Lux to Watt Calculator
Calculate the total wattage needed to achieve a target lux level in any room. This calculator combines illuminance requirements with LED efficacy to determine power consumption, annual cost, and number of fixtures. See also our Watt to Lux Calculator and Lux to Lumen Calculator.
How to Convert Lux to Watts
Converting lux to watts involves two steps: first converting lux to lumens (using room area), then converting lumens to watts (using bulb efficacy). This two-step process bridges the gap between lighting requirements (specified in lux) and electrical specifications (measured in watts). Here is the complete process:
- Determine the target lux level: Use lighting standards — 500 lux for offices, 300 lux for general areas, 750 lux for detailed work, or 150 lux for corridors and storage.
- Measure the room area in square meters: Calculate length × width. For irregular shapes, break into rectangles and sum the areas.
- Calculate total lumens needed: Multiply lux by area: Lumens = Lux × Area (m²). For example, 500 lux × 20 m² = 10,000 lumens.
- Determine the LED efficacy: Check the fixture specifications. Modern LEDs range from 80–200 lm/W. Use 100 lm/W as a conservative average for planning.
- Calculate watts: Divide lumens by efficacy: Watts = Lumens / Efficacy. For example, 10,000 lm / 100 lm/W = 100 watts total.
The result represents the total electrical power needed for all lighting fixtures combined. To find individual fixture wattage, divide by the number of planned fixtures. Remember to apply a maintenance factor (1.25–1.5) to the lumen calculation to ensure adequate light levels throughout the installation's lifetime.
Lux to Watt Formula
Watts = (Lux × Area) / Efficacy
Where:
Watts (W) = Total electrical power consumption
Lux (lx) = Target illuminance level
Area (m²) = Room floor area
Efficacy (lm/W) = Luminous efficacy of the light source
Expanded:
Lumens = Lux × Area (m²)
Watts = Lumens / Efficacy (lm/W)
Annual Energy Cost:
Cost = (Watts × Hours/day × 365) / 1000 × Rate ($/kWh)
Number of Fixtures:
Fixtures = Total Lumens / Lumens per Fixture
With Maintenance Factor (MF = 1.25):
Watts_adjusted = (Lux × Area × MF) / EfficacyThis formula combines the lux-to-lumens conversion (Lumens = Lux × Area) with the lumens-to-watts conversion (Watts = Lumens / Efficacy) into a single expression. The efficacy value is critical — using LED fixtures at 100 lm/W requires only one-seventh the wattage of incandescent bulbs at 14 lm/W for the same lux level. This is why LED retrofits dramatically reduce energy bills while maintaining or improving illumination quality.
Example Calculation
Problem: A 20 m² open-plan office needs 500 lux. Using LED panels with 100 lm/W efficacy, what total wattage is needed and what will it cost annually?
Step 1: Calculate total lumens
Lumens = Lux × Area = 500 × 20 = 10,000 lm
Step 2: Apply maintenance factor
Adjusted Lumens = 10,000 × 1.25 = 12,500 lm
Step 3: Calculate watts
Watts = 12,500 / 100 = 125 W
Step 4: Determine fixtures (using 40W LED panels at 4,000 lm each)
Fixtures = 12,500 / 4,000 = 3.1 → 4 panels
Actual wattage = 4 × 40W = 160 W
Step 5: Calculate annual cost (8 hrs/day, $0.12/kWh)
Annual kWh = 160 × 8 × 365 / 1000 = 467.2 kWh
Annual cost = 467.2 × $0.12 = $56.06/year
Answer: The 20 m² office needs 125W of LED lighting (4 panels at 40W each) to achieve 500 lux, costing approximately $56 per year in electricity at 8 hours daily use.
Lux to Watts Reference Table
Total watts needed for various room sizes and lux levels using LED fixtures at 100 lm/W efficacy.
| Room Area | 150 lux (W) | 300 lux (W) | 500 lux (W) | 750 lux (W) |
|---|---|---|---|---|
| 10 m² | 15 | 30 | 50 | 75 |
| 15 m² | 22.5 | 45 | 75 | 112.5 |
| 20 m² | 30 | 60 | 100 | 150 |
| 25 m² | 37.5 | 75 | 125 | 187.5 |
| 30 m² | 45 | 90 | 150 | 225 |
| 40 m² | 60 | 120 | 200 | 300 |
| 50 m² | 75 | 150 | 250 | 375 |
| 100 m² | 150 | 300 | 500 | 750 |
Values assume LED efficacy of 100 lm/W without maintenance factor. For CFL fixtures (60 lm/W), multiply by 1.67. For halogen (20 lm/W), multiply by 5. For incandescent (14 lm/W), multiply by 7.14. Add 25% for maintenance factor in practical installations.
Energy Efficiency Comparison
The choice of lighting technology dramatically affects the wattage needed to achieve any given lux level. Understanding these differences is crucial for energy budgeting, electrical circuit planning, and operating cost estimation. Here is how different technologies compare for a typical 20 m² office requiring 500 lux (10,000 lumens):
| Technology | Efficacy | Watts Needed | Annual Cost | Heat Output |
|---|---|---|---|---|
| LED | 100 lm/W | 100 W | $35.04 | 341 BTU/hr |
| CFL | 60 lm/W | 167 W | $58.40 | 569 BTU/hr |
| Halogen | 20 lm/W | 500 W | $175.20 | 1,706 BTU/hr |
| Incandescent | 14 lm/W | 714 W | $250.10 | 2,436 BTU/hr |
LED lighting requires 7× less power than incandescent for the same illumination, saving over $215 per year for a single 20 m² room. The reduced heat output also lowers air conditioning costs in warm climates — each watt of lighting adds approximately 3.41 BTU/hr of heat to the space that must be removed by HVAC systems.
Frequently Asked Questions
How many watts per square meter for LED lighting?
For LED lighting at 100 lm/W efficacy: Corridors (150 lux) need 1.5 W/m². Living rooms (200 lux) need 2 W/m². Offices (500 lux) need 5 W/m². Retail (750 lux) needs 7.5 W/m². Detailed work (1000 lux) needs 10 W/m². These are baseline values — add 25% for maintenance factor. The watts-per-square-meter metric is commonly used by electricians for quick circuit load calculations during building design.
Can I directly convert lux to watts?
Not directly — you need to know both the room area and the bulb efficacy. Lux measures light on a surface, while watts measure electrical power. The conversion requires two steps: first multiply lux by area to get lumens (the total light needed), then divide lumens by efficacy to get watts (the power needed to produce that light). Without knowing the area, you cannot determine total lumens; without knowing efficacy, you cannot determine watts.
What efficacy should I use for LED calculations?
For planning purposes, use 100 lm/W as a conservative average for modern LED fixtures. Budget LEDs achieve 80–90 lm/W, standard quality LEDs reach 100–130 lm/W, and premium high-efficacy LEDs achieve 150–200 lm/W. High-CRI LEDs (CRI > 90) typically fall in the 80–120 lm/W range. For the most accurate calculation, check the specific fixture's datasheet for its rated lumens and wattage, then calculate actual efficacy = lumens / watts.
How do I calculate lighting for an open-plan office?
For open-plan offices: Use 500 lux as the target (EN 12464-1 standard). Measure the total floor area. Apply the formula: Watts = (500 × Area) / Efficacy. Use a maintenance factor of 1.25. Plan fixture spacing based on the 1.5× mounting height rule (spacing between fixtures should not exceed 1.5× the mounting height above the work plane). For a 100 m² office with LED panels at 100 lm/W: 500 × 100 × 1.25 / 100 = 625W total, typically achieved with 16 LED panels at 40W each.
Does the formula account for natural daylight?
No, the basic formula calculates artificial lighting needed to achieve the target lux independently. In spaces with windows, daylight contributes significant illumination during daytime hours, reducing the artificial lighting requirement. Modern offices use daylight-responsive dimming systems that automatically reduce artificial light when natural light is sufficient. This can reduce lighting energy consumption by 30–60% in perimeter zones. For energy calculations, apply a daylight factor of 0.5–0.7 to perimeter zones within 5 meters of windows.
How much does it cost to light a room per year?
Annual lighting cost = (Total Watts × Hours/day × 365) / 1000 × electricity rate. For a typical 20 m² room at 300 lux with LED lighting (60W total) used 6 hours/day at $0.12/kWh: Cost = (60 × 6 × 365) / 1000 × 0.12 = $15.77/year. For an office at 500 lux (100W) used 10 hours/day: Cost = (100 × 10 × 365) / 1000 × 0.12 = $43.80/year. Commercial rates vary from $0.08–$0.25/kWh depending on location and demand charges.