For information, I found out you can top-up your electricity balance at Electra shops. There is one in the city centre (Plateau) about 50m SW of the US Embassy, on Rua Serpa Pinto. There's another in Achada de Santo Antonio, on the street at the side of Hotel Santiago, past the Jamaican food place, opposite the Cedeao offices. For some reason they weren't appearing on Google maps.