Accurate measurement of kinetic power of AGN jets is important to three areas of astrophysics: (1) jet generation mechanisms; (2) studies of jet dynamics; and (3) the role of AGN feedback in galaxy formation and evolution. However, measuring jet power is difficult. The lack of reliable empirical methods has resulted in the widespread use of radio luminosity as a proxy for jet power.We present a new method of measuring jet power in FR-II radio galaxies, based on the observed parameters of jet terminal hotspots. We compare the jet power - lobe luminosity relation derived for our sample with that obtained for FR-I radio galaxies based on X-ray cavity measurements. Somewhat surprisingly, we find approximate agreement between these relations, despite mounting evidence in the literature favouring vastly differing compositions and energy budgets in these two classes of radio galaxies.We use our hotspot measurements to show that FR-II radiative efficiency is sensitive to radio galaxy size, in agreement with predictions of dynamical models. Any observed jet power - radio luminosity relation is therefore sensitive to sample selection effects. This dependence explains, at least in part, the coincidence of the jet power - lobe luminosity relations for FR-I and FR-II sources. Our findings are important to interpretation of next-generation radio survey data.