The short answer

Lighting energy is input watts divided by 1,000, multiplied by bulb count, operating hours and use days. Compare similar lumens before estimating LED savings. Replacement payback divides the new purchase and installation cost by positive annual electricity savings; it does not predict bulb life.

Use the lighting & led payback calculator
On this page

Compare the light you need before the power you buy

The useful question is how much electricity provides suitable light for this room. Lumens describe light output; input watts describe electrical power. A bulb advertised as “60 W equivalent” does not necessarily consume 60 W. Enter the actual input rating from its packaging.

Check colour temperature, light distribution, dimming support and fitting compatibility too. Similar lumens do not guarantee the same result inside a shaded fitting. Energy Saving Trust’s lighting guidance explains these purchase checks; the EU light-source guidance explains the energy-label context.

A five-bulb replacement example

Every quantity below is an illustrative arithmetic assumption, not a product test or average household:

Input Current bulbs Replacement bulbs
Number of bulbs 5 5
Input per bulb 60 W 9 W
Daily use 4 hours 4 hours
Use days per year 365 365
Entered electricity price $0.20/kWh $0.20/kWh

Current energy is 438 kWh/year, costing $87.60. Replacement energy is 65.7 kWh/year, costing $13.14. The difference is 372.3 kWh and $74.46 per year. These figures compare the same entered lighting routine; they are not a universal saving for changing a bulb.

Recover the outlay on replacing working bulbs

Suppose each replacement costs $3 and installation adds $0. The new outlay is $15. Simple electricity payback is $15 ÷ $74.46 = approximately 0.201 years under the stated annual routine.

The original purchase price of working bulbs is already spent. Including it again would change the baseline. If you are choosing between two new lighting systems instead, compare their extra purchase cost and their relevant installation costs.

Repairs, replacement failures, finance and future electricity prices are excluded. An early calculated payback is not a warranty or a prediction of lamp life. If electricity savings are zero or negative, there is no positive electricity-savings payback.

Different rooms need separate schedules

A hallway used briefly, a kitchen used each evening and an outdoor lamp operating seasonally should not all inherit one schedule. Run separate scenarios for distinct groups of identical bulbs, then carry their annual kWh into separate rows of the household planner.

Count only the lamps that actually operate. A fitting with six sockets may use fewer bulbs. Smart lamps can draw power while their light is off; include that separately in operating modes or measure a complete routine. Do not add a standby estimate to a whole-day measurement that already includes it.

Make a useful lighting record

Keep the old and new model codes, actual input watts, lumens, purchase price and operating schedule. If measuring a dimmed fitting, record the setting and whether the meter covers one lamp or the whole circuit. Some hardwired lighting needs appropriate circuit measurement rather than a socket meter.

Use the lighting calculator to edit the example. Read lumens versus watts before using a replacement result as a buying comparison.

Prioritize rooms using actual schedules

The same bulb change can have different value in a briefly used cupboard and a kitchen used each evening. Record quantities and hours for each distinct schedule rather than averaging unrelated rooms into a single apparently typical bulb.

For the illustrative 60 W to 9 W change, the reduction is 0.051 kWh per operating hour for one bulb. At $0.20/kWh, one hour/day across 365 days saves $3.723/year; four hours/day saves $14.892/year. These are the same watt difference with different use, not different product efficiencies.

Include controls without overlapping the lamp energy

A complete circuit reading can include lamps, drivers and controls. Do not then add those same components as separate planner rows. A lamp-only rating, by contrast, may leave another component outside the stated boundary.

For a replacement decision, keep a suitable light-output and compatibility check with each group. The lighting source explains these suitability questions; a short payback result does not verify a fitting or forecast a product’s life.