AbstractThe purpose of the fusion programme is to provide broadly competitive electricity (with perhaps credit given for possible environmental, diversity and safety benefits). Fusion reactor design studies routinely predict electricity prices broadly comparable to conventional alternatives such as the PWR, despite the devices producing the electricity being typically ∼ 10 times bigger, more complicated and requiring more maintenance. In this paper this issue is considered. These low power densities, indicated both by reactor studies and confirmed by generic engineering and geometric arguments, are shown to be a rather fundamental feature of the D-T tokamak. Coupled with the complex construction and expensive materials required, and the probable low availability its serial complexity and high maintenance needs will cause, it is argued that D-T tokamak electricity prices will be a significant multiple of alternatives. This is the case even with all remaining physics issues assumed resolved successfully.