Last updated: 2026-05-16

Why Solar Panel Sizing Matters

Getting solar panel sizing right matters. Too small a system and you still draw heavily from the grid; too large and you overpay for capacity your roof will never fully use. This calculator translates your electricity bill and location into a concrete system specification.

How It Works

The calculation converts annual energy demand into a DC nameplate capacity, accounting for the hours of useful sunlight your location receives and the energy lost in a real installation:

where PSH is peak sun hours per day, $L$ is system losses in percent, and $W_p$ is panel wattage. ⌈·⌉ is the ceiling function — rounds up to the nearest whole panel (a fraction of a panel still requires a full physical unit). Installation cost is simply $P_{\text{sys}} \times 1000 \times \text{price per watt}$.

Peak Sun Hours by Region

Peak sun hours (PSH) is not the same as daylight hours — it is the equivalent number of hours at full 1,000 W/m² irradiance. A partly-cloudy 14-hour day may deliver only 4 PSH of usable energy.

For a precise local value, use NASA POWER, PVGIS (Europe), or the National Renewable Energy Laboratory's PVWatts tool.

System Losses Explained

A default of 20% losses is conservative but realistic for a typical residential roof over its lifetime:

• Inverter conversion: ~3–4%
• DC wiring resistance: ~2%
• Soiling and dust: ~2–5%
• Panel temperature (above 25°C test point): ~3–10% in summer
• Module mismatch and shading: ~2–4%
• Long-term degradation (~0.5%/year)

NREL's PVWatts uses 14% for a brand-new, perfectly-sited system. Use 20% for a realistic residential estimate.

Installed Cost Benchmarks (2024)

The US federal residential clean-energy credit (30% through 2032) reduces gross cost by roughly one-third. State and utility incentives vary widely.