Solar Panel Output Calculator

How to use this calculator

Enter six numbers and the calculator returns daily, monthly, annual, and 25-year energy output, plus dollar savings:

1. Array size (W) — total panel wattage. A 6 kW system is 6,000 W. Use the STC nameplate sum of every panel.
2. Peak sun hours per day — local average. Continental US ranges 3.5 (Seattle) to 6.5 (Phoenix). NREL’s PVWatts shows the value for any ZIP code.
3. System efficiency (%) — leave at 78% unless you know better. This is your derate factor.
4. Panel age (years) — 0 for a new install, 10 for a 10-year-old system.
5. Electricity rate ($/kWh) — your current utility rate. EIA’s residential average for 2026 is about $0.165/kWh.
6. Annual rate escalation (%) — historical US average is 2.8–3.2%/yr. Use 3% as a balanced default.

The formula

Solar output is a straightforward product of irradiance, area, and losses:

daily_kWh    = array_W × peak_sun_hours × derate / 1000
annual_kWh   = daily_kWh × 365
lifetime_kWh = sum over 25 years of annual_kWh × (1 - degradation)^year

A worked example for a 6 kW residential system in Denver:

• 6,000 W × 4.7 sun-hr × 0.78 = 22.0 kWh per day
• 22.0 × 30.4 = 669 kWh per month
• 22.0 × 365 = 8,022 kWh per year
• At $0.13/kWh Denver rate: $1,043 in year-1 bill savings
• Lifetime (25 years, 0.5% degradation, 3% rate escalation): about $37,500 in savings

Output reference table by system size

Using 4.8 peak sun hours, 78% derate, 0-year age, $0.165/kWh, and 3% annual rate escalation:

The pattern is linear in array size, so a 12 kW system produces exactly twice what a 6 kW system does under identical conditions.

What changes the output

Location and peak sun hours

Peak sun hours is the dominant variable. A 6 kW array in Phoenix (6.5 PSH) produces 30 kWh/day; the same array in Seattle (3.5 PSH) produces 16 kWh/day — a 47% difference. NREL’s National Solar Radiation Database is the authoritative source for any US ZIP code.

Tilt and orientation

A south-facing array at latitude tilt is the reference case. Off-axis tilts cost 2–8% per year. East/west orientation gives up roughly 10–15% versus south. The solar panel orientation calculator and tilt angle calculator quantify this for your specific roof.

Temperature

Hot panels lose power. Each °C above 25°C cell temperature drops output by about 0.4% (the “temperature coefficient” on the panel datasheet). On a 90°F (32°C) day with a black roof, cell temperatures reach 60–65°C — that’s 14–16% off rated power. This is why Phoenix doesn’t actually produce 2× what Seattle does despite the irradiance difference.

Shading

A single shaded panel in a string can drag down the entire string by 50%+ if the system uses a central inverter. Module-level optimisers (SolarEdge) or micro-inverters (Enphase) limit the loss to just the shaded panel. This is one of the biggest swings between “rated” and “real” output.

Soiling and snow

Dust, pollen, and bird droppings cost 2–5% of annual output in dry climates and 1–3% in rainy ones. Rain washes panels effectively. Snow blocks 100% of production until it slides off — which usually happens within 1–3 days on a tilted array. The solar panel cleaning cost calculator walks through whether annual cleaning is worth the cost.

Why this calculator uses a 78% derate

The default 78% derate is NREL PVWatts v6’s residential rooftop standard. It accounts for:

• Inverter efficiency: 96–97% for modern string inverters and micro-inverters
• AC + DC wiring losses: 2% on a properly sized run
• Soiling: 2% annual average for residential rooftops
• Mismatch and connection losses: 2%
• Light-induced degradation (LID): 1.5% over the panel’s life
• Temperature derating: ~10% effective annual average for a typical roof

Multiplied: 0.97 × 0.98 × 0.98 × 0.98 × 0.985 × 0.90 ≈ 0.78. Bump it to 82% if you have an unshaded ground-mount with an oversized inverter. Drop it to 72% for a hot rooftop, central inverter, and minor shading.

Common mistakes

• Using rated wattage to estimate annual production: A 6 kW system does NOT produce 6 kW × 24 hr = 144 kWh/day. Peak sun hours captures the actual integrated energy, which for residential US is 14–22 kWh/day on a 6 kW array.
• Forgetting to derate: Using 100% efficiency overstates output by 22–28%.
• Ignoring temperature: This single factor explains why hot-climate systems don’t proportionally outperform cool-climate ones.
• Treating all panels equally: Tier-1 mono-PERC panels (LG, REC, SunPower, Q CELLS) hit 21–22% efficiency. Cheap polycrystalline panels are 15–17%. Same nameplate, different real-world output because of temperature and degradation curves.

Sources

• NREL PVWatts v6 — system loss conventions and irradiance data by ZIP
• NREL National Solar Radiation Database — peak sun hours for any US location
• EnergySage 2026 Solar Production Database — actual production data for 30,000+ residential systems
• DOE Solar Energy Technologies Office — federal performance benchmarks
• SEIA Annual Industry Report 2026 — installed capacity, performance trends