Aqueous thermogalvanic cells, the solution analogs of solid-state thermoelectric devices, are compared for power generation. Measurements on the copper - copper formate - copper system yield thermoelectric powers which are higher than those exhibited by other copper systems. In these solutions three copper formate complexes are present. Practical cells were built and tested. The power output is largely limited by cell resistance, though mass and charge transfer contribute to the observed overvoltages. The coupling of this thermogalvanic system with an electrochemical photovoltaic effect (a photothermogalvanic cell) is briefly described.