Everything around the stack: water purification, compressors, heat exchangers, gas purification, cryogenic and pressure tanks.
Electrolysers need ultra-pure water (about 9-10 litres per kg of hydrogen before losses), cooling, rectifiers, gas drying and compressors. Ammonia plants add synthesis loops and refrigerated tanks. These are made and installed by process-equipment companies.
Balance of plant is a large share of an electrolysis plant's cost, and it is where Indian engineering firms already have scale. Thermax signed a licence with Norway's HydrogenPro in Aug 2025 to offer complete alkaline electrolyser systems, making the stacks' surroundings in India.
Steadier than stacks: the same water-treatment, heat-exchange and storage gear sells to refineries, chemicals and power plants. Green hydrogen is an add-on order stream for these companies, not their main business.