This document is applicable to gas-fired mCHP appliances producing domestic hot water and electricity. The electricity is generated in a process linked to the production of useful heat. It applies to a mCHP appliances marketed as single unit or as a package fully specified by a manufacturer that have: a gas heat input not exceeding 400 kW; an electrical output not exceeding 50 kW and a hot water storage capacity (if any) not exceeding 2000 l. prEN 13203-1:2021 sets out in qualitative and quantitative terms the performance in delivery of domestic hot water for a variety of uses. It also gives a system for presenting the information to the user. The present document sets out a method for assessing the energy performance of gas fired mCHP appliances. It defines a number of daily tapping cycles for each domestic hot water use, kitchen, shower, bath and a combination of these, together with corresponding test procedures, enabling the energy performances of different gas-fired appliances to be compared and matched to the needs of the user. When the mCHP generator does not supply domestic hot water in the summer period, the present standard is not applicable. prEN 13203-2:2021 is used for performance assessment of these generators.