"But Mouchot was learning the limitations of solar power. Sunlight is plentiful and free, but it comes as an intermittent flow, not a reliable stock. Mouchot's engines were useless at night or on cloudy days--and French skies were often cloudy. Even when the sun shone, the mirrors were costly. One skeptical engineer noted in a review of Mouchot's work that running a typical one-horsepower steam engine required "about two kilograms [4.4 pounds] of coal." To drive the same engine with the sun, Mouchot would need a mirror of about 320 square feet. Operating factory-scale machinery would require hundreds of giant mirrors--a huge expense. Meanwhile, French industry was not running out of fuel, as so many had predicted it would. Paris had signed a trade agreement with London, and the nation was awash with British coal."