While the precise wattage of a fluorescent light fixture depends on its design, fluorescent lights generally use around 25 percent as much energy as a comparable incandescent bulb. This is due to the fact that fluorescent fixtures produce far less heat, allowing them to convert more energy into visible light.
Incandescent bulbs produce light by feeding current through a resistor wire, usually made of tungsten. As the current flows through the wire, it heats up and begins to glow. Traditional incandescent bulbs only convert around 5 percent of the electricity that flows through the wire into light, and the rest is released as heat.
Fluorescent lights, on the other hand, have no filament resistor. Instead, the electric current excites mercury inside the bulb, vaporizing it and causing it to release ultraviolet photons. These photons interact with a phosphor coating on the inside of the bulb, producing light. Since less energy is converted to heat, a fluorescent lamp can produce the same light as an incandescent bulb while using a small fraction of the wattage.
Since the wattage of a bulb can be an imprecise way of measuring its light output, many manufacturers have taken to rating their bulbs by lumen output. A 60-watt incandescent, for instance, puts out around 800 lumens of light.