The calculation
kWh saved = number of fittings × watts saved per fitting × hours lit a year ÷ 1,000. Then multiply by the government's grid factor for the carbon, and by your unit rate for the money. Three numbers decide the answer:
- The old wattage, including control gear. Fluorescent fittings draw more than the tube's rating because the ballast uses power too. The label on the fitting, or your electrician, will tell you.
- The new wattage. From the LED product's data sheet.
- Hours lit. The single biggest variable. A warehouse lit around the clock saves far more from the same swap than a meeting room used two hours a day.
A worked example
An office replaces 60 fluorescent fittings, each drawing about 70 W with its control gear, with LED fittings drawing 30 W. The lights are on about 3,000 hours a year. These are example figures: use your own.
| Calculation | Result | |
|---|---|---|
| Before | 60 × 70 W × 3,000 h ÷ 1,000 | 12,600 kWh |
| After | 60 × 30 W × 3,000 h ÷ 1,000 | 5,400 kWh |
| Saved | 7,200 kWh | |
| Carbon, Scope 2 | 7,200 × 0.13096 | 943 kg, or 0.94 tonnes CO2e |
| Money | 7,200 × 29.1p | about £2,094 a year |
The price is the government's 2025 average for businesses using between 20 and 500 MWh of electricity a year, including the Climate Change Levy and excluding VAT. Your own unit rate is on your bill, and it may be higher or lower.
If you also count grid losses and the upstream emissions of producing the electricity, as Carbon Recycling does in Scope 3, the full saving is about 1.33 tonnes a year.
Add sensors and timers
The cheapest kWh is the one you never use. If occupancy sensors and timers in the example cut the lit hours from 3,000 to 2,400, that saves a further 1,080 kWh, or 141 kg CO2e, on top of the LED swap. Toilets, stores, corridors and meeting rooms are the usual places lights are left on in empty rooms.
Why the carbon saving shrinks every year
The kWh saved stay the same each year, but the carbon saved falls, because each kWh from the UK grid carries less carbon each year. The same 7,200 kWh would have been 1,840 kg CO2e at the 2019 factor of 0.2556, against 943 kg at the 2026 factor. That is good news for the planet, but it means you should calculate each year's saving with that year's factor, and not claim a 2019-sized saving in 2026. Our post on why your carbon footprint dropped in 2026 explains the grid factor change.
Fluorescent tubes are being phased out anyway
In 2021 the government announced plans to remove fluorescent lights from sale from September 2023, as part of tighter energy efficiency rules for lighting. If your building still has fluorescent fittings, LED is likely to be the replacement when they fail, so it often makes sense to plan the switch rather than wait for each tube to go.
Reporting the saving
- Carbon Reduction Plan. List LED lighting as a completed measure, with the year, the kWh saved and the tonnes. Our guide to PPN 006 Carbon Reduction Plans covers what evaluators look for.
- SECR. Companies reporting under SECR must describe the principal energy efficiency measures taken in the year, and the government's guidance recommends stating the energy saved. LED lighting is one of the examples in its own sample report.
- Green tariffs. The saving shows in your location-based Scope 2. See our post on green tariffs and location-based figures.
Lighting is often the simplest electricity measure in a plan. The next ones in this series are rooftop solar and replacing a gas boiler with a heat pump.
How Carbon Recycling helps
Enter your electricity bills month by month and the dashboard's emissions by month chart shows the drop after the switch, calculated with each year's government factor. Add the LED project as a completed measure in your Carbon Reduction Plan and describe it in the energy efficiency section of your SECR report, both in the app. For a quick check before you buy, our free kWh to CO2e calculator turns any kWh figure into tonnes.