How Many Watts Does an LED Street Light Need? 50 W, 100 W, 150 W or 200 W

“What wattage do we need?” is usually the first question in a street lighting inquiry, and the honest answer is: it depends on three numbers you already know. The width of the road, the height of the poles, and the lighting class your road owner requires. This guide turns those three numbers into a wattage, with worked examples you can check yourself.

Start from lumens, not watts

A watt measures power drawn, not light delivered. What lights the road is the luminaire’s lumen output, which is wattage × efficacy:

So an 80 W light at 130 lm/W does the same job as a 100 W light at 100 lm/W. When you compare quotes, ask for the measured luminaire lumens (from an integrating-sphere report, not the LED chip rating) and compare those.

The three inputs

1. Road width (W). The light has to cover it. Wider roads need more lumens per metre of road, or poles on both sides.
2. Mounting height (H). Higher poles spread the light further, so they can be spaced further apart, but each luminaire then needs more lumens.
3. Lighting class. EN 13201 classes set the target: about 7.5 lx for a minor road (M5), 15 lx for an urban arterial (M3), 22 lx for a main road (M2), 30 lx for a motorway (M1). Residential P classes run from 2 to 15 lx. See our guide to road lighting classes.

Four worked examples

We ran four typical roads through our free pole height and spacing calculator, assuming LED street lights at 130 lm/W and a maintenance factor of 0.8:

Two things stand out. First, in all four cases the spacing is limited by uniformity (3.5 × the pole height), not by light output: the poles cannot go further apart without dark patches between them, even though the luminaires could light a longer stretch. Second, that means the wattage in each row is more than the class needs. The calculator works out what would be enough at the same spacing: about 30 W for the residential road, about 82 W for the two-lane road, about 100 W for the 10 m road and about 160 W for the four-lane road.

Open this example in the pole spacing calculator and change the wattage to see where the average drops below the target.

What the extra watts cost per kilometre

Choosing the “enough” wattage rather than a round catalogue number looks like a small saving per light, but street lights burn about 4,400 hours a year. Per kilometre of road, at 12 hours a night and US$0.15/kWh:

Over 15 years and a network of tens of kilometres, that is a real budget line. In practice you would not choose exactly 82 W; you would pick the nearest available power and keep a margin for ageing (see below). Modular luminaires and programmable drivers make it easy to set the output to what the road needs rather than to a catalogue step.

Optics: putting the lumens on the road

Two luminaires with the same lumens can light the same road very differently. The light distribution decides how much of the output lands on the carriageway rather than the verge, the houses or the sky:

• Type II (in the IES classification): a fairly narrow, long distribution for narrow roads and paths, with poles at the roadside.
• Type III: a wider throw across the road, the most common choice for two-lane and urban roads lit from one side.
• Type IV: a forward throw for wide roads and areas lit from the edge, such as car parks.

A good road optic also controls glare (the threshold increment in EN 13201) and keeps uniformity high at wide spacing. That is why a well-designed 80 W luminaire can outperform a cheap 120 W one: it wastes less. Ask for the IES or LDT photometric file and have the road calculated with it.

Colour temperature, ageing and the maintenance factor

Colour temperature. Warm-white LEDs (3000 K) are slightly less efficient than neutral white (4000 K), typically by 5–10%. Many cities now specify 3000 K or lower for residential roads to reduce glare and sky glow, so allow a little more wattage for the same lumens.

Ageing. LEDs slowly lose output. A specification such as L80 at 60,000 hours means that at least 80% of the initial lumens remain after 60,000 hours. The maintenance factor (MF) in a lighting calculation accounts for this and for dirt on the cover; 0.8 is a common value for sealed LED street lights in normal environments, lower in dusty or polluted areas.

Constant light output (CLO). Some drivers start at a reduced current and raise it gradually over the years, so the light stays at the design level and the early-life energy is saved. It is a neat way to avoid over-lighting a new installation.

Quick wattage table

For budgets and first quotes, this table gives typical ranges for modern LED street lights at 120–140 lm/W:

The final value comes from a photometric calculation with the luminaire’s IES file, which checks uniformity and glare as well as the average.

Why not just buy the biggest wattage?

Over-lighting costs money three times: a higher price per luminaire, more electricity every night for 15 years or more, and, for solar projects, a bigger panel and battery. It also increases glare and light spill into houses. The right approach is to pick the lowest wattage that meets the class at the spacing the poles allow, then add a small margin for lumen depreciation.

Replacing old lamps

If you are retrofitting, the old lamp wattage is a useful sanity check but not the answer. A 250 W high-pressure sodium lamp draws about 290 W with its ballast; it is usually replaced by 100–120 W of LED. Our HPS to LED conversion chart gives the equivalents for 70–1000 W lamps, and the LED retrofit calculator shows the payback.

Choosing a luminaire that can follow the calculation

A modular street light makes wattage selection easy, because one housing covers a wide range: our LDM30C modular LED street light runs from 30 W to 350 W in seven housing sizes, so a project with several road types can use one fitting throughout. For solar roads, see our guide to solar street light pole height and spacing.

Send us your road widths, pole heights and the class from your tender. We will return the wattage for each road with a photometric layout.