Getting the battery and solar panel sizing right is the difference between a light that works reliably for years and one that fails after the first cloudy week. This guide explains the calculations with real examples.
The Fundamental Formula
Battery Capacity (Wh) = LED Power (W) × Hours per Night × Autonomy Days ÷ (DoD × System Efficiency)
Understanding Each Variable
| Variable | Description | Typical Value |
|---|---|---|
| LED Power | Rated wattage of the LED fixture | 30W, 60W, 100W, etc. |
| Hours per Night | How many hours the light operates | 10-12 hours (dusk-to-dawn) |
| Autonomy Days | How many days without sun the system should handle | 2-3 days (standard), 5 days (critical) |
| DoD (Depth of Discharge) | How much battery capacity you can safely use | 80% for LiFePO4, 50% for Lead-Acid |
| System Efficiency | Combined losses from wiring, controller, conversion | 85-90% |
Real-World Example: 60W Solar Street Light
Given:
- LED Power: 60W
- Operation: 12 hours/night (full brightness 4h, 50% dim 8h = equivalent 8h)
- Autonomy: 3 days
- Battery type: LiFePO4 (80% DoD)
- System efficiency: 85%
Step 1 — Calculate equivalent watt-hours per night:
Nightly energy = 60W × 8h = 480 Wh
Step 2 — Calculate required battery capacity:
Battery = 480 × 3 ÷ (0.80 × 0.85) = 2,118 Wh
At 12V nominal: 2,118 ÷ 12 = 176.5 Ah
Result: Choose a 12V 180Ah LiFePO4 battery ✓
Solar Panel Sizing
Panel Power (Wp) = Daily Energy Need (Wh) ÷ (Peak Sun Hours × Charging Efficiency)
Peak Sun Hours by Region
| Region | Average PSH | Worst Month PSH |
|---|---|---|
| Middle East / North Africa | 5.5 - 6.5 | 4.5 |
| Southeast Asia | 4.5 - 5.5 | 3.5 |
| Sub-Saharan Africa | 5.0 - 6.0 | 4.0 |
| Southern Europe | 4.0 - 5.0 | 3.0 |
| South America (tropical) | 4.5 - 5.5 | 3.5 |
Always design for the worst month, not the annual average. Source: NASA POWER / SolarGIS.
Panel Calculation Example
For the 60W light example, deployed in Nigeria (worst month PSH: 4.0):
Daily energy need = 480 Wh (from battery calculation)
Charging efficiency = 80% (accounting for MPPT losses, temperature derating, dust)
Panel Wp = 480 ÷ (4.0 × 0.80) = 150 Wp
Result: Choose a 150W-180W monocrystalline panel ✓
Quick Reference Table
| LED Wattage | Battery (LiFePO4) | Solar Panel | Typical Autonomy |
|---|---|---|---|
| 30W | 12V 60Ah | 80-100W | 2-3 days |
| 60W | 12V 120Ah | 150-180W | 2-3 days |
| 100W | 12V 200Ah | 250-300W | 2-3 days |
| 150W | 24V 150Ah | 350-400W | 2-3 days |
| 200W | 24V 200Ah | 450-500W | 2-3 days |
All values assume 10-12 hour operation with dimming, worst-month PSH ≥ 4.0. Adjust for your specific location and requirements.
Dimming: The Free Upgrade
Using motion sensors or timer-based dimming can reduce battery and panel requirements by 40-60%. A 60W light that dims to 30% when no movement is detected effectively operates at ~30W average, cutting the battery from 180Ah to 90Ah — a significant cost saving.
Need help sizing your system? Contact Minglu Lighting with your project specifications and we'll provide a free sizing calculation.
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