminishes unless hot-water demand is unusually high. Electricians should model annual surplus profiles using PV yield data and household consumption logs before recommending the device, and ensure clients understand that diversion cannot replace evening baseload met by batteries.

In multi-device scenarios—particularly PV-plus-EV combinations—load-management logic becomes critical. Myenergi's ecosystem approach simplifies configuration, but requires upfront investment in compatible hardware. Independent installers may prefer open-protocol solutions that allow mixing vendors, though this increases commissioning effort.

The eddi remains a pragmatic, low-cost entry point for self-consumption optimisation, provided the household load profile aligns. As tariff volatility increases and export rates remain depressed, thermal diversion will continue to complement—rather than replace—battery storage in the residential energy mix.