Sizing a solar street light comes down to three calculations: daily energy demand (LED wattage × the nightly run profile), battery capacity (demand × autonomy days ÷ usable depth of discharge), and panel wattage (demand ÷ worst-month peak sun hours × a 0.7 loss factor). A 60 W fixture on an adaptive profile needs roughly a 1,750 Wh battery and a 180 Wp panel in a 4-sun-hour climate. This guide walks each step with worked numbers, shows how climate changes the answer, and flags the shortcuts that produce lights that die at 2 a.m. in the rainy season.
Key Takeaways
- Size for the worst month’s peak sun hours (PSH), never the annual average — the same fixture is a different system in a different climate.
- An adaptive dimming profile (100% until midnight, 40% after) cuts the daily energy budget by roughly a third versus all-night full power.
- Battery: LiFePO₄ at 80% usable depth of discharge with 2–3 autonomy days is the quality baseline; 3–5 days for rainy climates.
- Panel: divide nightly demand by (PSH × 0.7) — the 0.7 covers controller efficiency, temperature derating, and dust.
- “600 W solar street light” marketing wattages are meaningless; specify delivered lumens and the run profile instead.
The Four Numbers That Size Everything
Every honest solar street light quotation starts by asking for four inputs:
- LED load (W). The fixture’s actual draw at its operating point — not the marketing number on the listing.
- Nightly run profile (h). Full power all night? Dimmed after midnight? Motion-boosted? The profile is the single biggest lever on system size.
- Peak Sun Hours (PSH) at the site in the worst month. PSH is the daily solar energy expressed as equivalent full-intensity hours; 4.0 PSH means the day delivers the energy of 4 hours at full sun.
- Autonomy days. How many sunless days the battery must bridge alone: 2–3 is typical, 3–5 for monsoon climates or roads where dark nights are unacceptable.
With those four, the arithmetic is short. Without them, any quotation is a guess wearing a spec sheet.
Step 1 — Daily Energy Demand
Take a 60 W LED street light running the most common adaptive profile:
| Period | Output | Hours | Energy |
|---|---|---|---|
| Dusk – midnight | 100% (60 W) | 5 h | 300 Wh |
| Midnight – dawn | 40% (24 W) | 7 h | 168 Wh |
| Total | 12 h | ≈ 468 Wh/night |
Running flat-out for 12 hours would demand 720 Wh — the adaptive profile cuts the energy budget, and therefore the battery and panel, by roughly a third. Motion-sensor boost profiles (20% baseline, 100% on detection) cut further on genuinely quiet paths. This is why a serious supplier’s first question is about your dimming profile, and why two “60 W” systems can legitimately differ by 40% in battery size.
Step 2 — Battery Capacity
Battery (Wh) = nightly demand × autonomy days ÷ usable depth of discharge
With LiFePO₄ chemistry (usable DoD ≈ 80%) and 3 autonomy days:
468 × 3 ÷ 0.8 ≈ 1,755 Wh → at 12.8 V nominal, ≈ 137 Ah.
Why LiFePO₄ won
| Chemistry | Cycle life | Usable DoD | Heat tolerance | Verdict |
|---|---|---|---|---|
| Lead-acid (GEL) | 500 – 800 | ~50% | Poor | Obsolete for quality systems |
| Ternary lithium (NMC) | 800 – 1,500 | ~80% | Moderate | Acceptable, thermally fussier |
| LiFePO₄ | 2,000 – 4,000+ | ~80% | Good | The quality baseline |
One under-appreciated fact: in hot climates, battery temperature — not sunlight — usually decides real service life. Chemistry ages exponentially with heat, so housings that shade and ventilate the pack, or bury it at the pole base in split designs, buy years of life. When comparing quotations, ask where the battery lives and what its cycle rating is at 45 °C, not just at 25 °C.
Step 3 — Panel Size
The panel must replace one night’s energy during one worst-month day, through real-world losses:
Panel (Wp) = nightly demand ÷ (PSH × 0.7)
The 0.7 combined factor covers charge-controller efficiency, panel temperature derating, wiring losses, and dust soiling.
Climate A — PSH 4.0 (typical worst month across much of the Middle East and North Africa): 468 ÷ (4.0 × 0.7) ≈ 167 Wp → specify 180 Wp.
Climate B — PSH 3.0 (tropical rainy season): 468 ÷ (3.0 × 0.7) ≈ 223 Wp → specify 240 Wp, and raise autonomy to 4–5 days.
Same fixture, same profile — a 33% larger panel and a bigger battery, purely from geography. The same fixture is a different system in a different climate, which is why reputable manufacturers configure battery and panel per project rather than selling one universal SKU. It is also why our solar lighting range quotes system configurations from your location’s sun-hours rather than from a fixed menu.
All-in-One vs Split: How Architecture Changes the Math
| All-in-one (AIO) | Split design | |
|---|---|---|
| Panel size limit | Capped by fixture footprint | Free — size to climate |
| Panel angle | Near-horizontal | Tilted to latitude |
| Worst-month yield | Loses 10 – 25% at higher latitudes | Optimized |
| Battery location | In fixture (hotter) | Pole-top, base, or buried (cooler) |
| Installation | Minutes, one unit | More parts, more flexibility |
| Best fit | Low latitudes, pathways, parks | Higher latitudes, rainy seasons, higher wattages |
The AIO’s near-horizontal integrated panel is fine at 15° latitude and costly at 40°, where a latitude-tilted split panel harvests decisively more in the short days of the worst month. Roads held to formal lighting classes — the P and M classes of EN 13201 road lighting design — usually justify split architecture for exactly this reason.
Reality Checks That Separate Good Systems From Disappointments
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MPPT over PWM. An MPPT charge controller harvests measurably more than PWM, especially in cool mornings and cloudy weather — the exact conditions that stress the energy budget.
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De-rate for dust. Desert and agricultural sites lose 10–20% of panel yield between cleanings; the 0.7 loss factor assumes normal soiling, not neglect. Plan a cleaning interval like any other maintenance task.
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Verify the LED claim. “600 W solar street light” listings routinely describe nothing measurable. Specify delivered lumens, luminous efficacy, and the run profile; ask for the photometric file just as you would for grid fixtures.
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Read the autonomy honestly. “Works 12 hours” describes one night, not resilience. The autonomy-days figure at your profile is the number that predicts rainy-week behavior.
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Check ingress and wind ratings. IP65 sealing for fixture and battery compartment, and panel mounts rated for local wind loads, are baseline items on coastal and open-road projects.
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Mind the structure. A latitude-tilted panel on a 6–8 m pole adds real sail area; coastal and typhoon-region projects should see a wind-load statement for the pole, bracket, and panel as part of the quotation, not as an assumption.
Procurement Checklist
- Site latitude and worst-month PSH stated in the quotation
- Run profile written out (hours × output %), not just “12 hours working time”
- Battery: chemistry, Wh (not just Ah), usable DoD, cycle life, and operating-temperature rating
- Panel: Wp, technology, tilt (split) or footprint constraint (AIO)
- Controller type (MPPT/PWM) and protection functions
- Delivered lumens + IES file for the lighting design
- Autonomy days at your profile, in writing
- Warranty terms for battery and fixture separately
Frequently Asked Questions
How do I calculate what size solar panel a street light needs? Divide the nightly energy demand by the worst-month peak sun hours times 0.7. A 468 Wh/night system in a 4-PSH climate needs about 167 Wp — round up to 180 Wp.
How many years does the battery last? Quality LiFePO₄ cycled nightly at 80% DoD in moderate temperatures typically delivers 5+ years. Heat is the main killer: shaded, ventilated battery compartments and honest thermal ratings extend life more than any spec-sheet number.
Can solar street lights work through a week of rain? Only if sized for it: 4–5 autonomy days plus a panel large enough to recover afterward. Adaptive and motion profiles stretch reserves dramatically in bad weather; a fixed full-power profile drains them fastest exactly when recharge is weakest.
What solar wattage replaces a 150 W grid street light? Typically a 50–60 W solar LED with proper roadway optics achieves comparable road illuminance — validate with a photometric layout exactly as for grid lighting, since pole spacing and mounting height still govern.
Are all-in-one solar street lights worse than split systems? Not worse — bounded. AIO installs in minutes and excels in high-sun, low-latitude sites for paths and perimeters. Split systems win where the energy budget is tight: high latitude, rainy seasons, higher wattage roads. Match architecture to climate, not to fashion.
Can you size a system for my site? Yes — send the location, road type, and desired run profile and we return the battery/panel configuration with the lighting layout as part of any solar quotation. Off-grid installations for roadways and agricultural sites follow exactly the method in this guide.
The Bottom Line
Solar street light sizing is honest arithmetic: profile the load, multiply for autonomy, divide by the worst month’s sun. The failures in the field are almost never mysterious — they are systems sized to the annual average, batteries cooked by heat, or marketing wattages nobody could measure. Hold every quotation to the four inputs and three formulas in this guide, and an off-grid road will still be lit at 4 a.m. in the wettest week of the year.