Solar Immersion Diverters Explained: Should NI Homeowners Get One?

A solar immersion diverter is one of those additions to a solar panel system that sounds technical but is actually remarkably simple. It takes surplus electricity that your panels generate (electricity that would otherwise be exported to the grid for a few pence per unit) and uses it to heat your hot water instead. I checked the numbers on this page against Northern Ireland tariffs on 23 September 2026, and the answer depends heavily on what heats your water today. In a home that heats water with the electric immersion, a diverter is one of the most cost-effective upgrades to a solar system. In an oil-heated home, which is 61% of NI homes (NISRA, 2024/25), it is worth far less than most guides claim.

How a solar immersion diverter works

The basic principle is this: during the day, your solar panels often generate more electricity than your home is using at that moment. Without a battery or a diverter, that surplus flows back to the grid, and your supplier pays you for it. Northern Ireland has no Smart Export Guarantee, so each supplier sets its own export rate. Power NI pays 11.88p per kWh for the year 1 October 2026 to 30 September 2027, and the rate resets each October; see our export payments guide.

A diverter monitors the electricity flowing through your system in real time. The moment it detects surplus generation (more being produced than consumed), it redirects that excess to the immersion heater in your hot water cylinder. The diversion happens proportionally, meaning it can send just a trickle of power or the full surplus, adjusting continuously as your household consumption and solar generation fluctuate throughout the day.

The result is that your hot water is heated by solar electricity that would otherwise have earned you 11.88p per unit. If you heat water with the immersion, you would otherwise pay your import rate for it later: 32.10p to 34.15p per kWh including VAT on Power NI’s domestic tariffs from 1 July 2026, and anywhere from 29.83p to 42.01p across the NI market (Consumer Council comparison table, 6 September 2026). The saving per diverted unit is the difference between your import rate and the export rate, so roughly 18p to 30p per kWh in NI. If oil heats your water, the sum is different, and I cover it below.

Popular diverter brands

There are several immersion diverters on the market, but three dominate the UK residential solar market.

myenergi eddi

The myenergi eddi is the most popular solar immersion diverter in the UK and the one most frequently recommended by NI installers. It costs around £400 to £500 including installation.

The eddi supports two heating loads, meaning it can heat your hot water cylinder first and then redirect any remaining surplus to a second load (such as a storage heater or towel rail). It has a clear display showing real-time diversion data, and it integrates with the myenergi app for remote monitoring. If you also have a myenergi zappi EV charger, they communicate with each other to prioritise power allocation intelligently.

Build quality is good, the installation is straightforward for any qualified electrician, and the warranty is solid. It is the safe choice for a reason.

iBoost+

The iBoost+ from Marlec is the main alternative to the eddi. At £300 to £400 installed, it is slightly cheaper. It does the core job well, diverting surplus solar to your immersion heater automatically and proportionally.

The iBoost+ includes a wireless sender that clips onto your meter tails (the cables connecting to your electricity meter) and a receiver unit that connects to your immersion heater. The wireless design makes installation simpler in some properties where running a cable between the meter and the cylinder would be awkward.

It also has a boost button, allowing you to manually heat water from the grid if needed, and a timer for scheduled heating. The monitoring is more basic than the eddi, with a simple display rather than a full app.

SolarEdge immersion controller

If your solar system uses a SolarEdge inverter, the SolarEdge immersion controller is worth considering. It integrates directly with the SolarEdge monitoring platform, so everything shows up in the SolarEdge app.

It is less commonly installed than the eddi or iBoost+, partly because it only works with SolarEdge systems. If you have a SolarEdge inverter, ask your installer whether this is an option.

How much can you save?

The savings from a diverter depend on three main factors: how much surplus solar you generate, how much hot water you use, and what you currently pay to heat it.

Here is a typical scenario for a Northern Ireland household with a 4kW solar system and no battery.

A 4kW system on a south-facing NI roof generates roughly 3,330 kWh per year (see our solar system sizes and costs guide). Without a battery, roughly 50% to 60% of that is surplus (not consumed directly in the home). That gives you approximately 1,670 to 2,000 kWh of potential surplus per year.

A typical hot water cylinder requires around 3 to 5 kWh per day to heat, or roughly 1,100 to 1,800 kWh per year. Not all of the surplus will be available when the cylinder needs heating, but in practice a diverter captures enough surplus to cover a significant portion of your hot water needs, particularly from March through October when solar generation is highest.

A diverter typically covers around 50% to 80% of a household’s hot water over a year. If your water is heated by the electric immersion, that is roughly 550 to 1,440 kWh, and at about 20p net per unit (Power NI 32.10p minus the 11.88p export) it is worth about £110 to £290 a year, checked on 1 October 2026.

With the diverter costing £300 to £500 installed, the payback period in an electrically heated home is typically 2 to 4 years. That is comfortably faster than a battery, which typically takes 8 to 12 years to pay back. In an oil-heated home the payback is far longer, for the reasons in the oil section below.

Diverter vs battery: which should you choose?

This is one of the most common questions NI homeowners ask, and the answer depends on what you want to achieve.

	Immersion Diverter	Battery Storage
Typical cost	£300 - £500	£3,000 - £6,000
Payback period	2 - 4 years	8 - 12 years
Annual saving	£120 - £320 (electric-heated water); far less on oil	£300 - £600
Uses surplus for	Hot water only	Any electricity use
Complexity	Simple add-on	More complex installation
Best for	Reducing hot water costs	Reducing overall electricity imports

If your primary goal is to make the most of surplus solar without a large upfront investment, a diverter is hard to beat. It is cheap, simple, and delivers a fast return.

If you want to reduce your overall dependence on grid electricity, particularly for evening and overnight use, a battery does more. It costs more and takes longer to pay back, but it addresses a wider range of electricity consumption.

Many homeowners choose both. The system can be configured so that surplus solar first powers the home, then charges the battery, then diverts to the immersion heater, and only exports to the grid when all three needs are met. This maximises self-consumption across the board.

Who should get a diverter?

A solar immersion diverter is a good fit if:

• You have a hot water cylinder with an immersion heater (most common in homes with a conventional boiler or oil-fired system)
• You generate surplus solar electricity during the day
• You want a quick, affordable way to use more of your solar generation
• You heat water using oil, your boiler is old or fires just for hot water in summer, and oil stays above about £935 for 900 litres (see the oil section below)
• You do not want to invest in battery storage, or you want a complement to a battery

A diverter is not suitable if:

• You have a combi boiler with no hot water cylinder (there is no immersion heater to divert to)
• You already have a battery that absorbs most of your surplus (though there may still be a small benefit)
• Your solar system is very small and generates little surplus

Oil-heated homes: a smaller gain than you will be told

61% of Northern Ireland homes heat with oil (NISRA, 2024/25), so this matters more here than anywhere else in the UK. In an oil home a diverted unit saves oil, not electricity, and you still give up the 11.88p export payment on it.

At the Consumer Council’s 17 September 2026 price of £1,007.16 for 900 litres, a useful kWh of oil heat from an 85% efficient boiler costs about 12.8p. Take off the 11.88p export and each diverted unit is worth about 1p, or about £9 a year on 1,000 kWh diverted. With an older 70% boiler it is about 4p a unit, about £36 a year. At the August 2026 price of £778.15 for 900 litres, a good boiler heats the water for less than the export payment, so diverting loses money. As a rule of thumb, with an 85% boiler a diverter only beats exporting when 900 litres costs more than about £935.

The full workings, with the sensitivity at different oil prices and the heat pump comparison, are in solar panels in an oil-heated home. A diverter can still be worth fitting in an oil home if your boiler is old, if it fires just for hot water in summer, or if the export rate falls in October. Just do not expect the £100-plus a year an all-electric home gets.

Installation: what to expect

Installing a solar immersion diverter is a straightforward job for a qualified electrician or solar installer. It typically takes 2 to 4 hours and involves:

1. Fitting the diverter unit near your consumer unit or hot water cylinder
2. Connecting a current clamp to your meter tails (to monitor grid import/export)
3. Wiring the diverter to your immersion heater element
4. Configuring the diverter settings (boost schedules, priority levels)
5. Testing the system to confirm surplus is being diverted correctly

Most solar installers can fit a diverter at the same time as your panel installation, which is the most cost-effective approach. If you already have solar panels, any MCS-certified installer or qualified electrician can retrofit one.

There is no need for any changes to your solar panel system, inverter or grid connection. The diverter sits alongside the existing system and simply makes better use of what is already there.

Compare solar quotes with diverter options

If you are getting solar panels in Northern Ireland, ask your installer about including an immersion diverter in the quote. Many offer it as a standard add-on. Our free comparison tool matches you with up to three local MCS-certified installers who can advise on the best combination of panels, battery and diverter for your property. It takes about two minutes and there is no obligation.