Electric fields have previously been used in microfluidic devices for the manipulation, sorting and mixing of microemulsions. Here, an active system for on-demand electrocoalescence of water droplets in oil is presented. The platform does not require precise electrode alignment nor droplet-droplet or droplet-electric field synchronisation. Droplets can be reliably merged in pairs at a rate up to 50 fusion events per second. The fusion mechanism is based on the balance between viscous, electric and interfacial stresses at the droplet interface and depends upon the flow behaviour in the microchannel. Experimental results show that, under different conditions of frequency, applied potential and size of the droplets with respect to the channel geometry, diverse types of droplet coalescence occur. The fusion mechanism and general trends which enabled different merging results are proposed. This system has potential for being applied and multiplexed for high throughput, emulsion-based applications in the field of combinatorial reactions and screening bioassays.