The conventional performance analysis of various technologies consists in calculating their energy balance. In addition, particularly when trying to optimize a system, the establishment of their exergy balance is of great interest, as it allows one to quantify the irreversibilities. Exergy balance poses no particular problem but needs to be done carefully otherwise errors can be committed. Various tools to conduct exergy balances are proposed: a methodological guide and a spreadsheet for simple systems, the use of productive structures for complex systems.