A new method is proposed for determining typical 1-year weather data from a multi-year record for evaluation of solar energy systems. The procedure is very straightforward and can be utilised with east: when determining the long-term performance of a solar hot water system (SHWS). It is made up of a concatenation of 12 months individually selected from a multi-year database. The criterion for the selection is the minimisation of error in the monthly solar gain prediction of the system. Considering this criterion, the 'typicality' of the weather pattern is taken into account, in addition to its influence on the behaviour of the solar system. A comparison is made between the new method and others frequently referred to in the literature. Based on simulation results for yearly, monthly and daily power delivered, six indicators have been calculated. These indicators quantify the different behaviours of the system when 'historical' and typical weather data are applied. The all inclusive comparison shows that the new method for deriving typical weather data leads to an accurate evaluation of the long-term performance of a SHWS. (C) 2001 Elsevier Science Ltd. All rights reserved.