How to choose a solar panel for street lighting

Solar street lighting looks like a simple product until you have to guarantee that the light is still on at 4 a.m. in the third week of a wet monsoon. The module choice is where most of that guarantee is either earned or lost.

Start from the night, not the day

Work out the energy the luminaire consumes in one night:

Luminaire watts × hours of operation = watt-hours per night

A 24W LED running from dusk to dawn for 11 hours needs 264Wh. If the fitting dims to half output after midnight, the real figure is lower — use the actual dimming profile, not the peak rating, or you will oversize the whole system and lose the tender on price.

Then decide autonomy

Autonomy is the number of days the light must keep working with effectively no charging. For most Indian street lighting, two days is a sensible design point and three is common in tenders for high-rainfall districts. Autonomy multiplies the battery size directly, so it is the single most expensive line in the specification. Do not choose it casually, and do not let it be assumed.

Size the battery, then the array

1. Battery capacity = (watt-hours per night × days of autonomy) ÷ (system voltage × usable depth of discharge). Lead-acid is usually taken to 50% DoD; LiFePO₄ goes considerably deeper.
2. Array size = daily watt-hours ÷ effective peak sun hours, plus a margin. Use the peak sun hours for the worst month at the site, not the annual average, and add 20–30% for controller losses, dust, cable drop and panel ageing.
3. Recharge check. The array must replace a night's consumption and claw back the autonomy reserve within a reasonable number of good days. An array sized for exactly one night's use never recovers from a bad week.

The constraints that are not electrical

This is where street lighting differs from every other application:

• The bracket. The module has to physically fit the pole-top bracket. A perfectly sized panel that fouls the luminaire arm is not a solution.
• Wind loading. A large panel on a tall pole is a sail. Structure and foundation have to be designed for it, and a smaller, higher-efficiency module sometimes wins for that reason alone.
• Tilt and orientation. Fixed tilt on a pole is usually set for winter performance, since that is the limiting season.
• Theft and vandalism. Fixing method and hardware matter as much as the electrical spec in some locations.
• Uniformity across the tender. Hundreds of poles need identical modules with identical documentation. A mixed batch creates a maintenance problem for the next fifteen years.

Which wattage, in practice

Rather than publish a table that pretends every site is the same, here is the honest version: the wattage falls out of the arithmetic above, and it is usually somewhere between 40Wp and 150Wp for a single-luminaire pole, moving higher for twin fittings, longer autonomy or high-mast applications.

The InnovaEDGE line was built for exactly this job: ten wattages from 40Wp to 335Wp in the 12V class, so the array can be matched to the actual load instead of rounded up to the nearest stock item. And because we make them in Bengaluru, a tender that needs three hundred identical modules with identical documentation is a production run, not a sourcing exercise.

Before you quote

• Confirm the luminaire wattage and its dimming profile
• Confirm the required autonomy in days — in writing
• Check the bracket dimensions against the module you intend to use
• Check the controller is rated above array Isc and above Voc in cold conditions
• Confirm what documentation the tender requires, and when

Send us the luminaire, the hours and the autonomy and we will size the array, battery and controller with you — usually while your customer is still on the phone.