Screen Power Consumption
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The black screen and the white screen draw almost the same amount of power: the black slightly less than the white (approximately 300 mW versus 320 mW).
The real power draw is the backlight (approximately 2.5 watts). The backlight at maximum is the largest power consumer of the entire system. Because most customers really use the light at maximum, Apple quotes battery life with the light on full brightness. We can't speak for how other manufacturers quote their battery life. However, the black versus white background argument is not valid in discussing battery life.
Battery Size Versus Weight
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Not all batteries are exactly the same, but Apple uses a state-of-the-art NiCad battery. In this class of battery, capacity and weight are directly related. Competitors with higher system weight may have larger batteries, and therefore, longer battery life.
Truth in Testing
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Apple tries to provide a real battery life description. We hear many reports of portable computers not meeting their reported battery life. Sometimes we suspect the testing procedure. Additionally, every manufacturer tests with a fully charged battery. NiCad charging is somewhat tricky; therefore, consumer reports of less than expected battery life need to be interpreted as a function of the NiCad charging sequence, battery history, and other factors. Apple uses an intelligent charging system to provide the best recharging possible.
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