Lux to Watt & Lighting Power Density (LPD

Lux to Watt Conversion, The Lumen Method & Lighting Power Density (LPD)

Converting surface brightness (Lux) to electrical power (Watts) requires factoring room geometry, luminaire efficacy, and room surface reflections:

• **

• Universal Lux to Electrical Watt Formula**:

P_Watts = E_lux × A_m^2η_lm/W × (CU × LLF)

Where:

• E_lux: Target maintained illuminance

• A_m^2: Illuminated floor area in square meters

• η: Luminaire luminous efficacy in Lumens per Watt

• CU × LLF: Combined optical room utilization and maintenance factor (typically 0.55 to 0.70)

• **

• Lighting Power Density (LPD) Metric**:

LPD = Total Installed Lighting WattsGross Illuminated Area

1 W/m^2 = 0.0929 W/ft^2 | 1 W/ft^2 = 10.7639 W/m^2

• **

• Standard ASHRAE 90.1 Lighting Power Density (LPD) Allowances**:

• Open Plan Office: 0.61 W/ft^2 = 6.57 W/m^2

• Classroom / Lecture Hall: 0.68 W/ft^2 = 7.32 W/m^2

• Hospital Patient Room: 0.52 W/ft^2 = 5.60 W/m^2

• Retail Sales Area: 0.84 W/ft^2 = 9.04 W/m^2

• Warehouse Storage Area: 0.33 W/ft^2 = 3.55 W/m^2

How to Use This Calculator

Enter your Required Illuminance (Lux / lx = lm/m²) (e.g. 500 in lx), Illuminated Floor / Work Area (e.g. 50 in area), Area Unit — choose from Square Meters (m² - SI Metric), Square Feet (ft² - Imperial), Luminaire Luminous Efficacy (lm/W) (e.g. 120 for commercial LED troffer in lm/W), Optical Efficiency Factor (CU × LLF: 0.1 to 1.0) (e.g. 0.60 for typical room reflectance and dirt depreciation in factor), and Architectural & ASHRAE 90.1 Benchmark — choose from Custom Lux & Area, Residential Kitchen (400 lx @ 15 m² @ 100 lm/W) → 100.0 W (LPD = 6.67 W/m²), ASHRAE Commercial Office (500 lx @ 50 m² @ 120 lm/W) → 347.2 W (LPD = 6.94 W/m² = 0.645 W/ft²). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Total Fixture Lumens Needed: Phi_total = (Lux * Area_m2) / Optical_Efficiency

Electrical Power = Total Lumens / Efficacy (Watts)

Worked Example

An electrical building designer calculates that lighting a 60 m² classroom to 300 Lux with 130 lm/W LED panels draws 230.8 Watts, easily satisfying LEED Gold sustainability certification requirements.

Understanding Your Result

Unit conversion precision: round to the same number of significant figures as your original measurement. Converting 6 feet to meters gives 1.8288 m — but if your measurement was approximate (±1 inch), expressing more than 2 decimal places is false precision. For international trade and technical specifications, SI units (meters, kilograms, liters) are the standard. US customary units (feet, pounds, gallons) differ from Imperial UK units: 1 US gallon = 3.785 L vs. 1 UK gallon = 4.546 L.