PV output of multi-junction cells is strongly influenced by spectrum change. The influence of Sun height was quantitatively analyzed, considering seasonally and daily changes of spectrum. The new model also considered the presence of clouds. The influence of daily random fluctuation of spectrum was shown averaged out in the integration of yearly PV output fluctuation. It was suggested that the overall mismatch loss by the change of Sun height and Suit orbital would be <4% for III-V multi-junction cells for concentration application, when the bandgap of each junction was well balanced. The sensitivity of spectrum fluctuation was shown enlarged with the discrepancy of current matching conditions. (C) 2002 Elsevier Science B.V. All rights reserved.