Regarding Rowan Hooper's Feb. 8 article, "City ecology explains Japan's low birthrate": In addition to the argument running a bit more like numerology than science, Rowan Hooper quotes "natural" human energy consumption as 8,400 kilojoules (kJ) per day and then proceeds to convert that to 100 watts per day, presumably to provide the value in more familiar units. Unfortunately, the watt is not a unit of energy but of power, or energy per unit time. Thus a "watt per day" would be some sort of odd energy acceleration. Eight thousand four hundred kJ per day is actually a bit less than 100 watts.
ian clark
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