For a typical Irish home that's occupied in the evenings, somewhere between 5 kWh and 10 kWh usually fits, and 10 kWh is a common sweet spot. The logic is that a 10 kWh battery is usually big enough to carry you from late afternoon, when generation tails off, through to the cheap EV/night rate that kicks in around 2am — so you rarely buy expensive peak-rate units.
Smaller 5 kWh batteries suit lower-consumption homes or anyone mainly load-shifting a modest evening load. Going bigger (15–20 kWh) only makes sense if you have high usage — an EV charging at home, a heat pump, electric cooking, or a busy household — and a large enough array to refill it. An oversized battery in a small home just sits partly empty and never earns back the extra cost.
One technical catch worth flagging: a battery's stated capacity is not its discharge rate. A "5 kWh" battery might only push out 2–2.5 kW at a time, which won't cover a kettle plus an oven plus a tumble dryer all at once. If high simultaneous loads matter to you, ask the installer about the continuous discharge rate, not just the kWh figure. The smart move is to size against your own usage data rather than a neighbour's setup.