What is the Lighting kWh Usage Calculator?
This calculator estimates how much electrical energy your light bulbs consume over a chosen period, measured in kilowatt-hours (kWh) — the unit your utility uses for billing. By entering the number of bulbs, their wattage, daily usage hours and the number of days, you instantly see total consumption and an optional cost estimate. It works for any country since kWh is a universal unit; just enter your local electricity rate.
How to use it
Enter the number of bulbs, the watts printed on each bulb, how many hours per day they are switched on, and the period in days. Optionally add your electricity rate (price per kWh) to estimate the bill. The result shows total kWh, total wattage, daily energy use, and projected cost.
The formula explained
First, total wattage = number of bulbs × watts per bulb. Energy in watt-hours is total wattage × hours per day × days. Dividing by 1000 converts watt-hours into kilowatt-hours: $$\text{kWh} = \frac{\text{totalWatts} \times \text{hoursPerDay} \times \text{days}}{1000}$$. Multiply the kWh by your rate to get cost.
Worked example
Suppose you have 10 bulbs of 60 W each, used 5 hours a day for 30 days. Total wattage = \(10 \times 60 = 600\) W. Energy = \(600 \times 5 \times 30 = 90{,}000\) Wh = 90 kWh. At a rate of 0.15 per kWh, the cost is \(90 \times 0.15 = 13.50\).
FAQ
Why switch to LED bulbs? LEDs use far fewer watts for the same brightness — replacing a 60 W incandescent with a 9 W LED cuts that bulb's energy use by about 85%.
What rate should I enter? Use the price per kWh from your electricity bill, often labelled the unit rate or supply charge.
Does this include standby power? No. It only models the time bulbs are actively on; smart fixtures may draw a small standby load.
