A diverter such as an Eddi sends your surplus solar — power you'd otherwise export cheaply — into your immersion to heat your hot water tank instead. Whether it's worth it depends heavily on how your house heats water and what your export rate is. It's genuinely a balance, and a couple of nuances decide it.
The case for: a diverter is relatively inexpensive (often around €550), it's set-and-forget, and it turns "nearly free" summer surplus into useful hot water, reducing the gas, oil or grid electricity you'd otherwise spend heating it. People who are out all day with no other daytime load like that it quietly mops up excess. The case against: in the darkest winter months there's little surplus to divert, so it does almost nothing exactly when you most want hot water; and if your export (CEG) rate is decent, you might earn more selling the surplus than you save by heating water with it. Some owners conclude a simple immersion timer on a cheap night/EV rate (heating the tank at, say, 2–5am for around 8–14c/kWh) is the more economical play.
A common middle path: use a night-rate immersion timer to heat the bulk of your water cheaply overnight, then let solar top it up during the day, exporting the rest. If you already have a gas combi and no tank, or solar thermal panels, the diverter's value drops further. The right answer is specific to your tariff, your tank, and your daytime habits — there's no one-size-fits-all here.