Ignore phrases like “100-watt equivalent,” which are used to compare brightness. You want the actual number of watts the bulb uses.

Example 1: A typical incandescent bulb draws 60 watts of power, or 60 / 1000 = 0. 06 kilowatts. Example 2: A typical fluorescent bulb uses 15 watts, or 15 / 1000 = 0. 015 kW. This bulb only uses ¼ as much power as the bulb in example 1, since 15 / 60 = ¼.

Example 1: Your 0. 06 kW bulb is turned on for 6 hours a day, every day. In a 30-day month, that’s a total of (30 days/month * 6 hrs/day) = 180 hours per month. Example 2: Your 0. 015 kW fluorescent bulb is on for 3. 5 hours a day, 2 days a week. In one month, it will be on for roughly (3. 5 hours/day * 2 days/wk * 4 wks/month) = 28 hours per month.

Example 1: The incandescent bulb uses 0. 06 kW of power and is on for 180 hours a month. Its energy usage is (0. 06 kW * 180 hours/month) = 10. 8 kilowatt-hours per month. Example 2: The fluorescent bulb uses 0. 015 kW and is on for 28 hours a month. Its energy usage is (0. 015 kW * 28 hours/month) = 0. 42 kilowatt-hours per month.

Example 1: Your power company charges 10 US cents per kWh, or $0. 10. The incandescent bulb uses 10. 8 kWh/month, so running it is costing you ($0. 10/kWh * 10. 8 kWh/mo. ) = $1. 08 per month. Example 2: At the same cost of $0. 10 per month, the low-use fluorescent bulb costs you ($0. 10/kWh * 0. 42 kWh/mo. ) = $0. 042 per month, or about four cents.

Light-emitting diodes (LEDs) are even more efficient and have a lifespan of 50,000 hours (almost six years of constant use). Over their whole lifespan, they save about $7 US per year. [5] X Research source

Poorly made CFL bulbs can burn out quickly. The best options have an EnergyStar logo in the US, or a rating of A+ or higher on the European Union energy label. If you’re lucky, the packaging will list “lumens,” a measure of brightness. If not, use this estimate: a 60 watt incandescent bulb, 15 watt CFL, or 10 watt LED are all roughly the same brightness. Look for a color descriptor. “Warm white” is closer to the yellow glow of an incandescent. “Cool white” heightens the contrast, which may feel harsh in living spaces. “Directional” LED lights focus light on a small area instead of illuminating a whole room.