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. Free hot water from energy you have already generated. It is a straightforward concept, and for many Northern Ireland homeowners, it is one of the most cost-effective upgrades to a solar system.

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 you receive an export payment of typically 4p to 8p per kWh.

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 only a few pence per unit. Instead of exporting at 6p per kWh and then paying 28p to 32p per kWh to heat water from the grid later, you are using that surplus directly. The saving per diverted unit is the difference between your import rate and your export rate, which in NI is typically 20p to 28p per kWh.

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 ecosystem.

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, providing a seamless experience within 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 in NI generates roughly 3,400 kWh per year. Without a battery, roughly 50% to 60% of that is surplus (not consumed directly in the home). That gives you approximately 1,700 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.

Most NI homeowners with a diverter find it provides roughly 50% to 80% of their hot water needs over the course of a year. At current electricity rates, this translates to savings of approximately £100 to £200 per year on water heating costs.

With the diverter costing £300 to £500 installed, the payback period is typically 2 to 4 years. That is comfortably faster than a battery, which typically takes 8 to 12 years to pay back.

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	£100 - £200	£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 is the more comprehensive solution. 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 and want to reduce your oil consumption
• 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 particular benefit

Northern Ireland has a much higher proportion of oil-heated homes than the rest of the UK. If you heat your water using oil, a solar immersion diverter is especially valuable. Instead of burning oil to heat water (at a cost of roughly 7p to 10p per kWh equivalent), you are using free surplus solar electricity.

For oil-heated homes, the saving is effectively the full cost of the oil you would have burned to heat water. Over a year, that can easily reach £150 to £250, making the payback period even shorter.

If you are an oil-heated home with solar panels and no diverter, this is probably the single best value upgrade you can make to your system.

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.