Tidal energy has been used since Roman times, although the use of tidal currents as a possible industrial energy source is a more modern concept. Unlike wind power, however, there is still no consensus on the most appropriate technology for resource exploitation. Similarities with wind power can cause errors of interpretation, especially when attempting to assess resource potential. The energy availability estimates that have been produced have, until now, only taken account of the apparent current flow speeds before the extraction of energy. As demonstrated in this paper, this may not be appropriate, especially for large energy extraction rates, which alter the underlying hydraulic nature of the flow environment. In a simple hypothetical channel linking two infinite oceans, a maximum extraction of 10 per cent of the apparent raw kinetic flux would appear to be acceptable. The limitations on sea loch type environments may, however, be less restrictive.