This paper considers the relative technical and economic performance, for selected sites in Southeast Asia, of PV plants using crystalline and thin-film PV module technology. Technical performance estimates are based on a forensic analysis of in-field data for two grid-connected PV installations in Thailand using polycrystalline and thin-film PV modules. These two case studies help to validate the performance simulation approach for the other considered countries with similar environmental conditions. The case studies show that Mott MacDonald’s yield analysis approach demonstrates acceptable accuracy for energy yield assessment in a grid-connected PV plant, at least under the observed environmental conditions, which are most relevant to Southeast Asia plants with polycrystalline and thin-film PV modules installed. The findings presented in this paper are relevant to project developers and investors who have an interest in selecting solar PV technologies for Southeast Asian regional conditions.