The SEAI grant is for the solar PV panels themselves, not a blanket subsidy on every component. Batteries are not funded by the solar grant, but they do get 0% VAT when supplied and installed with the panels under the same contract, which is a meaningful saving in itself. Add a battery later, though, and it isn't zero-rated: it's 13.5% or 23% VAT depending on how much of the price is goods, and usually 23% for a battery on its own. So there's a real financial reason to include storage at install time if you know you want it.
For other extras, ask the installer to show the VAT on each line of the quote. You don't need to qualify for any grant to get the 0% VAT on the panels and a battery bought with them: the zero rate and the grant are two separate things that often stack.
So the picture is: a grant of up to €1,800 for the panels, 0% VAT on the panels and a battery installed with them under the same contract, and 13.5% or 23% VAT on a battery added afterwards. A separate €600 battery grant is expected in Budget 2027 on 6 October 2026, but it isn't confirmed and its rules aren't published, including whether it covers a battery added to an existing system. If you're weighing whether to include a battery now or later, the VAT difference is part of the maths.