This study investigated and compared energy conservation levels between listed companies in energy-intensive industries and non-energy-intensive industries in Thai capital markets. It also tested the impact of energy conservation on firm performance using companies in the two industries. The sample for the study was sourced from 552 companies in the Stock Exchange of Thailand (SET) and 169 companies in the Market for Alternative Investment (MAI). The data was collected from the companies' annual reports spanning the period from 2016 to 2020. Descriptive analysis, independent sample t-test, and unbalanced panel data analysis were used to analyze data. The findings revealed that energy conservation scores for Thai-listed companies were generally stable, averaging between 0.45 and 0.46. It was also revealed that the energy conservation of companies in energy-intensive industries was significantly greater than that of companies in non-energy-intensive industries, with average scores of 0.55 and 0.43, respectively. Additionally, the study found that energy conservation has a positive impact on the firm performance of energy-intensive industries, while no significant impact in energy-intensive industries was recorded. The findings demonstrate that stakeholder and legitimacy theories can help explain how energy conservation benefits companies in terms of increased firm performance.