How to size a solar street light for a real road
A working method for sizing solar street lights: pole height, spacing, lux target, lumen output, battery and array, with a worked example you can copy.
Most failed solar street light projects were never lighting projects. They were procurement projects: someone specified 100 W because 100 W sounded stronger than 60 W, nobody calculated the road, and eighteen months later half the poles are dark and the other half are on their second battery.
This is the method we use when a customer sends us a road width and a budget instead of a model number. It takes about fifteen minutes with a calculator.
1. Start from the road, not the wattage
Two numbers describe the road: carriageway width and the lighting class the road actually needs. Everything else follows.
A practical starting rule for pole height is 0.5 to 0.8 times the carriageway width. A 7 m road wants an 8 m pole, not a 4 m one. Spacing then falls between 3 and 4 times the mounting height: an 8 m pole lands at 24 to 32 m spacing.
- Residential and village access: 3 to 5 lux average
- Collector road with pedestrians: 8 to 12 lux
- Arterial road: 15 to 25 lux
- Roundabout or junction: 25 lux and above, and expect to need more poles
Those figures are average maintained illuminance on the carriageway, not peak lux under the fixture. Peak lux is a number vendors like because it looks large.
2. Convert lux into lumens
Lumens needed equals target lux times lit area, divided by two efficiency terms: utilisation (how much of the output actually lands on the road) and maintenance factor (dirt, LED depreciation, ageing optics).
Lumens = (lux x width x spacing) / (utilisation x maintenance)utilisation 0.30 to 0.40 for a typical Type II optic; maintenance 0.70 to 0.80Worked example: a 7 m carriageway, 8 m poles, 30 m spacing, 10 lux target.
(10 x 7 x 30) / (0.35 x 0.75) = 8,000 lmRound up, then pick a 150 lm/W module: about 60 WThat is how you end up at a 60 W luminaire on an 8 m pole, which is a common and correct answer. It is also why a 150 W unit on the same road is not a better product, just a faster way to drain a battery.
3. Size the battery from watt-hours, not amp-hours
Amp-hours mean nothing without the voltage. Convert everything to watt-hours first, then apply depth of discharge and round-trip efficiency.
Wh per night = lumens-driven watts x burning hoursThen divide by 0.85 for driver and controller lossesA 60 W fixture running 11 hours draws about 660 Wh before losses, roughly 780 Wh after them. A 12.8 V 36 Ah pack holds 460 Wh, so at full output it covers well under one night. That is not a defect; it is arithmetic, and it is why every credible design uses a dimming profile.
A profile of 100 percent for the first five hours, then 40 percent for the remaining six, gives 300 + 158 = 458 Wh. Now the same pack covers the night with a little margin. Autonomy claims should always be read against the profile: three to five overcast days almost always means three to five days at the dimmed output, not at full brightness.
4. Size the array from peak sun hours
Panel wattage depends on where you are, not on the fixture. Use local peak sun hours (PSH) for the worst month, not the annual average.
| Region | Worst-month PSH | Panel for 460 Wh/night |
|---|---|---|
| West Africa (Sahel) | 5.0 to 6.0 | 120 to 150 Wp |
| East Africa | 4.5 to 5.5 | 150 to 180 Wp |
| Gulf and Arabian Peninsula | 4.5 to 6.0 | 120 to 170 Wp |
| South and Southeast Asia | 3.5 to 4.5 in monsoon | 180 to 230 Wp |
The monsoon row is the one that catches people. A design that works in February fails in July, and the failure looks like a battery fault when it is really a panel sizing fault.
5. What actually kills installations
- Shade. One tree branch across a panel can cost more than a 20 percent array derate. Walk the route at the height the panel will sit.
- Tilt and orientation. Five degrees off is minor; facing the wrong way is fatal. In the northern tropics, face south; south of the equator, face north.
- Dust and soiling. In Sahelian and Gulf conditions, budget for cleaning twice a year and add 10 percent to the array.
- Corrosion. Coastal air destroys cheap brackets long before the electronics fail. Hot-dip galvanising is not a premium feature there, it is a requirement.
- Theft. A battery in an unlocked pole base is a battery that will leave. Lockable enclosures and anti-tamper fasteners are worth the cost.
6. The specification checklist
If a supplier cannot answer these six questions with numbers, the datasheet is marketing rather than engineering.
- What is the maintained lux on the carriageway at the stated spacing and mounting height?
- What is the battery capacity in Wh, and what dimming profile was assumed for the autonomy claim?
- What is the worst-month peak sun hours figure the array was sized against?
- What is the lumen maintenance after 25,000 hours?
- What is the ingress and impact rating, and which test house issued it?
- What spare-part ratio and lead time apply to a repeat order?
Send us the road width, the mounting height you can achieve and the lighting class you need. We will come back with a configuration and the arithmetic behind it, so you can check the work.
Send us the road.
Width, length, mounting height and the lighting class you need. We will reply with a configuration and the arithmetic behind it.