Solar Sizing Cheat Sheet
Size solar PV arrays and batteries — panel output, peak-sun hours, and system losses.
Reference
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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