With a view to improving climate prediction, we test various models for estimating solar radiation at sea from standard meteorological surface observations against long series of radiation measurements at several Ocean Weather Stations and at an offshore island meteorological station. The widely-used Budyko formula is found to err by up to 32% in estimating long-term mean insolation, and has even larger errors in reproducing seasonal variations. Various models which estimate solar radiation from solar elevation and from hourly cloud amount and type, using empirical or simple physical formulae, are evaluated. None of these models estimate mean insolation better than an existing formula which relies only on noon solar elevation and daily mean cloud amount, although the hourly models do reproduce seasonal, monthly and daily variations better. None of the formulae using standard surface observations is able to achieve the 10 Wm−2 accuracy in long-term mean insolation which is required for climate prediction.