Renewable energy sources are assessed on the basis of the practically available size of the reserve and the economic competitiveness of its development. Solar water heating panels are expensive, but attention to passive solar heating and heat retention in house design is inexpensive and rewarding. Photovoltaic cells remain expensive for United Kingdom use but are likely to be useful in small specialised applications. Offshore wave power needs bulky and expensive structures, though economic generation may be possible at favoured points on rocky shores. Wind power is nearly competitive with conventional electricity generation and offers an attractive export market even with a small United Kingdom base. Total power promises to be economic but it involves very large installations where the obstacle is the huge initial investment. Geothermal energy from hot, dry rocks has yet to show acceptable heat generation rates. The most promising renewable energy source at present is man-made; domestic, industrial and agriculture waste used as fuel.