Lumens to Watts Converter
Convert a light output in lumens to the wattage you need for each bulb technology.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Estimate power as lumens divided by luminous efficacy. Enter the lamp’s lumens-per-watt value from its packaging or technical data; efficacy varies by model and operating conditions. Annual energy assumes the entered daily hours for 365 days.
Worked example
calculated with the default values below
Light output 800 lm · Light output per watt (from product data) 100 lm/W · Hours used per day 5 h · Electricity rate 0.17 $/kWh Equivalent wattage 8.0 W · Energy per year 14.60 kWh · Running cost per year 2.48 $
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
watts = lumens ÷ efficacy; kWh/year = watts ÷ 1000 × hours/day × 365; cost = kWh × electricityRate
- lumens
- light output (lm)
- W
- equivalent power (W)
- h
- daily use (hours)
- r
- electricity rate per kWh
- kWh
- annual energy use
- efficacy
- user-entered luminous efficacy (lm/W)
Efficacy is user-entered in lm/W from product data; watts = lumens ÷ efficacy. Annual use assumes entered hours/day for 365 days.
Frequently asked questions
How do I convert lumens to watts accurately?
Divide the bulb’s lumen output by its published luminous efficacy in lm/W. This tool asks for that product-specific value because efficacy varies across lamps and operating conditions; the annual energy estimate also depends on use hours and electricity price.