We describe and validate a simple PV simulation model capable of predicting average PV output as a function of array geometry (slope and azimuth) and location. This simulation tool is used in the Clean Power Estimator-a web-based PV economic evaluation program available in the US and several countries. Results show that the simplified model accurately captures array geometry, seasonal and daily PV output variations when benchmarked against a standard PV simulation program. This paper concludes that web-based Clean Power Estimator simulations have a sound physical basis. (C) 2004 Elsevier Ltd. All rights reserved.