Conversion of solar energy to electrical energy is effected using combination of (a) a photoelectrochemical membrane cell in which (i) photosensitisers are illuminated with solar energy to produce excited sensitisers and electrons, the sensitisers being located in a redox electrolyte comprising R/O couples adjacent to an electron transferring membrane separating the redox electrolyte from a redox aq. anolyte comprising A+n/A+n-1 couples (redox electrolyte couples with rH more-vc than the decompsn. potential of the sensitisers; redox anolyte couples having rH more +vc than the excited state level of the sensitisers or the flatband potential of a semiconductor sensitiser), (ii) the electrons are passed through the membrane (I) oxidising the sensitisers and reducing the redox anolyte couples, (iii) the oxidised sensitisers are regenerated by redn. in the redox electrolyte producing oxidised redox electrolyte couples, and (iv) the oxidised redox electrolyte couples are electrochemically regenerated at the surface of a -vc electrode in electronic communication with the redox electrolyte and (through an external bias circuit) in electronic communication with a +vc electrode in electronic communication with the redox aq. anolyte, while O2 produced at the +vc electrode is vented; (b) the reduced redox aq. anolyte couples are passed to a redox-O2 cell in the redox-O2 cell (i) the reduced redox anolyte couples are passed through a porous flowthrough anode so that they are oxidised and become suitable for recycle to the photoelectrochemical membrane cell or electron acceptor redox anolyte couples, (ii) O2 is passed through a diffusion cathode to a catholyte of the redox-O2 cell to form H2O, and (iii) electrical energy is withdrawn from the redox-O2 cell by an external load circuit between the porous anode and the diffusion cathode-an ionically conductive separator between the redox-O2 cell catholyte and anolyte completing the circuit; and (d) the oxidised redox anolyte coupler are recycled to the anolyte side of the photoelectrochemical membrane cell.