The short answer

Lumens describe light output, while input watts describe electrical power. Compare suitable brightness, fitting, dimming and light quality before pricing a replacement. An equivalent-incandescent wattage is a brightness comparison, not the LED’s electrical input; label kWh per 1,000 hours has another stated basis.

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Give brightness and electrical power different jobs

Use lumens when assessing how much light a bulb produces, and actual input watts when pricing electricity. An “equivalent” wattage refers to another lighting technology’s approximate brightness and is not necessarily the power consumed by the new product.

Energy Saving Trust explains why light output and suitability matter. The EU light-source label uses a stated energy basis that should remain attached to the figure when transferred into a calculation. Efficiency letters alone do not supply watts or household kWh.

Read the actual energy unit

For a source declaring 9 kWh per 1,000 hours, the average input represented by that declaration is 9 W: 9 kWh ÷ 1,000 hours × 1,000 W/kW. This is an illustrative unit conversion, not a real product record.

Do not enter 9 as kWh/year unless the document actually states that annual quantity. A household schedule still needs quantity, hours and use days. At one bulb, four hours/day and 365 days, a 9 W assumption gives 13.14 kWh/year.

Suitable light is more than one number

Compare fitting size and type, distribution, colour temperature, colour rendering and dimming support where those matter to the room. A different shade or beam can change useful illumination even when a headline lumen value is similar.

Check an existing dimmer and fitting against the product requirements. The cost tool records your confirmation that the candidates are suitable; it cannot determine compatibility from wattage. Keep the source models with a saved comparison so you can revisit that decision.

Choose the busiest matching schedules first

A replacement saving grows with the hours the lamp actually operates. Under an illustrative 60 W to 9 W change, one bulb saves 0.051 kWh per operating hour. At $0.20/kWh, that is $0.0102 per hour.

For one hour/day over 365 days, annual savings are $3.723. Four hours/day gives $14.892 under the same assumptions. Those figures help explain why different rooms have different payback, even for the same purchase price. They do not tell you to reduce useful illumination.

Keep standby and dimming within the boundary

Smart lamps or controls can use electricity when the lamp appears off. Include a separately documented mode if it lies outside the lit-hours estimate. A complete-day circuit measurement may already include that energy, so adding it again would overstate use.

A dimmed lamp’s average input can differ from its maximum rating. Measure or use a documented setting-specific value rather than assuming that half perceived brightness means exactly half input power.

Make the comparison reviewable

Record the exact bulb/fitting, source watts, lumen context, quantities and operating schedule. The lighting tool then estimates electricity and simple replacement payback. The cost guide shows a complete five-bulb example with its purchase assumptions.

Use separate scenarios for different room schedules, and keep installation charges explicit. A calculated payback says nothing about a manufacturer’s warranty, product lifetime or whether a fitting is appropriate for a particular use.