A field trial has been initiated by The Energy Saving Trust (United Kingdom) on 88 domestic solar thermal water heating systems to provide evidence on how actual systems perform in real homes in the UK and the Republic of Ireland. This field trial took place in 2008. Conclusions are that well-installed and properly used systems provided around 60% of a household’s hot water. The trial also found examples where systems were not properly configured or used, and where the contribution from solar was as low as 9%. The median across all systems was 39%.