Is it cheaper to add a battery at install or later?

Adding the battery WITH the panels is usually cheaper, because the 0% VAT rate on domestic solar applies to the panels and the battery when they are supplied and installed together under the same contract. A battery added later doesn't qualify for that 0% rate: it's 13.5% or 23% VAT, usually 23% for a battery on its own, which can add several hundred euro or more for the same hardware. So even if you are unsure, get the battery priced as part of the initial install, and at minimum fit a hybrid (battery-ready) inverter now so you are not also paying to swap the inverter later. A battery is not essential for a low-usage home, but it is better to decide before the install than to default to adding one afterwards. One thing to weigh in October 2026: a €600 battery grant is expected in Budget 2027 on 6 October, but it isn't confirmed, and it isn't known yet whether it will cover batteries fitted with new panels, added later, or both.