What is Luminous Efficacy? - The Most Important LED Efficiency Metric

Q: Is higher efficacy always better?

For energy efficiency: yes. But very high efficacy claims (above 180 lm/W at affordable price points) should be scrutinised - either the claim is for chip-level (not luminaire-level) efficacy, or the optical design is sacrificing beam quality for raw lumen count. A light with 160 lm/W but poor beam optics may illuminate the sky rather than the road. Efficacy must be paired with the correct photometric distribution for the road type.

Q: My existing street lights say 150W sodium but I want to replace with 40W LED. Is this correct?

Check the lumen output of the sodium lamp (typically 13,500–17,500 lumens for 150W HPS) versus the LED (40W at 140 lm/W = 5,600 lumens). A 40W LED is significantly less bright than a 150W HPS in total lumens. However, the LED delivers its lumens directly to the road (directional optics) while HPS wastes 40–50% upward. A photometric calculation is needed to verify whether 40W LED achieves the required IS 1944 lux for your specific road.

Q: Does efficacy change over time as the LED ages?

Yes - this is lumen depreciation. A light starting at 140 lm/W will produce only 98 lm/W (70% × 140) at L70 end of life. For IS 1944 compliance, road lighting must meet the minimum lux at the end of design life - so the design should use maintained efficacy (70–80% of initial), not initial efficacy. This is another reason to start with high initial efficacy - even at 70% depreciation, a 150 lm/W light outperforms a 90 lm/W new light.

What is Luminous Efficacy? - The Most Important LED Efficiency Metric

Luminous efficacy - measured in lumens per watt (lm/W) - is the single most important number for comparing LED street lights. It tells you how much useful light you get for every watt of electricity consumed. Understanding it lets you compare products honestly and avoid paying more electricity for less light.

By Xera Tech Team · Updated 12 Jun 2026 · 5 sections

The Simple Definition

Luminous efficacy = lumen output ÷ power input (lm/W). A 40W LED producing 5,200 lumens has an efficacy of 5,200 ÷ 40 = 130 lm/W. A 40W LED producing 3,600 lumens has an efficacy of 90 lm/W. Both consume 40W of electricity, both cost the same to run - but the 130 lm/W light produces 44% more road illumination. To get the same road brightness from the 90 lm/W light, you would need a 57W light - costing 43% more electricity, every night, for 15 years.

Historical Context - Why Efficacy Matters Now

Incandescent bulb: 10–15 lm/W. Fluorescent tube: 60–80 lm/W. High-pressure sodium (HPS): 80–130 lm/W (but omnidirectional - road-delivered efficacy is 50–70 lm/W). Early LED (2010): 70–90 lm/W. Quality LED today (2025): 130–170 lm/W. A current quality LED delivers 2–3× more light per watt than a sodium lamp. This improvement is entirely captured in the efficacy number - not the wattage. A government tender that specifies '100W LED to replace 250W sodium' is correct only if the LED achieves 130+ lm/W. A 100W LED at 80 lm/W (poor quality) is a failed upgrade.

System Efficacy vs Luminaire Efficacy

Chip-level efficacy: measured at the LED chip junction under controlled conditions - always the highest value (up to 200+ lm/W for premium chips). Luminaire efficacy (LER - Luminaire Efficacy Rating): accounts for optical losses in the housing and lens. Typically 10–15% lower than chip efficacy. This is the value in the product specification sheet. System efficacy: accounts for driver losses (typically 5–10% additional loss). The system efficacy value is what actually reaches the road. Always ask for luminaire efficacy - not chip efficacy. The ERDA photometric test report measures luminaire efficacy directly and is the authoritative reference.

What to Specify and How to Verify

Minimum efficacy for Indian tenders: 130 lm/W for general municipal LED street lights (IS 16905 and BIS tenders). 140+ lm/W for premium tenders and projects where lifecycle cost matters. 150+ lm/W for NHAI and export-quality products. How to verify: the ERDA or NABL photometric test report (LM-79 test) reports total luminaire lumens and input power - divide for efficacy. Do not trust specification sheets without supporting test reports. Xera Tech's products achieve 140–160 lm/W luminaire efficacy across the street light and flood light range - independently verified by ERDA.

Frequently Asked Questions