One question that comes up regularly from people who have recently installed plug-in solar panels is whether they should add a solar diverter — a device that routes excess solar generation to their hot water cylinder rather than letting it flow back into the grid unpaid.
It is a good question, and the answer is more nuanced than most of the marketing material suggests. Let me give you the honest version.
What a solar diverter does
Your plug-in solar panels generate electricity whenever the sun shines. That electricity feeds into your home’s wiring through the inverter and is used by whatever appliances happen to be running at the time.
If your panels are generating more electricity than your home is currently using, the surplus flows back into the grid. Under current UK rules, plug-in solar users cannot access the Smart Export Guarantee, so that exported electricity is effectively given away for nothing.
A solar diverter sits at your consumer unit and monitors the flow of electricity. The moment it detects you are generating more than you are consuming, it activates your immersion heater, diverting the surplus into heating water in your hot water cylinder. The cylinder stores that heat for hours, so even a short burst of surplus generation at midday can contribute meaningful hot water for the evening.
The most popular options in the UK
MyEnergi Eddi — The Eddi is probably the best-known solar diverter in the UK. It is well-built, integrates with the MyEnergi app, and plays nicely with other MyEnergi products like the zappi EV charger and the libbi home battery. If you are building out a fuller home energy management setup, the Eddi fits into that ecosystem well. Price: around £350 to £400 installed by a qualified electrician, which is required since it connects to the consumer unit.
Marlec iBoost — The iBoost has been around for longer and has a reputation for reliability and straightforward installation. It works on the same principle as the Eddi, costs slightly less (around £250 to £300 installed), and does the job without a lot of additional features. If you just want a diverter and nothing else, the iBoost is perfectly good.
SOLiC 200 — A third option that sometimes comes up in UK solar forums. Similar functionality to the above, slightly cheaper, and with a simpler display. Worth considering for budget-conscious installations.
The catch for plug-in solar users
Here is the thing that the diverter manufacturers do not prominently advertise: the amount of surplus you can divert depends entirely on how much more you are generating than you are currently consuming.
A typical UK household has a baseline consumption of around 200W to 400W at any given time — fridges cycling on and off, a router, standby devices, perhaps a TV. Call it 300W on average.
A single 400W plug-in panel at peak summer output generates around 350W to 380W of actual AC power through its micro-inverter. Subtract your 300W baseline consumption and you have 50W to 80W of genuine surplus to divert.
An immersion heater is typically a 3kW device. It takes 3,000 watts to heat your water. Your 50W to 80W of surplus will heat water so slowly that the benefit is negligible.
Even with a twin-panel 800W setup — the most common plug-in configuration — your peak surplus on a clear summer day might be 400W to 500W. That is better, but still not huge. The Eddi will make some use of it, but the hot water contribution is a fraction of what you would get from a proper rooftop system with several kilowatts of panels.
When a diverter does make sense
There are circumstances where a diverter makes more sense for plug-in solar users.
You have a large array. If you have gone beyond the standard single or twin panel setup — multiple strings, a larger micro-inverter capacity, the full 3.68kW that G98 allows — then your surplus generation on a sunny day can be genuinely substantial, and a diverter starts to pay for itself.
You are already in the MyEnergi ecosystem. If you have a zappi EV charger or are considering one, the Eddi integrates with it. The combined system manages your whole energy profile together, which can be worth more than the sum of the parts.
You have no immersion timer. A diverter responds automatically to changing generation; a timer does not. If you find managing a timer annoying or your generation hours vary significantly with season, a diverter removes the mental overhead.
The timer alternative: 80% of the benefit for 5% of the cost
For most people with a single or twin plug-in solar panel setup, my honest recommendation is a plug-in timer, not a solar diverter.
Set a mechanical or smart timer on your immersion heater to run between 11am and 2pm. On days when your panels are generating well, a meaningful portion of the electricity used for heating water is coming from solar. On overcast days, the immersion runs on grid electricity at the same time it would have anyway, and you have at least chosen a midday window when grid prices (on Agile or similar tariffs) are often cheap.
A mechanical immersion timer costs about £15. A smart plug with scheduling costs £20 to £30. Either achieves 70 to 80 percent of what a full diverter would do for a small plug-in solar setup, without the £300 to £400 installation cost or the need for a qualified electrician to wire it into your consumer unit.
The diverter earns its place when you scale up. For now, the timer is the pragmatic choice.
Heat pumps: a different calculation
If you have an air source heat pump rather than an immersion heater, the calculation changes somewhat. Heat pumps work most efficiently when run continuously at low output rather than in short bursts, and they cannot absorb surplus solar generation the same way an immersion can. A solar diverter will not help with a heat pump.
The better approach with a heat pump and plug-in solar is the same timing strategy described above: run your heat pump during the solar generation window so that your heating draw is offset by solar production. The heat pump buffers the energy in your home’s thermal mass or in a buffer tank, and your solar contribution reduces the grid draw during those hours.
Summary
Solar diverters are excellent pieces of kit for households with full rooftop solar systems generating significant surplus. For plug-in solar users with one or two panels, the honest advice is: start with a timer on your immersion, see how it goes, and revisit a diverter when you have expanded your setup enough that the surplus generation genuinely justifies the hardware cost.
If you do decide you want a diverter, the MyEnergi Eddi and the Marlec iBoost are both solid choices. Either needs installation by a qualified electrician — budget £250 to £400 for the unit and another £100 to £200 for installation depending on your setup.