Understand your estimate
How to use this result
Compare lighting electricity on a matched routine and suitable light output, then check how long positive electricity savings take to cover the replacement outlay. Input watts and equivalent-incandescent watts are different quantities.
Method reviewed · Calculation methods · Evidence and source policy
Find the right inputs
Current and replacement bulbs
Actual input W and confirmed suitable lumens/fittings
Use comparable light output, dimming and fitting context.
Quantity and schedule
Bulbs, hours/day and use days/year
Use separate scenarios for different rooms or schedules.
Replacement outlay and tariff
Price/bulb, total installation and price/kWh
The cost of keeping already-owned working bulbs excludes their old purchase price.
The calculation
Annual energy = watts ÷ 1,000 × quantity × hours/day × days. Annual saving = (current − replacement kWh) × tariff. Simple payback = new outlay ÷ positive annual saving.
A worked arithmetic example
Illustrative arithmetic: five 60 W bulbs versus five 9 W bulbs, four hours/day and 365 days, save 372.3 kWh/year. At $0.20/kWh the saving is $74.46. A $15 replacement outlay has about 0.201 years of simple payback.
What the estimate can tell you
The confirmation is a visitor review, not an automated fitting or lighting-design test. Payback does not forecast lamp life. Smart-control standby or an unentered installation charge can change the relevant cost scope.
Questions about this calculation
Can a bulb with more lumens still be compared?
Assess whether it provides the suitable service you actually need. Matching a task is more useful than ranking watts for different brightness or distribution.
What if I am changing bulbs only after failure?
That is a choice between new purchases rather than replacing working bulbs. Compare the extra price of suitable new options and their electricity difference.