Plasmonic nanoparticles attract great attention owing to their strong light-to-heat conversion properties. Fundamental understanding of their photothermal performance is critical to develop solar-to-heat systems. Here, we use gold nanoparticles as the photothermal agent. Solar water evaporation are explored through a hanging droplet containing the nanoparticles. These nanoparticles induce multiple scattering events, increasing photon absorption and concentrating the light within the mesoscale domain, leading to an intense collective heating that trigger the evaporation. The droplets with different initial-particle-concentrations lead to evaporation at various rates (e.g. K constant in D2-law), and an optimal initial-particle-concentration can be expected. With steam releasing from the surface, shrinking of the droplet increases the particle concentration in the domain that accelerates the surface evaporation. The surface evaporation rate increases with the increasing concentration of nanoparticle, reaching an optimized value at a threshold concentration and then stable. K values demonstrate a nonlinear dependence over the time, reflecting complex heat-transfer physics behind the phenomenon. We assume that the collective heating is controlled by two parameters: one that relates the morphology properties of the droplet and the other that characterizes the gap between nanoparticles. This work provides an important insight on the evaporation dynamics of plasmonic droplets, and stands for a basis to design the plasmonic solar heating systems.