Hi Mark,
Solar panels include all systems that are installed in, or on the site of, a building and that are:
• solar collectors such as evacuated tube or flat plate systems, together with associated pipework and equipment, such as circulation systems, pump, storage cylinder, control panel and heat exchanger
• photovoltaic (PV) panels with cabling, control panel and AC/DC inverter
So, if the replacement inverters are a composite part of a solar panel and, the solar panel needs them to operate then the replacement of a part specifically designed to allow the continued use and operation of the solar panel would be zero-rated when installed in accordance with the guidance shown in paragraph 2.7 of the following guidance:
2. Installations of energy-saving materials
Thank you.