A BER rates a home’s energy performance from A (most energy-efficient) to G (least efficient), with sub-ratings such as A1, A2 and A3. The rating is calculated by a SEAI-registered BER assessor using factors including insulation, heating, ventilation, windows, hot-water systems and renewable energy, rather than one fixed list of requirements for each grade. A BER certificate is generally needed when a property is built, sold or rented, subject to limited exemptions. The assessor’s advisory report can suggest upgrades, but the final rating depends on the property and assessment.