Lumens per watt: what they really measure

Lumens per watt (lm/W) express luminous efficacy: how much light, in lumens, is produced for every watt of electrical power drawn. The higher the value, the more efficient the source.

The key is understanding what the stated value refers to. A datasheet may quote any of three different levels:
– LED efficacy: the value for the individual chip, measured in the laboratory. It is the highest figure and the least representative of a real installation;
– strip or module efficacy: includes losses in the circuit and the strip’s components;
– system efficacy: also includes the driver, profile and diffuser. It is the only value that truly describes how much light reaches the space.

Where efficiency is lost

Between the rated figure and the light actually delivered, several losses come into play:
– the driver: no driver converts 100% of the energy it receives; some is dissipated as heat;
– the diffuser: an opal diffuser evens out the light but absorbs part of it, and the more opaque it is, the more it absorbs;
– temperature: an LED running hot, for example in an unsuitable profile, emits less light for the same power.

A simple example. A strip with a rated efficacy of 120 lm/W, powered by a driver with 90% efficiency and installed in a profile whose diffuser transmits 80% of the light, delivers a system efficacy of:
120 × 0.90 × 0.80 = approximately 86 lm/W
Almost 30% lower than the starting figure. This is why, when comparing different solutions, it is essential to compare like with like at system level.

Power factor: the figure that is often overlooked

Power factor (PF) is a value between 0 and 1. It indicates how much of the power drawn from the mains is actually converted into useful work, in this case light.

In technical terms, it is the ratio between active power (W), the power that produces light, and apparent power (VA), the power the mains supply actually has to deliver.
– PF close to 1 (e.g. 0.95): the driver draws current from the mains in proportion to the power it uses.
– Low PF (e.g. 0.5–0.6): for the same wattage, the driver draws far more current than necessary.

Why it matters

A low power factor does not increase the watts consumed, but it has real consequences for the installation:
– more current in the cables, meaning higher losses and the need for larger conductor sizes;
– oversized protective devices and circuits, particularly in installations with many drivers;
– higher energy bills, because business customers are often charged for reactive power above certain thresholds;
– disturbance on the electrical network, which accumulates when drivers are numerous.

In professional installations, specifying drivers with a high power factor is a design decision, not a detail.

From rated figure to real-world consumption: a worked example

Let’s look at how the numbers change in a real case.

The installation: 50 metres of 10 W/m LED strip in a retail space. Strip load: 500 W.

Operation: 12 hours a day, 300 days a year, i.e. 3,600 hours per year.

1. Driver efficiency

The strip draws 500 W, but the installation also draws the driver’s losses from the mains.
– Driver with 90% efficiency: 500 ÷ 0.90 = 556 W drawn from the mains.
– Driver with 80% efficiency: 500 ÷ 0.80 = 625 W drawn from the mains.

On an annual basis:
– 90% driver: 556 W × 3,600 h = approximately 2,000 kWh per year;
– 80% driver: 625 W × 3,600 h = 2,250 kWh per year.

The difference is approximately 250 kWh per year. Assuming an energy cost of £0.25/kWh, that is around £62 per year, or more than £600 over ten years, for a single line of light.

2. Power factor

Now let’s compare two drivers with the same efficiency (90%) but different power factors, on a 230 V supply:
– PF 0.95: 556 W ÷ 0.95 = 585 VA, a current of approximately 2.5 A;
– PF 0.60: 556 W ÷ 0.60 = 926 VA, a current of approximately 4 A.

The watts consumed are identical, but with a low PF the circuit has to carry almost 60% more current. Multiplied across every driver in a building, the effect on cabling, protective devices and energy bills becomes significant.

How to assess the efficiency of an installation correctly

A few practical guidelines for designers and electrical contractors.

Think in terms of system efficacy. Compare solutions by considering strip, driver, profile and diffuser together, not just the lm/W figure of the light source.

Check driver efficiency. It has a direct impact on annual consumption and should be read from the datasheet, ideally at the actual load conditions.

Check the power factor. In installations with many drivers, specify products with a high PF.

Size the driver correctly. An oversized driver operates far from its peak efficiency point; an undersized one operates under stress.

Manage heat properly. An adequate aluminium profile keeps the LEDs at the right temperature and preserves efficiency over time.

Frequently asked questions about LED energy efficiency

What does lm/W mean?

Lumens per watt: it indicates how much light a source produces for every watt of power it draws. The higher the value, the more efficient the source.

What is the difference between LED efficacy and system efficacy?

LED efficacy refers to the chip alone. System efficacy also includes the driver, profile and diffuser, and describes the light actually delivered to the space.

What is the power factor of an LED driver?

It is the ratio between active power and apparent power. A value close to 1 indicates that the driver draws only the current it needs from the mains.

Does a low power factor increase consumption in watts?

No, but it increases the current drawn from the mains. This results in higher cable losses, more heavily loaded circuits and protective devices and, for business customers, possible reactive power charges.

How do you calculate the annual consumption of an LED installation?

Divide the light source’s power by the driver efficiency to obtain the power drawn from the mains, then multiply it by the annual operating hours.

Want to assess the efficiency of your next installation?

From the choice of strip to the sizing of the drivers, every component affects real-world consumption. The LEDCO technical team will help you design an efficient system, from the first metre to the last.

→ Contact the LEDCO technical team