RPI-X regulation sounds like a simple formula, but this article starts by examining whether it really is. Then, avoiding the interesting, but difficult, counterfactual question of how electricity prices have differed from what they would have been without privatisation and regulation by OFFER1 it demonstrates that no precise description is available of how they have in fact moved. The figures that it ends up with are therefore only inadequate approximations. Thus regulation cannot, and available price measures do not, apply recognised state-of-the-art principles.