A detailed majority and minority carrier current balance model for surface photovoltage (SPV) is presented. A rigorous treatment of carrier junction generation and surface recombination is shown to be essential. We show that under weak above band gap photoexcitation, standard constant open-circuit voltage and constant photon flux SPV measurements can both reveal the bulk minority carrier diffusion length, provided the initial band bending Vbi is sufficiently high. However, when Vbi is small, approximate SPV expressions are inapplicable and both standard SPV manifestations will then yield incorrect diffusion length values. We also show that for cases of low surface recombination velocity, the SPV technique can be used to measure the width of the space charge region and Vbi; however, when the surface recombination velocity is large, this approach cannot be used as majority carrier backdiffusion can play a dominant role under high photon energy excitation conditions.