Acrylic prepolymer comprising: from 10 to 50% by weight of units derived from at least one alkyl acrylate, the alkyl group having from 4 to 12 carbon atoms, from 30 to 60% by weight of units derived from at least one alkyl methacrylate, the alkyl group having from 1 to 5 carbon atoms, and from 10 to 40% by weight of units derived from methyl acrylate. The prepolymer is used to encapsulate photocells, connected to one another by conducting wires and positioned on a support plate, by casting a resin into the space located between the support plate and a second protective plate, the resin being obtained by mixing 100 parts of the acrylic prepolymer, from 0.1 to 4 parts of a vanadium arenesulphonate and from 0.5 to 4 parts of a free-radical initiator, at a temperature between 10.degree. and 70.degree. C. and for a sufficient time to solidify the polymeric resin at the temperature selected.