A simple and low-cost method for analyzing amorphous silicon solar cells and modules, which have low values of the fill factor (FF), is proposed. Low fill factors can occur mainly because of 3 reasons: (a) excessive recombination due to "bad" intrinsic layers; (b) shunts and (c) very high series resistance. The method described here allows one to discriminate between (a), (b) and (c). It consists of measuring the J-V curves at different light intensities, varying typically from 0.05 to 1 sun. It has been called the "variable intensity method (VIM)". Here, one plots R(sc)=partial derivative V/partial derivative J (at V=0) and R(oc)=partial derivative V/partial derivative J (at J=0) as a function of J(sc). From the slope of the R(sc)-J(sc) curve, one derives the "collection voltage V(coll)"; from the asymptotic value of R(sc) for low values of J(sc) ( < 0.1 mA/cm(2)) one obtains the "true" shunt resistance R(shunt); from the asymptotic value of R(oc) for high values disc J(sc) (around 10 mA/cm(2)) one obtains the "true" series resistance R(series). This paper shows quantitatively how too low values of V(coll) and of R(shunt) as well as how too high a value of R(series) lead to a low value of FF for both cells and panels/modules. (c) 2010 Elsevier B.V. All rights reserved.