Calculate solar panel power output, daily and annual energy production (kWh), and system performance ratio. Free solar power calculator for installers and homeowners.
Understanding how much power your solar array actually produces — versus what the nameplate rating says it should produce — is one of the most critical skills in solar design and sales. Nameplate capacity tells you what panels produce under ideal lab conditions; real-world output depends on your location, orientation, tilt, temperature, and system losses.
This solar power calculator gives you a complete energy production profile for any system. Enter your system size, peak sun hours, system efficiency, tilt and azimuth angles, panel temperature coefficient, and local ambient temperature. The calculator outputs daily and annual energy production, performance ratio, capacity factor, and a breakdown of all system losses — so you can see exactly where production is being left on the table.
Whether you're sizing a new system, validating production estimates in a proposal, or troubleshooting an underperforming installation, this tool delivers the production numbers solar professionals rely on every day.
Real-World Production Modeling
Calculates actual energy output using tilt, azimuth, temperature coefficient, and system derating factors — not just nameplate × sun hours.
Performance Ratio & Capacity Factor
Outputs industry-standard KPIs — performance ratio and capacity factor — so you can benchmark any system against industry norms at a glance.
Monthly Production Table
Generates a month-by-month energy production table with seasonal variation factors, giving you the detail needed for financial models and proposals.
Building a Sales Proposal
Generate accurate production estimates before running financial models. Use daily kWh and annual kWh outputs as the foundation for savings, payback, and ROI calculations in your proposal.
Validating Existing System Performance
Compare the calculator's expected output to your monitoring system's actual production. A significant gap between expected and actual output signals shading, soiling, inverter, or degradation issues.
Comparing Panel or Inverter Options
Run the calculator for different panel temperature coefficients or inverter efficiencies to quantify the real-world annual production difference between equipment options before making a recommendation.
Enter System Size (kW DC)
Input your system's total DC nameplate capacity in kilowatts. For example, 20 panels × 400W = 8.0 kW DC. This is the starting point for all output calculations.
Enter Peak Sun Hours
Find your location's average peak sun hours per day from NREL data or enter a custom value. U.S. values range from 3.5 (Seattle) to 6.5 (Phoenix). This directly scales your daily production output.
Set System Efficiency (%)
Enter your overall system efficiency, which accounts for inverter losses, wiring resistance, soiling, and mismatch. The default of 80% aligns with NREL PVWatts standard derating. Adjust up to 85% for premium systems or down to 75% for high-temperature climates.
Enter Tilt and Azimuth Angles
Enter the panel tilt angle in degrees (0° = flat, 90° = vertical) and azimuth angle (180° = true south in the Northern Hemisphere). The calculator applies an irradiance correction factor based on these inputs to adjust production from the peak sun hours baseline.
Enter Temperature Coefficient & Ambient Temperature
Input your panel's temperature coefficient (%/°C, typically -0.35% to -0.45%) and the average ambient temperature for your location. The calculator estimates cell operating temperature and applies the appropriate power derating.
Review All Output Metrics
Instantly see daily kWh, annual kWh, performance ratio, capacity factor, and a complete losses breakdown. Use the monthly production table to review seasonal variation and export values for financial models.
Each output metric tells a different part of your system's production story. Here's what they mean and how to use them.
Daily Energy Production
e.g. 38.4 kWh/day
Average daily energy output after all derating factors. Divide by system size to get specific yield (kWh/kWp/day) for location benchmarking.
Annual Energy Production
e.g. 14,016 kWh/yr
Total annual output used for utility offset calculations, savings projections, and interconnection applications. Compare against annual household consumption for offset %.
Performance Ratio
e.g. 80.0%
Ratio of actual output to theoretical maximum. Industry standard is 75–85%. Values below 75% suggest shading issues, soiling, or equipment problems worth investigating.
Capacity Factor
e.g. 18.5%
Percentage of maximum possible output if the system ran at full power 24/7. Residential solar averages 15–22%. Higher values indicate sunnier locations or optimized geometry.
Temperature Derating Loss
e.g. -5.2%
Power loss due to cell operating temperature exceeding 25°C (STC). Larger losses in hot climates (Phoenix, Miami). Use panels with lower temperature coefficients to minimize this.
System Losses Breakdown
Inverter + Wiring + Soiling
Itemized loss percentages for inverter efficiency, DC wiring resistance, soiling/dust, and module mismatch. Use to identify where design improvements have the highest yield impact.
All calculations follow the NREL PVWatts methodology, extended with tilt/azimuth irradiance correction and NOCT-based temperature derating. Here are the core formulas used.
Step 1 — Tilt/Azimuth Irradiance Correction Factor
POA_factor = cos(|tilt - optimal_tilt|) × cos(|azimuth - 180°|) × 0.05 + 0.95
Effective PSH = Peak Sun Hours × POA_factor
Step 2 — Cell Temperature & Temperature Derating
Cell Temp (°C) = Ambient Temp + (NOCT - 20) × (Irradiance / 800)
Temp Derating = Temp_Coeff × (Cell Temp - 25)
(where NOCT ≈ 45°C for most silicon panels)
Step 3 — Daily Energy Production
Daily kWh = System kW × Effective PSH × System Efficiency × (1 + Temp Derating)
Step 4 — Performance Ratio & Capacity Factor
Performance Ratio (%) = (Daily kWh / (System kW × PSH)) × 100
Capacity Factor (%) = (Annual kWh / (System kW × 8,760)) × 100
Worked example: A 10 kW system in Dallas, TX (5.5 PSH/day, 30° tilt south-facing, 28°C avg ambient, -0.40%/°C temp coeff, 80% system efficiency). Cell temp = 28 + (45-20) × (1000/800) = 59.25°C. Temp derating = -0.40% × (59.25 - 25) = -13.7%. Effective PSH ≈ 5.5 × 1.00 (optimal tilt). Daily kWh = 10 × 5.5 × 0.80 × (1 - 0.137) = 38.0 kWh/day. Annual = 13,870 kWh. Performance ratio = 38.0 / (10 × 5.5) = 69.1% (note: temperature loss dominates in hot climates).
Data Sources & Standards
Estimated annual production for a 10 kW DC system at optimal tilt, south-facing, 80% system efficiency.
| City / State | Peak Sun Hrs/Day | Daily Output (kWh) | Annual Output (kWh) | Performance Ratio | Capacity Factor |
|---|---|---|---|---|---|
| Phoenix, AZ | 6.5 | 52.0 | 18,980 | 80% | 21.7% |
| Las Vegas, NV | 6.3 | 50.4 | 18,396 | 80% | 21.0% |
| Dallas, TX | 5.5 | 44.0 | 16,060 | 80% | 18.3% |
| Denver, CO | 5.4 | 43.2 | 15,768 | 80% | 18.0% |
| Atlanta, GA | 4.9 | 39.2 | 14,308 | 80% | 16.3% |
| Chicago, IL | 4.2 | 33.6 | 12,264 | 80% | 14.0% |
| New York, NY | 4.0 | 32.0 | 11,680 | 80% | 13.3% |
| Boston, MA | 3.9 | 31.2 | 11,388 | 80% | 13.0% |
| Seattle, WA | 3.5 | 28.0 | 10,220 | 80% | 11.7% |
Peak sun hours sourced from NREL NSRDB. System efficiency 80% per PVWatts standard derating. Annual output = daily output × 365.
Don't Confuse Peak Sun Hours with Daylight Hours
Peak sun hours are not the number of hours the sun is up. They represent equivalent hours at 1,000 W/m² irradiance. A location with 12 hours of daylight may have only 5.0 peak sun hours. Always use NREL NSRDB or PVWatts data — not a general "sunny day" assumption.
Account for Cell Temperature, Not Ambient Temperature
Solar cells run 20–30°C hotter than the air around them under full sun (NOCT effect). Enter your ambient temperature and the calculator handles the NOCT offset. Never apply temperature derating directly against ambient temp — you'll underestimate losses significantly.
Use Plane-of-Array (POA) Irradiance When Available
If you have actual POA irradiance data from a weather station or monitoring system for the installation site, use it directly instead of the tilt/azimuth correction factor. POA data is always more accurate than a calculated correction.
A High Performance Ratio Doesn't Mean Maximum Output
A PR of 85% is excellent — but a system in Seattle with 85% PR produces far less energy than a system in Phoenix with 80% PR, because Phoenix has far more peak sun hours. Always evaluate both PR and absolute kWh production together, not PR alone.
Solar power output is calculated using: Daily kWh = System Size (kW) × Peak Sun Hours × System Efficiency (%). For example, a 10 kW system with 5.0 peak sun hours and 80% efficiency produces 10 × 5.0 × 0.80 = 40 kWh per day. For a more precise result, also apply a tilt/azimuth irradiance correction factor and temperature derating based on your panel's temperature coefficient and local climate.
Performance ratio (PR) is the ratio of your system's actual energy output to the theoretical maximum it would produce under ideal irradiance conditions. A PR of 80% means the system delivers 80% of its nameplate potential. Industry-standard PR for well-designed systems ranges from 75–85%. Values below 75% typically indicate shading losses, excessive soiling, inverter clipping, or module degradation that warrants investigation.
Capacity factor is the percentage of maximum possible output if the system ran at full nameplate capacity continuously 24 hours a day, 365 days a year. Solar systems typically have capacity factors of 15–22% for residential installations. A 10 kW system in Phoenix that produces 18,980 kWh/year has a capacity factor of 18,980 / (10 × 8,760) = 21.7%. Higher capacity factors indicate sunnier locations or better-optimized system geometry.
Solar panels lose efficiency as temperature rises above their 25°C standard test condition (STC). Most monocrystalline silicon panels have a temperature coefficient of -0.35% to -0.45% per °C. In hot climates where panels can reach 65–70°C on summer afternoons, this translates to a 16–20% power loss from temperature alone. This is why hot, sunny climates like Arizona don't always outperform milder climates as much as raw sun hours might suggest.
A well-designed residential solar system typically achieves 77–82% overall system efficiency. This accounts for inverter efficiency (~96%), DC wiring losses (~1–2%), AC wiring losses (~0.5–1%), temperature derating (~3–5%), soiling (~1–3%), module mismatch (~1–2%), and system availability (~0.5%). NREL's PVWatts default is approximately 86% for the inverter and 14% combined DC/AC losses. If your system consistently measures below 75% efficiency, investigate equipment and design factors.
Tilt angle affects how much solar irradiance hits the panel surface throughout the year. The optimal fixed tilt for maximum annual production is roughly equal to the site's latitude. For a location at 35° latitude, a 35° tilt facing due south maximizes annual output. Deviating 15° from the optimal tilt typically reduces annual production by 2–4%. Flat roofs (0° tilt) can lose 10–15% compared to an optimally tilted array but may have lower installation complexity and wind loading.
Typical solar system losses include: inverter efficiency losses (3–5%), DC wiring resistance (1–3%), AC wiring resistance (0.5–1%), temperature derating (3–8% depending on climate), soiling and dust accumulation (1–4%, more in arid regions), module-to-module mismatch (0.5–2%), shading from obstructions (0–25% — the biggest variable), and system downtime/availability (<1% for new systems). Total combined losses typically range from 15–25% of DC nameplate capacity, giving an effective system efficiency of 75–85%.
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