Solar Panel Sizing Calculator
Enter monthly kWh usage, peak sun hours, and system losses to find the right rooftop solar system size, panel count, annual output, and estimated cost.
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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:
Edaily=365Emonthly×12 Psys=PSH×(1−L/100)Edaily N=⌈WpPsys×1000⌉ Eannual=Psys×PSH×365×(1−L/100)where PSH is peak sun hours per day, is system losses in percent, and 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 .
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.
| Location | Typical PSH/day |
|---|---|
| Northern Europe, Pacific Northwest | 2.5 – 3.5 |
| UK average, US Northeast | 3.5 – 4.5 |
| Central Europe, US Midwest | 4.5 – 5.5 |
| US Southwest, North Africa, Australia | 5.5 – 7.0 |
| Arabian Peninsula | 6.0 – 7.5 |
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)
| Country | Typical installed cost |
|---|---|
| United States | ~$3.00/W before incentives |
| Germany | ~€1.30/W |
| Australia | ~$1.20–1.50 AUD/W |
| India | ~₹40–55/W |
The US federal residential clean-energy credit (30% through 2032) reduces gross cost by roughly one-third. State and utility incentives vary widely.
Frequently Asked Questions (FAQ)
What are peak sun hours?
Peak sun hours (PSH) is the equivalent number of hours per day at the standard test-condition irradiance of 1,000 W/m². It is not the same as daylight hours — a sunny 12-hour day with mixed conditions may only deliver 5 peak sun hours of equivalent full-strength sun. Use a local solar atlas, NASA POWER, or PVWatts for a site-specific number.
Why subtract 20% for system losses?
Real installations lose energy to: DC→AC inverter conversion (~3–4%), DC wiring (~2%), soiling/dust (~2–5%), partial shading, panel temperatures above the 25°C test point (~3–10% in summer), module mismatch, and degradation over time. 14% is NREL's PVWatts default for a brand-new, perfectly-sited system; 20% is a realistic figure for a typical residential install across its lifetime.
How many panels will fit on my roof?
A 400 W panel is roughly 1.8 m × 1.1 m (≈ 2 m²). Allowing for setbacks, vents, and shading, a typical asphalt-shingle roof fits 10–25 panels on the south-facing slope. If the calculator suggests more than will fit, consider higher-wattage panels or a battery + grid-tied hybrid that draws from grid in winter.
How long until the system pays itself back?
At $3/W installed and $0.15/kWh retail, a system that produces all of a home's electricity typically pays back in 8–12 years before incentives. With the US 30% federal tax credit, payback drops to 6–9 years. Lifetime is 25–30 years, so most of that is net savings.
Disclaimer
Estimates are first-order and assume south-facing rooftop with little shade. Actual sizing depends on roof orientation, tilt, shading, local irradiance, regulatory caps, net-metering rules, and panel/inverter selection. Get a quote from a certified installer before purchasing.
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