For most of human history, candles and small oil lamps lit the dark, so it’s no wonder that measurements of light would be based somehow on candles. 

One candlepower was originally defined as a pure spermaceti candle of 1/6 pounds burning at 120 grains (a little under 8 grams) per hour. Spermaceti comes from the sperm whale, and at one time was the source of the best candles. Americans dominated the sperm whale fishery, so generations of Americans read their Bible and learned their sums by sperm whale candles.

Candlepower is an out of date term. In current usage light is measured by candelas or lumens. They are not interchangeable. Lumens measure the total light output in all directions—a lot of light is wasted in the sense of not being directed in a useful direction (like a lamp at night lets you read but also shines on the walls and ceiling). Candelas measure the beam intensity in one direction.

People buying light bulbs almost never read the packaging other than noting the wattage, but the light the lightbulb will emit is measured in candelas and lumens.

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