KEY TAKEAWAY
The project conditions should determine the solar lighting configuration.
Solar street light brightness should be specified as illuminance on the road surface in lux, not as fixture lumens. As a planning band, residential and access roads are commonly designed around 3-10 lux average, collector roads around 7-15 lux and busy arterial roads around 15-30 lux, with the applicable national standard always taking precedence. Once the lux target, road width, mounting height and pole spacing are fixed, the required luminaire output in lumens follows from a short utilisation-and-maintenance calculation, and the luminaire's efficacy in lumens per watt then sets the size of the solar panel and battery.
Lumens, lux and candela: the three numbers buyers mix up
A luminaire's lumen rating describes how much visible light it emits in total. Lux describes how much of that light lands on a surface: one lux is one lumen spread over one square metre. Candela describes intensity in a single direction, which is what an optical distribution actually shapes. Road-lighting requirements are almost always written in lux or in road-surface luminance, because what matters is what a driver or pedestrian can see, not what the fixture produced in a laboratory.
The distinction has practical consequences. The same 10,000 lumen luminaire can give a comfortable, uniform result on a 6 metre residential street at 6 metres mounting height and an unacceptable one on a 12 metre arterial road at 10 metres with the same spacing. Lumens are an input; lux, uniformity and glare are the outputs a project is judged on.
- Lumen (lm): total light output of the luminaire
- Lux (lx): illuminance, or lumens per square metre delivered on a surface
- Candela (cd): luminous intensity in one direction, shaped by the optics
- Candela per square metre (cd/m2): road-surface luminance, used by most traffic-route classes
- Lumens per watt (lm/W): efficacy, the conversion rate from electrical power into light
Why two lumen figures can describe the same luminaire
Suppliers quote lumens on different bases. An LED or chip lumen figure is measured on the bare light source at a reference junction temperature. A luminaire or system lumen figure is measured after the optics, diffuser, driver losses and real operating temperature have taken their share. The second number is always lower, and it is the only one usable for design.
Reluxlight publishes system luminous flux per model rather than chip output, for example 5,100 lm at 30 W and 20,400 lm at 120 W in the all-in-two street-light series. Ask every supplier for the basis of a quoted figure, and for a photometric file in IES or LDT (EULUMDAT) format rather than a single number: a file describes the distribution, a number does not.
Typical lighting levels by road and area class
Public lighting levels are set by standards, not by preference. In Europe, EN 13201 assigns roads to M classes for motorised traffic, specified in road-surface luminance, and to P classes for pedestrian and low-speed areas, specified in illuminance. CIE 115 is the international recommendation those classes derive from, and North American projects normally work from the IES RP-8 recommended practice. Many countries then publish a national annex or a municipal specification that takes precedence over all of them.
The bands below are the planning ranges commonly used for a first configuration when a formal class has not yet been assigned. Use them to reach the right product band, then confirm the class, maintained value and uniformity criteria against the standard that applies to the site.
| Road or area class | Typical average maintained illuminance | Typical uniformity expectation | Typical mounting height | Indicative luminaire output |
|---|---|---|---|---|
| Pedestrian path, park walkway | about 2-5 lux | continuity matters more than the ratio | 3-4.5 m | roughly 1,500-4,500 lm |
| Residential / access road | about 3-10 lux | overall uniformity often 0.2-0.4 | 4-6 m | roughly 3,000-8,000 lm |
| Collector / distributor road | about 7-15 lux | overall uniformity often 0.4 or better | 6-9 m | roughly 8,000-17,000 lm |
| Arterial / main urban road | about 15-30 lux | overall uniformity 0.4 or better, plus longitudinal criteria | 9-12 m | roughly 17,000-34,000 lm |
| Parking, yard and perimeter areas | about 5-20 lux by activity level | no dark gaps between poles | 5-10 m | roughly 5,000-32,000 lm |
Illuminance or luminance?
Traffic-route classes increasingly specify luminance in candela per square metre because it correlates better with what a driver perceives, and it depends on the road surface reflection properties as well as on the light delivered. Pedestrian and area classes stay with illuminance in lux, because the visual task is different.
If a tender specifies luminance, an illuminance calculation is not a substitute: the road-surface classification has to be part of the model, and the calculation has to be run for the driver's viewing position.
Uniformity is usually the harder requirement
It is easy to hit an average lux number with bright pools under each pole and darkness between them. Standards therefore also fix uniformity: overall uniformity, the ratio of minimum to average illuminance, and on traffic routes longitudinal uniformity along the driver's line of sight.
Uniformity is driven by spacing, mounting height, overhang and optical distribution, not by wattage. That is why adding power rarely repairs a failing layout, and why the spacing-to-height relationship is a photometric decision as much as a civil one.
Glare and obtrusive light cap the useful maximum
Standards also limit glare and light trespass into surrounding properties and the night sky. A higher-output luminaire aimed poorly produces complaints and a worse visual result than a lower-output luminaire with the correct distribution. Road optics exist for this reason: the Reluxlight street-light series use Type II-M or Type III-M distributions so that flux lands on the carriageway rather than in the surroundings.
From a lux target to a lumen requirement
Once the class, geometry and pole arrangement are fixed, an approximate lumen requirement follows from the definition of illuminance. The first-pass form is: required luminaire lumens = (target average lux x area served per luminaire) / (utilisation factor x maintenance factor).
Area served per luminaire is the road width multiplied by the pole spacing for a single-sided or staggered arrangement, and half of that for an opposite (twin) arrangement, because two luminaires share the same stretch of road. The utilisation factor is the fraction of luminaire output landing on the assessed area, commonly 0.3-0.5 for street optics. The maintenance factor covers lumen depreciation and dirt over the maintenance cycle, commonly 0.7-0.85.
Worked example: 6 metre residential road
A 6 m carriageway with single-sided poles at 25 m spacing serves 150 square metres per luminaire. At an 8 lux target with a 0.4 utilisation factor and a 0.8 maintenance factor, the requirement is (8 x 150) / (0.4 x 0.8), or about 3,750 lm.
A 30 W configuration at 5,100 lm system flux covers that with headroom for a lower utilisation factor or a more conservative maintenance assumption. The headroom is not wasted: it lets the controller dim during low-traffic hours and still meet the target when it matters.
Worked example: 10 metre collector with opposite poles
A 10 m carriageway with opposite poles at 30 m spacing also serves 150 square metres per luminaire, because the area is halved between the two sides. At a 20 lux target with the same factors, the requirement is about 9,375 lm, which sits between the 60 W (10,200 lm) and 80 W (13,600 lm) configurations of the split solar street light series.
At that point the decision is no longer about average lux but about which option meets the uniformity and glare criteria at the chosen height and overhang, which only a simulation can answer.
Why the arithmetic is only a first pass
The formula assumes an average utilisation that a real optical distribution may not deliver at the chosen mounting height, tilt and overhang. Before a large order, run the layout in a photometric tool using the manufacturer's IES or LDT file with the real pole positions, arm length, road width, surface classification and maintenance factor.
The simulation is what proves uniformity, glare rating and the value at the worst point on the surface, and it is the document a tender reviewer will ask for. Request it before the commercial discussion rather than after it.
Efficacy in lm/W: the number that sizes the solar system
On a grid-connected road, efficacy mostly determines the electricity bill. On a solar system it determines the size of everything else. Every watt the luminaire consumes has to be collected by the panel, stored in the battery and carried through several days of poor weather, so an efficacy gain compounds through panel area, battery capacity, pole loading and shipping volume.
The Reluxlight street-light series are published at 170 lm/W system efficacy across the all-in-one, all-in-two and split platforms, which is why a 60 W configuration is rated at 10,200 lm and a 200 W split configuration at 34,000 lm. Because that figure is stated at system level, it can be used directly in the lumen calculation above without a separate allowance for driver and optical losses.
Compare efficacy on the same basis
Efficacy claims are only comparable when the measurement conditions match, and five questions settle most disagreements.
- Is the figure LED-only or whole-luminaire (system) efficacy?
- At what drive current and junction or ambient temperature was it measured?
- Does it include the driver, optic, diffuser and any sensor losses?
- Is it day-one output or a maintained value after depreciation?
- Which colour temperature and colour rendering index does it correspond to?
Colour temperature and CRI cost efficacy
Warmer colour temperatures and higher colour rendering both reduce lumens per watt for the same LED platform, so a comparison has to hold the optical specification constant. The Reluxlight street series offer 2700K to 6500K with CRI of 70 or above, with an 80-plus CRI option listed on the integrated series, so fix the operating point in the specification before flux figures from different suppliers are placed side by side, and keep one colour temperature across the project for a consistent street scene.
Design for maintained output, not day-one output
LED output declines slowly over service life, and dirt on the optic reduces delivered light further. LED lumen maintenance is characterised using the LM-80 measurement method and projected with TM-21, and results are normally expressed as an L-value such as L80 or L70 at a stated number of hours and temperature.
The design should meet its target at the end of the maintenance period, not on the day of handover, which is what the maintenance factor represents. Heat is the main accelerator of depreciation, so thermal design and the specified ambient range, published as -25 to 65 degrees Celsius on the all-in-two and split series, are part of the brightness question.
Dusk-to-dawn or motion-based: choosing an operating profile
Brightness is not one value across the night. The operating profile decides how much light is delivered in each hour, and it is the main tool for reconciling a lux target with a finite battery. Four families are used in practice, often in combination.
| Profile | How it behaves | Energy vs constant output | Best fit | Watch-outs |
|---|---|---|---|---|
| Dusk-to-dawn, full output | Constant output from switch-on to switch-off | Highest; the 100% reference | Roads that must hold a class all night | Largest panel and battery for the same light |
| Scheduled (time) dimming | Steps output down after peak hours on a timer | Commonly 40-70% of constant output | Roads and campuses with predictable traffic peaks | The class may set a floor; night length changes by season |
| Motion-triggered (PIR or microwave) | Holds a low standby level, raises to full output on detection | Commonly 20-50% on low-traffic sites | Paths, car parks, perimeters, intermittently used roads | Detection zone and hold time need tuning on site |
| Adaptive / weather-reactive | Controller reduces output as state of charge falls | Varies with conditions | Any site where some light matters more than full light | Backup-night ratings depend on this reduction |
What the Reluxlight controllers support
The all-in-two series ships with a default staged profile of three hours at 100 per cent, two hours at 80 per cent and seven hours at 25 per cent, with PIR or microwave sensing available as an option. The all-in-one and split series support dusk-to-dawn operation, time-based dimming, remote programming and optional PIR sensing.
Whichever profile is chosen, write it into the specification. It changes both the delivered lux at a given hour and the energy result, so a proposal that omits it cannot be compared with one that includes it.
Sensor settings are a design decision, not a factory default
Motion control only saves energy without generating complaints when these values match how the site is actually used. Agree them during design and record them at commissioning.
- Standby level, commonly in the 20-30 per cent range of full output
- Hold time after the last detection, commonly in the 15-60 second range
- Detection range and the mounting height the sensor is rated for
- Sensitivity in wind, rain and close to moving vegetation
- Whether neighbouring luminaires should raise output together to avoid a strobing corridor
Turning brightness into an energy budget
The energy check is short arithmetic and the fastest way to test whether a brightness target and a battery specification belong together. Nightly consumption is the sum of the power at each stage of the profile multiplied by its hours, plus a small allowance for controller standby and sensor operation.
Take a 60 W configuration on the default staged profile: three hours at 60 W, two hours at 48 W and seven hours at 15 W gives 180 + 96 + 105, or roughly 381 Wh per night. The all-in-two 60 W model pairs with a 12.8 V 54 Ah LiFePO4 pack, which is about 691 Wh of nominal storage; with a conservative usable depth of discharge, the working reserve is nearer 550 to 620 Wh.
That is on the order of one and a half nights at the full default profile, which is why autonomy is published as adaptive backup nights rather than as a fixed figure. A three-to-five night rating assumes the controller steps output down after a day without meaningful charging. That is a legitimate design, but the assumption must be stated: three nights at full brightness and three nights in a reduced reserve mode are not the same product.
Charging is the other half of the budget. A panel does not collect its rated wattage all day; the usable measure is peak sun hours, the equivalent number of hours at full rated irradiance, which varies by location and season. The split series is published with a 7-9 peak-sun-hour charge-recovery assumption, so a cloudier or higher-latitude project should expect to need a larger panel, a larger battery, a more conservative profile, or all three.
- What operating profile is the backup-night rating based on?
- What depth of discharge and end-of-life capacity are assumed?
- What peak-sun-hour figure was used, and for which month?
- What does the controller do on the second and third day without charge?
- Is the panel oversized enough to refill the battery after a bad-weather run, not just to cover one night?
How to specify brightness in a tender or RFQ
A specification suppliers can price consistently states a target, a geometry and a profile. Without all three, the cheapest bid is the one that assumed the least.
- Lighting class or target average maintained lux, plus the uniformity criteria
- Road or area dimensions, pole height, spacing, overhang and arrangement
- Operating hours and the required dimming or sensor profile
- Backup nights and the acceptable reduced-output behaviour in bad weather
- Colour temperature, colour rendering and any glare or light-trespass limits
- Whether a photometric simulation with IES or LDT files is required with the bid
FREQUENTLY ASKED QUESTIONS
Questions buyers ask before configuration
How many lumens does a solar street light need?
There is no single answer, because lumens are an input and the requirement is lux on the surface. Residential and access roads are commonly designed around 3-10 lux average, collector roads around 7-15 lux and arterial roads around 15-30 lux; convert that target to lumens using road width, pole spacing, a utilisation factor of roughly 0.3-0.5 and a maintenance factor of roughly 0.7-0.85, then confirm the class against the local standard.
Is a brighter solar street light always better?
No. Above the required level, extra output adds glare, light trespass into homes and unnecessary energy demand, which on a solar system means a larger panel and battery for no visual benefit. Uniformity and correct optical distribution usually improve a road more than additional lumens.
What does 170 lm/W mean on the Reluxlight street-light series?
It is the published system efficacy for the complete luminaire rather than for the bare LED, which is why a 60 W configuration is rated at 10,200 lm and a 200 W split configuration at 34,000 lm. Being a system-level figure, it can be used directly in a lumen calculation without separate driver or optical loss allowances.
How much energy does motion-based dimming save?
It depends on how often the sensor triggers. On low-traffic paths, car parks and perimeters, a low standby level with short hold times commonly lands in the 20-50 per cent range of constant full-output consumption. On a busy road it saves little, and where a lighting class must be held continuously it may not be permitted.
Do I need a photometric simulation, or is a lux table enough?
A table is enough to choose a product band. A simulation is needed whenever the project has a formal lighting class, a uniformity or glare requirement, an unusual geometry or a commercial quantity, because those criteria depend on the real optical distribution rather than on average assumptions.
Which colour temperature should be specified?
The Reluxlight street series can be supplied from 2700K to 6500K with CRI of 70 or above, and the integrated all-in-one and all-in-two series list an 80-plus CRI option. Cooler light is commonly specified on traffic routes, warmer light next to homes and in landscape settings; warmer and higher-CRI versions produce fewer lumens per watt, so fix the operating point before comparing flux figures between suppliers.