This paper presents a year round energy performance monitoring results of two solar water heaters with 4 m(2) flat plate and 3 m(2) heat pipe evacuated tube collectors (ETCs)operating under the same weather conditions in Dublin, Ireland. The energy performance of the two systems was compared on daily, monthly and yearly basis. Results obtained showed that for an annual total in-plane solar insolation of 1087 kWh m(-2), a total of 1984 kWh and 2056 kWh of heat energy were collected by the 4 m(2) FPC and 3 m(2) ETC systems respectively. Over the year, a unit area of the FPC and ETC each generated 496 kWh m(-2) and 681 kWh m(-2) of heat respectively. For 3149.7 kWh and 3053.6 kWh of auxiliary energy supplied to the FPC and ETC systems their annual solar fractions (SFs) were 38.6% and 40.2% respectively. The annual average collector efficiencies were 46.1% and 60.7% while the system efficiencies were 37.9% and 50.3% respectively for the FPC and ETC respectively. Economic analysis showed that both solar water heating (SWH) systems are not economically viable with NPVs ranging between -(sic)4,264 and -(sic)652 while simple payback periods (SPPs) varied between 13 years and 48.5 years. (C) 2011 Elsevier Ltd. All rights reserved.