Fog harvesting is an application of aerosol filtration to capture water droplets from fog by using nets, called fog collectors. It is a promising technique for freshwater supply in arid and semi-arid regions. In this study, we investigate the collection efficiency of harp-like fog collectors consisting of vertical fibres. We conduct well-controlled laboratory scale experiments on model fog collectors and develop a predictive theoretical model accounting for the liquid distribution on the fibres and for aerodynamic effects. Without using any adjustable parameter, we obtain a good agreement between our theoretical model and laboratory scale experimental results. Furthermore, we perform collection efficiency measurements on an in-situ pilot scale fog collector, accompanied by a description of the fog's microphysical properties. Comparing the results of our pilot scale and laboratory scale experiments and the theoretical model developed, we propose a method to accurately evaluate the efficiency of our collector, with simultaneous measurements of collected water volume and fog characteristics.