Key numbers
| STC | 1 000 W/m², AM1.5, cell 25 °C — panel rated power is at STC |
|---|---|
| NOCT | 800 W/m², 20 °C ambient, 1 m/s wind — more realistic rating |
| Peak sun hours | Daily kWh/m² equivalent of 1 kW/m² — 4–6 for most of US |
| Panel area | Typical 400 W residential panel ≈ 2 m² (200 W/m²) |
| System loss | Allow 20–25% (wiring, inverter, temp, soiling) |
System sizing
| Daily energy | E (Wh) = P_array (W) × peak_sun_hours × (1 − losses) |
|---|---|
| Array size | P_array = daily_load_Wh / (PSH × efficiency) |
| Battery (daily cycle) | Capacity Ah = daily_Wh / (V_bank × DoD × η) |
| Days of autonomy | Multiply battery by N days for cloudy weather |
| Charge controller amps | Array short-circuit current × 1.25 safety margin |
Battery depth of discharge
| Chemistry | Max DoD | Cycle life (typical) |
|---|---|---|
| Lead-acid (flooded) | 50% | ~500 cycles |
| Lead-acid (AGM) | 50% | ~800 cycles |
| LiFePO₄ | 80–90% | 2 000–5 000 cycles |
| Li-ion (NMC) | 80% | ~1 000 cycles |
Notes
- Peak sun hours vary by month — size for worst month (typically December in N. Hemisphere).
- Panel temperature rises reduce output ~0.3–0.5% per °C above 25 °C.
- Use MPPT charge controllers above ~100 W arrays — 20–30% more harvest than PWM.
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