The SEAI grant is for the solar PV panels themselves, not a blanket subsidy on every component. Batteries are not grant-funded — but they do benefit from 0% VAT when bought as part of a solar PV installation, which is a meaningful saving in itself. Add a battery later to an existing system, though, and it attracts the standard 13.5% VAT rather than the zero rate, so there's a real financial reason to include storage at install time if you know you want it.
The same VAT logic applies to other extras bought as part of the PV install: doing it all together under the one solar installation generally gets you the 0% rate. You don't need to qualify for any grant to benefit from the 0% VAT on the PV system and the batteries bought with it — the zero rate and the grant are two separate mechanisms that often stack.
So the picture is: grant (€1,800) for the panels, plus 0% VAT across the PV install including batteries and diverters bought at the same time, versus 13.5% VAT on a battery added afterwards. If you're weighing whether to include a battery now or later, the VAT difference is part of the maths — buying it with the panels can be cheaper even though the grant itself doesn't touch the battery.