For a lot of households this is the smarter-money option, and it comes up constantly among Irish solar owners. The idea is to heat the bulk of your hot water overnight on a cheap night or EV rate — heating the tank at around 2–5am at roughly 8–14c/kWh using a simple immersion timer — so you start each day with a full, hot tank for little money. Solar then tops the tank back up during the day as you use water, and you export the rest for CEG payments.
The economics often favour this over a diverter because of one inconvenient truth: hot water from a diverter is essentially "free" surplus, but if your export rate is healthy, that same surplus might earn you more sold to the grid than it saves by heating water. Heating overnight at a low fixed rate, then exporting daytime surplus, can come out ahead — especially for homes with a good EV tariff. A diverter still has a place (it's cheap and effortless), but it's not automatically the most economical route.
The setup many owners settle on: night-rate immersion timer for the base load, solar top-up through the day, and a manual or short boost in the evening only if needed. If you have a thermostatic mixer valve, you can heat the tank hotter overnight (storing more energy) and mix down for use. Run it against your own tariff — the right answer turns on your night rate versus your export rate.