- Energy per night = watts × hours at each dimming level. A 30 W LED at full power for 12 hours uses 360 Wh.
- Dimming to 50% after the first 4 hours cuts that to 240 Wh, about one third less.
- Days of autonomy = nights the battery can run with no charging at all. Three is a common minimum.
- LiFePO4 batteries are typically used to about 80% depth of discharge in street lights.
- Location-specific solar radiation data, including monthly values, is freely available from NASA POWER.
Why so many solar street lights fail in July
Most failed solar street lights we are called to look at were not faulty. They were undersized. The panel and battery were chosen for sunny March, so after four grey days in July the battery is empty and stays half-charged till September. People then conclude ‘solar doesn't work here’, which is unfair to a technology that works well when sized properly.
The fix is simple: size for the worst month and add a margin to recover after cloudy spells.
Step 1: Work out energy per night
Nights in Maharashtra run roughly 11 to 13 hours depending on the season. We use 12 hours for the example. The big decision is the dimming profile.
| Profile | Calculation | Energy per night |
|---|---|---|
| Full power all night | 30 W × 12 h | 360 Wh |
| 100% for 4 h, then 50% for 8 h | 30 W × 4 h + 15 W × 8 h | 240 Wh |
| 100% for 4 h, 30% for 8 h | 30 W × 4 h + 9 W × 8 h | 192 Wh |
For roads that are nearly empty after 11 pm, 50% dimming is barely noticeable to people but saves a third of the battery and panel. Many solar poles also add a motion sensor that brings the light back to full when someone walks by.
Step 2: Size the battery
Formula: battery Wh = nightly energy × days of autonomy ÷ usable depth of discharge, plus a margin for ageing and temperature.
| With dimming | Without dimming | |
|---|---|---|
| Nightly energy (incl. ~5% controller losses) | 252 Wh | 378 Wh |
| × 3 nights autonomy ÷ 0.8 DoD × 1.1 margin | 1,040 Wh | 1,559 Wh |
| At 12.8 V LiFePO4 | about 81 Ah → use 80 Ah | about 122 Ah → use 120 Ah |
In the Konkan belt and ghat areas, where cloudy spells run longer, consider four or five nights of autonomy.
Step 3: Size the panel
Formula: panel Wp = nightly energy ÷ (design peak sun hours × system efficiency). Then add a recovery margin so the battery can refill after a cloudy spell.
- Design peak sun hours: use your worst month's daily solar radiation (in kWh/m²/day, which equals peak sun hours). We assume 3.5 for a monsoon month in this example; look up your own district on NASA POWER.
- System efficiency: about 0.75 covers heat, dust, controller and battery losses.
- Recovery margin: about 30% extra.
| With dimming | Without dimming | |
|---|---|---|
| Nightly energy | 252 Wh | 378 Wh |
| ÷ (3.5 h × 0.75) | 96 Wp | 144 Wp |
| + 30% recovery margin | 125 Wp → use 120 to 150 Wp | 187 Wp → use 180 to 200 Wp |
The final comparison
| Item | 30 W with dimming | 30 W full power |
|---|---|---|
| Battery (12.8 V LiFePO4) | 80 Ah | 120 Ah |
| Panel | 120 to 150 Wp | 180 to 200 Wp |
| Relative cost | Lower | Noticeably higher |
| Panel size on a decorative pole | Easier to hide in a design | Larger, harder to integrate |
Placement matters as much as size
- Face the panel south. A tilt roughly equal to your latitude (about 16° to 21° across Maharashtra) works well year-round and helps rain wash off dust.
- Check for shade from trees, buildings and hoardings at all times of year. Even partial shade on one corner of a panel cuts output sharply.
- Keep the battery in a ventilated, shaded enclosure. Heat shortens battery life.
- Use anti-theft fasteners on panel and battery box.
What to ask a solar pole supplier
- Show me the sizing sheet: nightly energy, autonomy, depth of discharge, design sun hours, efficiency.
- Which month and which data source did you use for sun hours?
- What is the dimming profile, and can it be changed on site?
- Battery chemistry, rated cycles and warranty years.
- Charge controller type (MPPT is preferred) and its protections.
- Is the LED fitting BIS registered, and what is its IP rating?
For the money side of solar vs grid, see our 10 and 15-year cost comparison.
References
- NASA POWER — Prediction of Worldwide Energy Resources (solar irradiance data by location) — NASA Langley Research Center
- MNRE scraps second phase of AJAY solar streetlights program (27 April 2020) — Mercom India
- IS 10322 (Part 5/Section 3) — Luminaires for Road and Street Lighting — Bureau of Indian Standards
- IEC 60529 — Degrees of Protection Provided by Enclosures (IP Code); Indian adoption IS/IEC 60529 — International Electrotechnical Commission / BIS
Frequently Asked Questions
What battery size does a 30 W solar street light need?
With 50% dimming after the first 4 hours and three nights of autonomy, about 80 Ah at 12.8 V LiFePO4. Without dimming, about 120 Ah.
What panel size does a 30 W solar street light need?
In our monsoon-month example, 120 to 150 Wp with dimming, or 180 to 200 Wp without dimming. Use your own district's sun data.
Why do solar street lights stop working in the rainy season?
Usually undersizing. If the system was designed for average sunshine, a few cloudy days drain the battery and it cannot recover. Size for the worst month with a recovery margin.
Which direction should the solar panel face?
South, tilted at roughly your latitude (about 16° to 21° in Maharashtra), with no shade at any time of year.
डिमिंग से क्या फायदा है?
रात 10-11 बजे के बाद रोशनी आधी करने से बैटरी और पैनल का साइज़ लगभग एक-तिहाई कम हो जाता है, और सड़क पर फर्क मुश्किल से दिखता है।
कोकणात किती दिवसांचा बॅटरी बॅकअप घ्यावा?
कोकण आणि घाट भागात ढगाळ दिवस जास्त असतात, म्हणून 4 ते 5 रात्रींचा बॅकअप विचारात घ्या.
