An extension of previous studies, which did not treat quantitatively possible effect of diffuse skylight on the enhancement of energy gain of solar thermal collectors when sun-tracking instead of fixed position is used, involved consideration of possible contribution of the former to the overall solar power and energy received on the ground under different conditions (time of a year, degree of overcast). The effect on the efficiency of two types of solar thermal collectors: a standard (fixed) one and one of adjustable orientation, is considered. Also, possible advantage of orienting adjustable-position collectors towards maximum light (light-tracking) instead of the astronomic position of the sun (sun-tracking) was discussed, as to conclude that little is to be gained and even that only at the sky totally covered by clouds. Some experimental proof of the validity of conclusions is presented. (c) 2005 Elsevier Ltd. All rights reserved.