Solar Street Lights Sizing 101: Panels, Batteries, and Autonomy Days Explained

Solar Street Lights Sizing 101:

Panels, Batteries, and Autonomy Days Explained

A fast, practical guide to solar street light sizing. Use this to brief your team and sanity-check vendor proposals. Target keywords: solar street light sizing, LiFePO4.

Key concepts you must set first

• Irradiance (Peak Sun Hours, PSH): Use worst-month PSH at the tilted plane for your site, not the annual average. Example: 4.5 PSH (good sun), 2.5–3.0 PSH (higher latitudes/winter).

• Autonomy days: How many nights the system must run with zero charging (cloud, storms). Typical 2–3 days for urban reliability; 4–5 for critical or remote sites.

• MPPT vs PWM: MPPT harvests more energy (10–25% vs PWM), handles higher PV voltages safely, and is the default for modern systems. Size it for array Voc in cold and adequate charge current.

• LiFePO4 basics: High cycle life and safe chemistry. Design DoD 60–80% for long life; capacity drops in cold (apply 0.8–0.9 derating below 0–10°C). Common packs are 12.8 V (4S) or 25.6 V (8S).

Sizing workflow (the 5-step method)

1. Define the nightly load

• Build a dimming profile (e.g., 100% early evening, reduced late night).

• Nightly energy (Wh) = sum of Power × Hours for each dim level.

2. Account for system losses

• Driver efficiency, controller, wiring, soiling. A practical overall round-trip for load side is 0.80–0.90.

• Required energy from battery (Wh) = Nightly load / efficiency.

3. Size the PV array

• PV Wh harvest/day ≈ Wp × PSH × ηPV, where ηPV (temp, MPPT, wiring, soiling) ≈ 0.70–0.80.

• Wp required ≈ Nightly battery energy / (PSH × ηPV). Add 20–30% margin.

4. Size the LiFePO4 battery

• Storage Wh needed = Nightly battery energy × Autonomy days.

• Nominal battery Wh = Storage Wh / allowable DoD. Convert to Ah using pack voltage (12.8 V or 25.6 V).

• Apply temperature derating if the battery will be cold.

5. Pick the MPPT controller

• Current rating: Icharge ≥ 1.25 × (Array Wp / Battery V).

• Voltage rating: Controller Voc limit > coldest-day array Voc. Follow module datasheet temp coefficients.

Worked example A (temperate site, compact pole-top)

• Load: 30 W LED, 100% for 5 h, 50% for 7 h → 30×5 + 15×7 = 255 Wh/night.

• Losses: overall 0.85 → Battery must supply ≈ 255/0.85 ≈ 300 Wh/night.

• Site: worst-month PSH = 4.5; ηPV = 0.75.

• PV size: 300 / (4.5 × 0.75) = 300 / 3.375 ≈ 89 Wp. Add 25% → ≈ 110–120 Wp. Choose a 120 W panel.

• Battery: Autonomy 2 days → 300 × 2 = 600 Wh storage.

  • DoD 80% → Nominal Wh ≈ 600 / 0.8 = 750 Wh.

  • At 12.8 V → Ah ≈ 750 / 12.8 ≈ 59 Ah. Choose 12.8 V 60 Ah LiFePO4.

• MPPT: Icharge ≥ 1.25 × (120/12.8) ≈ 11.7 A → a 15 A, 12 V MPPT with sufficient Voc headroom.

Worked example B (higher latitude winter, higher reliability)

• Load: 40 W LED, 100% for 4 h, 50% for 8 h → 40×4 + 20×8 = 320 Wh/night.

• Losses: 0.84 → Battery ≈ 320/0.84 ≈ 381 Wh/night.

• Site: worst-month PSH = 2.5; ηPV = 0.75.

• PV size: 381 / (2.5 × 0.75) = 381 / 1.875 ≈ 203 Wp. Add 30% → ≈ 260–270 Wp (two ~135 W modules or one 260 W).

• Battery: Autonomy 3 days → 381 × 3 ≈ 1143 Wh storage.

  • Cold climate derating 0.8 and DoD 70% for longevity → Nominal Wh ≈ 1143 / (0.8 × 0.7) ≈ 2047 Wh.

  • Use 25.6 V pack to cut current: Ah ≈ 2047 / 25.6 ≈ 80 Ah → choose 25.6 V 80 Ah LiFePO4.

• MPPT: Icharge ≥ 1.25 × (260/25.6) ≈ 12.7 A → a 20 A, 24 V MPPT. Verify cold Voc for the chosen module string.

Practical tips and pitfalls

• Size to the worst month, not annual averages. If vandal- or blackout-critical, increase autonomy by one day.

• Avoid PWM for anything beyond very small, low-cost units. MPPT gives headroom that often downsizes batteries.

• Mount and tilt panels for winter sun and keep them unshaded. Include soiling/wind loads in the mechanical spec.

• Set LVD/HVD correctly in the controller to protect LiFePO4 (follow the battery BMS recommendations).

• Use 25.6 V systems for 40 W+ luminaires to limit cable losses and controller current.

• Verify all datasheets: LED driver efficiency, module NOCT/temperature coefficients, and BMS low-temp charge limits.

• Plan for maintenance: clean panels, check connections, review data logs; LiFePO4 ages slower but not zero.

Takeaway

For reliable, efficient solar street light sizing: calculate real nightly Wh with dimming, design to worst-month PSH, use MPPT, and size LiFePO4 for your autonomy target and DoD. A quick check is PV Wp ≈ Nightly Wh / (PSH × 0.75) and Battery Wh ≈ Nightly Wh × Autonomy / DoD. This keeps your solar street light sizing lean, robust, and ready for real-world conditions.