How this calculator works
A bulb's energy use is its watts divided by 1,000 (to get kilowatts) times the hours it is on. Over a year that is watts ÷ 1,000 × hours a day × 365. Multiply the kWh by your pence per kWh and divide by 100 for pounds, and subtract the LED figure from the old figure for the saving. Dividing the cost of the new bulbs by the monthly saving gives the payback time.
Worked example: 20 halogen spotlights changed from 50 W to 5 W LEDs, on 3 hours a day. Each halogen uses 50 × 3 × 365 ÷ 1,000 = 54.8 kWh a year, each LED 5.5 kWh. For 20 bulbs the saving is 985.5 kWh, which is £266.09 a year at 27p per kWh. 20 LED bulbs at £3 cost £60.00, so they pay for themselves in about 2.7 months. A single bulb saves £13.30 a year.
Assumptions and sources
The assumptions behind your numbers (prices, efficiencies and usage) are listed in the notes directly under your results, and they update when you change an input.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: your own prices, equipment and habits will move them.
LED saving per bulb by brightness
Brightness in lumens, the incandescent bulb it replaces and the range of LED watts in the US Department of Energy FEMP table (lumens are the same unit in the UK). Saving per bulb per year at 3 hours a day and 27p per kWh.
| Brightness | Old incandescent | LED watts | Saving per bulb per year |
|---|---|---|---|
| 450 lumens | 40 W | 4.5 to 7.0 W | £9.76 to £10.50 |
| 800 lumens | 60 W | 5.9 to 10.5 W | £14.63 to £15.99 |
| 1,100 lumens | 75 W | 9.5 to 14.0 W | £18.03 to £19.37 |
| 1,600 lumens | 100 W | 10.5 to 18.0 W | £24.24 to £26.46 |
Tips before you buy
- Compare lumens, not watts. A 60 W incandescent bulb gives about 800 lumens, and an LED with the same lumens uses a fraction of the watts. The box shows the lumens and the watts.
- Start with the bulbs that are on longest: kitchen, living room, outdoor and hallway lights. A bulb on 6 hours a day saves twice as much as one on 3 hours.
- Check the cap (B22 bayonet, E27 or E14 screw, GU10 spotlight) and "dimmable" on the box if the light is on a dimmer. A non-dimmable LED on a dimmer flickers or buzzes.
- Halogen spotlights (GU10) are often 35 or 50 W. Match the lumens on the box and enter the real watts of the bulbs you have and the ones you are buying.
- A plug-in energy monitor shows what an existing lamp really draws, if the old bulb has no label.
Tools & materials for this project
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Frequently asked questions
How much does it cost to run a 50 watt halogen bulb for a year?
At 3 hours a day and 27p per kWh, a 50 W bulb uses 54.8 kWh and costs £14.78 a year. A 5 W LED costs £1.48.
How much do LED bulbs save compared to halogen?
Check the lumens on the box and compare watts at that brightness: the US Department of Energy range for 450 lumens is 4.5 to 7 W, against 40 W or more for an old incandescent. At 5 W instead of 50 W the energy cut is 90%. The money saved depends on your price per kWh and the hours the bulb is on.
How many lumens is a 60 watt bulb?
About 800 lumens. A 40 W bulb is about 450 lumens, 75 W about 1,100 and 100 W about 1,600.
How do I find the price per kWh on my bill?
Look for the unit rate in pence per kWh on your bill or in your online account. Do not include the standing charge, which is a daily fee and does not change with the bulbs.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check the Building Regulations and your supplier's guidance before ordering.