Three quotes is the number that works — enough to triangulate a fair price without dragging the process out forever. The key is making them genuinely comparable: ask each installer to quote like-for-like on panel make and wattage, number of panels, inverter brand and type (string vs hybrid), battery size if any, and whether BER certification and the SEAI grant paperwork are included. Two quotes that look different on price often differ because one bundles things the other leaves out.
When the prices come back close together, that's usually a good sign you're being quoted fairly rather than ripped off. When one is a clear outlier, dig into why — is it a cheaper generic inverter, fewer panels, a different roof type, or extras like scaffolding bundled in or stripped out? The rough-guide numbers (≈€1k/kWp, ≈€550 for an Eddi, ≈€1k per 2 kW of battery, after grant) are a handy yardstick for sanity-checking any individual line.
Beyond price, weigh the things that don't show on the quote: communication, lead times, after-sales support, and whether the work is tidy and compliant. It's worth stressing that good post-installation support matters as much as the upfront figure — a slightly dearer installer who actually answers the phone when something needs adjusting can be the better buy.