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 as one PV system. Retrofitting a battery on its own later typically does not qualify for that 0% rate, so you pay VAT on top — often a few hundred euro 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.