Photovoltaic-thermal (PV/T) systems integrate photovoltaic and solar thermal technologies into one single system with dual production of electricity and heat energy A typical arrangement is the direct attachment of PV modules onto a solar thermal collector surface. For a given collector surface area, the overall system energy performance is expected higher than the conventional "side-by-side" PV and solar thermal systems. In the development of PV/T collector technology using water as the coolant, the most common design follows the sheet-and-tube thermal absorber concept. Fin performance of the thermal absorber has been identified as one important factor that affects much the overall energy performance of the collector Accordingly, an aluminum-alloy flat-box type PV/T collector prototype was constructed and tested in Hong Kong. Our test results indicate that a high combined thermal and electrical efficiency can be achieved. The primary-energy-saving efficiency for daily exposure approaches 65% at zero reduced temperature operation. With a simple and handy design, the product is considered to be very suitable for domestic application.