Solar Diverters: Sending Surplus Power to Hot Water

A solar diverter is one of the simplest and most cost-effective ways to use surplus solar electricity. Instead of exporting excess generation to the grid for a modest payment, a diverter sends it to your immersion heater, giving you free hot water. Here's how they work and whether one makes sense for your setup.

How Solar Diverters Work

A solar diverter monitors electricity flow at your meter point using a CT clamp. When it detects that your solar panels are generating more than your home is consuming (i.e., you're about to export), it diverts the surplus to a connected load — typically your immersion heater.

The diversion is proportional: if you have 500W of surplus, the diverter sends 500W to the immersion. If surplus rises to 2kW, it sends 2kW. This smooth, variable control means virtually no electricity is wasted to export.

The Key Principle

• Without a diverter: surplus solar goes to the grid at around 12p/kWh (export tariff as of early 2026)
• With a diverter: surplus heats your water, saving ~24p/kWh (what you'd pay to heat water from the grid)

The saving is the difference between export value and import cost — typically 12–16p per kWh diverted.

Do You Need a Hot Water Cylinder?

Yes. Solar diverters only work if you have a hot water cylinder with an immersion heater. This includes:

• Traditional vented cylinders (gravity-fed systems)
• Unvented (pressurised) cylinders
• Thermal store cylinders
• Heat pump cylinders with immersion backup

Solar diverters do NOT work with:

• Combi boilers (no cylinder)
• Instantaneous water heaters
• Systems without an immersion heater element

If you have a combi boiler and no cylinder, a solar diverter isn't an option. You'd need to install a hot water cylinder first (£500–1,500), which may or may not be cost-effective depending on your circumstances.

Top Solar Diverters

myenergi Eddi

The most popular solar diverter in the UK. Made by myenergi (the same company behind the Zappi EV charger).

• Diverts up to 3.68kW to one or two heater loads
• WiFi connected with app monitoring
• Can prioritise between two loads (e.g., hot water first, then underfloor heating)
• Works with the myenergi ecosystem (Zappi, Libbi, Hub)

myenergi Eddi Solar Diverter

£185

max power w

3000

modes

power_divert,timed_boost

outputs

2

priority

configurable

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Solar iBoost+

A well-established, simpler alternative.

• Diverts surplus to immersion heater
• Wireless sender clips to your meter tails
• LCD display shows energy diverted
• Timer function for boost heating

Solar iBoost+ Immersion Heater Controller

£150

max power w

3000

modes

auto_divert,manual_boost

outputs

1

buddy unit available

true

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Costs and Payback

Payback Calculation

Assume you divert 1,500kWh per year to hot water (realistic for a 4kW system):

• Value of diverted electricity: 1,500 x 24p = £360 per year (what you'd pay to heat water from the grid)
• Minus lost export income: 1,500 x 12p = £180 per year (what you'd have earned exporting)
• Net annual benefit: £180

At a total cost of £400–600, payback is 2.2–3.3 years. That's a strong return.

Solar Diverter vs Battery Storage

This is a common question. Both use surplus solar, but differently:

A solar diverter is far cheaper and has much faster payback, but it only helps with hot water. A battery is more versatile but costs 5–10x more. Many households install a diverter first and add a battery later when prices fall further.

Can You Have Both?

Yes. A diverter and battery can work together. The typical priority order:

1. Power your home's immediate needs
2. Charge the battery
3. Divert surplus to hot water
4. Export any remaining surplus to the grid

Most hybrid inverters and smart diverters can be configured to work in this sequence.

Installation

Solar diverter installation is relatively simple:

1. CT clamp fitted to your meter tails (measures grid import/export)
2. Diverter unit mounted near your consumer unit or immersion heater
3. Wiring from the diverter to the immersion heater circuit
4. Configuration — setting temperature limits, priority loads, and WiFi connection

A qualified electrician can install a solar diverter in 1–2 hours. Some solar installers include diverter installation as part of a solar package.

Beyond Hot Water

While immersion heating is the primary use, some diverters can also power:

• Underfloor heating — Electric underfloor heating elements can be connected as a second priority load
• Towel rails — Electric towel radiators
• Storage heaters — Charging storage heaters with solar surplus during the day

The myenergi Eddi supports two loads with configurable priority, making it versatile for multi-load setups.

Who Benefits Most?

Solar diverters make the most financial sense for:

• Households with a hot water cylinder and immersion heater
• Systems without battery storage (the diverter captures value that would otherwise go to low-rate export)
• Households that use significant hot water (families with regular baths/showers)
• Systems with export limitations where surplus would otherwise be curtailed entirely

They make less sense for:

• Households with combi boilers (no cylinder)
• Systems with large batteries that already capture most surplus
• Very small solar systems (less surplus available to divert)

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