A study is being undertaken in the United Kingdom of means of developing the potential of the large wave-energy resource around the coast, in particular, that to the west facing the Atlantic. Derivation of the mean annual energy to be expected involved knowledge, not only of the wave climates, conversion efficiency characteristics of the proposed devices and of the power transmission system, but also of factors reflecting the availability overall. The paper sets out a first simplified approach to the quantifying of reliability for each stage of the process, and, after establishing an appropriate method of analysis, gives a summary of the preliminary results obtained.