What you need to know
- Add the wattage of every lamp that is on at the same time.
- Compare light output in lumens, not only watts.
- Use hours per day and your electricity price to convert lighting into a monthly cost.
Use your own measurements
Use the linked BricoDato resource to check these inputs in your own situation. The savings from LED lighting depend on the watts actually replaced and the number of hours used.
Add the complete lighting load
Multiply the number of bulbs by the wattage of each lamp. If rooms use different lamps or hours, calculate them separately instead of forcing one average.
Compare lumens, not only watts
An LED replacement should provide the brightness you need. A lower-watt lamp is not a saving if the room then needs more fittings or no longer has enough useful light.
Target the longest-running lights first
Replacing or reducing use in lamps that run many hours gives a larger annual saving than focusing on rarely used rooms.
Keep light quality in the decision
Colour temperature, colour rendering and dimmer compatibility can matter as much as raw efficiency.
What does replacing four bulbs save?
The savings from LED lighting depend on the watts actually replaced and the number of hours used. Keep brightness and the required light distribution comparable: a dimmer lamp is not necessarily an equivalent replacement.
Example to adapt
Replace four 40 W older lamps with four 8 W LEDs, used 4 hours per day. Power drops from 160 W to 32 W, saving 128 W. Over 30 days that is 0.128 kW × 4 × 30 = 15.36 kWh, or €3.07 at an illustrative €0.20/kWh. The calculation excludes lamp purchase cost.
| Configuration | Power for four lamps | 30-day energy |
|---|---|---|
| 40 W lamps | 160 W | 19.2 kWh |
| 8 W LED lamps | 32 W | 3.84 kWh |
| Difference | 128 W | 15.36 kWh |
Checks before acting
- Compare lumens and beam angle when choosing replacement bulbs.
- Check fitting, dimmer compatibility, colour temperature and colour rendering.
- Prioritise rooms used for many hours before seldom-used cupboards.
- Compare expected saving with purchase cost and replacement life.
Final check
Count the lamps that are actually on together, then record their input watts and hours rather than assuming all bulbs run continuously. For four lamps changing from 35 W to 6 W at three hours per day, the daily difference would be 4 × 29 × 3 ÷ 1,000 = 0.348 kWh under those assumptions. Also compare lumen output, beam width and whether the new lamp works with the existing driver and dimmer.
- Count all bulbs in the group.
- Use real operating hours.
- Compare equivalent lumens.
- Check dimmer and fitting compatibility before buying.
Keep a small job sheet
Record bulb count, old and new watts, hours/day, lumens and purchase price.
Turn the guide into a decision
Measure the longest-running fittings first, and verify flicker, useful light at the work surface and temperature limits. Use your actual electricity tariff and annual hours to calculate a payback, including replacement transformers if needed. Keep a photo of the old lamp specification so you can check the replacement socket and beam when another bulb fails.