Light Bulb Cost Calculator

How it is calculated

Energy is power times time, so kWh per year = W × hours per day × bulbs × 365 / 1000, where W is the power of one bulb in watts and the division by 1000 turns watt-hours into kilowatt-hours. The yearly cost is kWh per year × rate, with the rate in your currency per kWh. The monthly cost uses the same formula with 30 days instead of 365, so twelve months of it comes to slightly less than the yearly figure (360 days against 365). Hours per day can be at most 24; watts, hours, rate and bulbs must be above zero, and the bulb count must be a whole number, otherwise there is no result. The tool assumes every bulb draws its full rated watts for the same hours each day. Dimmers, standby draw and the heat a bulb adds or removes from a room are not included. The cost is shown with the dollar sign whatever currency your rate is in.

Common mistakes

• Entering the rate in cents instead of in currency units. 15 cents must be typed as 0.15; typing 15 makes the cost a hundred times too large.
• Putting the total wattage of a fixture in the watts field and also entering the number of bulbs. The watts field is for one bulb, because the tool multiplies by the bulb count.
• Using the hours the room is occupied instead of the hours the lights are really on. Lights left on in empty rooms count. More than 24 hours a day is rejected.
• Entering the equivalent brightness of an LED (for example 60) instead of its real draw (for example 9). Use the watts printed on the LED itself.
• Multiplying the monthly figure by 12 and expecting the yearly total. The month is 30 days, so the product is 360 days of cost.