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Greenhouse cultivation is an energy intensive process therefore it is worthwhile to introduce energy saving measures and alternative energy sources.Here we show that there is scope for energy saving in fan ventilated greenhouses.Measurements of electricity usage as a function of fan speed have been performed for two models of 1.25m diameter greenhouse fans and compared to theoretical values.Reducing the speed can cut the energy usage per volume of air moved by more than 70%.To minimize the capital cost of low-speed operation, a cooled greenhouse has been built in which the fan speed responds to sunlight such that full speed is reached only around noon.The energy saving is about 40% compared to constant speed operation.Direct operation of fans from solar-photovoltaic modules is also viable as shown from experiments with a fan driven by a brushless DC motor.On comparing the Net Present Value costs of the different systems over a 10 year amortization period (with and without a carbon tax to represent environmental costs) we find that sunlight-controlled system saves money under all assumptions about taxation and discount rates.The solar-powered system, however, is only profitable for very low discount rates, due to the high initial capital costs.Nonetheless this system could be of interest for its reliability in developing countries where mains electricity is intermittent.We recommend that greenhouse fan manufacturers improve the availability of energy-saving designs such as those described here.