Why Battery Sizing Beats Wattage
When a solar street light fails early, it is almost never the LED — it is the battery running out of usable capacity. Correct sizing means the battery delivers full nightly load through cloudy stretches without deep-discharging itself to death. This guide gives you the exact math.
The Core Formula
Required battery capacity (Wh):
Battery Wh = (Nightly load Wh × Autonomy days) ÷ (DoD × Temperature derating × Inverter/driver efficiency)
Where:
- Nightly load Wh = LED power (W) × equivalent full-power hours. With motion dimming (30% base, 100% on trigger), a 60 W light averages ~35–40 W over a 12-hour night.
- Autonomy days = rainy/cloudy days the battery must bridge: 2 days minimum, 3 for coastal/tropical zones.
- DoD (depth of discharge) — LiFePO4 tolerates 80–90% but designing at 80% extends cycle life toward 6,000+.
- Temperature derating — capacity drops in heat over time and in cold immediately; use 0.85 for tropical climates as an aging + heat factor.
- Driver efficiency — 0.90 typical.
Worked Example 1: 60 W All-in-One, Estate Road (Lagos)
- Nightly load: 60 W × 12 h with dimming ≈ 480 Wh (assume average 40 W)
- Autonomy: 2 days
- DoD 0.8, temp factor 0.85, driver 0.9
Battery = (480 × 2) ÷ (0.8 × 0.85 × 0.9) ≈ 1,570 Wh → a 12.8 V system needs ~123 Ah, so specify 12.8 V 150 Ah LiFePO4 (1,920 Wh) for margin. This matches Woneng's 6060P-class integrated units.
Worked Example 2: 150 W Split System, Highway (Kano)
- Nightly load: 150 W × 11 h, full power no dimming (highway spec) ≈ 1,650 Wh
- Autonomy: 2 days, but add harmattan dust factor on panel harvest (size panel +20%)
- DoD 0.8, temp 0.85, driver 0.9
Battery = (1,650 × 2) ÷ (0.8 × 0.85 × 0.9) ≈ 5,400 Wh → specify 25.6 V 210 Ah LiFePO4 (5,376 Wh) — or 2× 25.6 V 105 Ah packs in parallel for serviceability.
Panel Sizing: The Other Half of the Equation
Battery capacity is useless if the panel cannot refill it. Rule:
Panel W ≥ (Nightly load Wh ÷ Peak sun hours ÷ 0.75 system efficiency) × 1.2 dust/cloud margin
Lagos ≈ 4.5 peak sun hours: 480 Wh ÷ 4.5 ÷ 0.75 × 1.2 ≈ 170 W panel for the 60 W light. Most integrated 60 W lights carry 60–90 W panels and rely on dimming — which is why dimming strategy is a spec item, not a feature bullet.
Chemistry Choice: Lead-Acid vs LiFePO4 vs NMC
| Lead-acid (GEL) | LiFePO4 | NMC lithium | |
|---|---|---|---|
| Usable DoD | 50% | 80–90% | 80% |
| Cycle life | 300–600 | 6,000+ | 1,500–2,500 |
| Heat tolerance | Medium | Excellent | Poor (thermal runaway risk) |
| Cost per usable kWh over life | Highest | Lowest | Medium |
| Recommended for Africa | No | Yes | Avoid in hot climates |
Sizing Mistakes That Kill Batteries Early
- Sizing at 100% new capacity — LiFePO4 fades to ~80% by end of life; size for year-5 capacity, not day-1.
- Ignoring dimming in the load calc — or ordering "dimmable" lights whose controller doesn't actually implement staged dimming.
- Battery in the lamp head without thermal separation — every 10 °C above 35 °C roughly halves cycle life.
- One autonomy number for the whole country — coastal South Nigeria and Sahel North have different cloud regimes; specify per site.
Woneng sizes every project quotation with this exact method and publishes the full calculation sheet. Send your road spec — pole height, spacing, lux target, location — and receive a complete sizing sheet with panel, battery and controller recommendation.
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