Solar Panel Output Calculator
Calculate the daily energy output (kWh) of a solar panel from its rated wattage, daily peak-sun hours, and efficiency. Use it during system sizing or to estimate production from a specific panel under your local sunlight conditions.
Last updated: May 2026
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About this calculator
The calculator estimates daily energy production using a simplified model: Output (kWh) = (Panel Wattage × Sun Hours × Efficiency/100) / 1000. Variables: Panel Wattage is the panel's rated DC output in watts at Standard Test Conditions (typically 350-450W for modern residential panels); Sun Hours is daily peak-sun-equivalent hours (the equivalent number of hours per day the panel operates at full 1000 W/m² irradiance — typically 4-6 in most US locations); Efficiency is a system-level efficiency multiplier as a percentage, capturing real-world losses below rated capacity (typical 75-85%, capturing inverter losses ~3-5%, wiring/DC losses ~2-3%, soiling ~3-5%, temperature derating ~5-15%, mismatch losses ~2%). Edge cases: very high temperatures (panel surface above 50°C) cause significant performance loss — panels lose about 0.4% efficiency per °C above STC (25°C); shading even a small portion of a panel can dramatically reduce output because cells are series-connected; very dusty conditions can cut output 5-15%; snow cover stops production entirely until it slides or melts off. The formula treats efficiency as a single multiplier — real-world systems have many separable loss factors that vary throughout the day and year. For accurate annual production modeling, use detailed simulation tools like PVWatts (NREL's free online tool), PVsyst, or SAM (System Advisor Model). Quick rules of thumb: 1 kW of installed solar in a good US location produces about 1,300-1,800 kWh/year; in a poor location 900-1,200 kWh/year; in exceptional locations (Arizona, Hawaii) 1,800-2,200 kWh/year.
How to use
Example 1 — Modern residential panel in good sun. 400W panel, 5.5 sun hours/day (typical mid-latitude US), 80% efficiency. Step 1: 400 × 5.5 = 2,200 Wh raw. Step 2: × 0.80 = 1,760 Wh = 1.76 kWh/day. Verify ✓. Annual production = 1.76 × 365 ≈ 643 kWh/year from one panel. A 20-panel residential array would produce about 12,860 kWh/year — covering most of an average US home's electricity consumption (~10,500 kWh/year). Example 2 — Older panel, lower-sun location. 280W panel, 3.8 sun hours/day (Pacific Northwest winter), 75% efficiency (older system with degradation). Step 1: 280 × 3.8 = 1,064 Wh. Step 2: × 0.75 = 798 Wh = 0.80 kWh/day. Verify ✓. Annual = 0.80 × 365 ≈ 292 kWh/year per panel. A 20-panel array produces 5,840 kWh/year — about half the typical US household consumption, requiring a larger array or supplemental grid electricity for full coverage.
Frequently asked questions
What are "peak sun hours" and how do I find mine?
Peak sun hours (PSH) is the daily equivalent of full-strength solar radiation hours, defined as solar irradiance at 1,000 W/m² (the same level used to rate panels). A location receiving 5 sun hours per day actually has the sun up much longer — perhaps 10-14 hours — but the integrated solar energy equals 5 hours of full-intensity sun. This single number captures latitude, season, weather, and average cloud cover. Typical annual averages: Arizona/Nevada/Hawaii 6.0-7.0 hours/day; Southern California, Texas, Florida 5.5-6.0; Mid-Atlantic, Midwest, Northeast 4.0-4.5; Pacific Northwest, Alaska 3.0-4.0; Northern Europe 2.5-3.5. NREL provides authoritative US data through PVWatts (free online tool) — enter your address and it gives monthly and annual sun hours adjusted for actual weather data from the closest TMY (Typical Meteorological Year) station. Always use annual average sun hours for system sizing, not best-case (summer) or worst-case (December), because grid-tied solar smooths over seasons.
What efficiency values are realistic for panels and systems?
Two different efficiencies are commonly confused. Panel efficiency is the percentage of incident solar energy converted to electricity at Standard Test Conditions — currently 18-22% for residential silicon panels (premium models 23-24%), 25-27% for emerging perovskite/silicon tandem cells, and 38%+ for laboratory multi-junction cells used in satellites. This is built into the panel wattage rating. System efficiency (also called Performance Ratio, PR) captures real-world losses below the rated wattage: inverter losses 3-5%, wiring/DC losses 2-3%, soiling 3-5%, temperature derating 5-15%, mismatch 1-3%, total typically 75-85%. The calculator's 'efficiency' input combines all these — use 80% as a typical value, 75% for older or non-ideal systems, 85% for new well-designed systems with microinverters and zero shading. Many ground-truth solar production reports use the PVWatts default of 14% combined losses (i.e., 86% efficiency), which is close to best-achievable for new equipment.
What are the most common mistakes when estimating solar production?
The biggest is using the rated panel wattage as actual hourly output — even at high noon on a sunny day, real-world output is typically 70-85% of rating due to temperature and other losses. The second is forgetting seasonal variation; winter production in northern latitudes can be 30-40% of summer due to shorter days and lower sun angle, but annual averages are what matters for grid-tied systems. The third is overlooking shading — even a small shadow on a single cell can reduce a panel's output by 30-50% because series-connected cells limit each other; microinverters or power optimizers help but don't fully solve this. The fourth is using nameplate efficiency for old panels; modules degrade 0.5-0.7% per year, so a 15-year-old 250W panel is producing more like 225W under the same conditions. The fifth is missing the orientation/tilt effect — south-facing (in Northern Hemisphere) at latitude tilt is optimal; east/west-facing produces 15-25% less; north-facing 50-70% less; large deviations destroy economics.
When should I NOT use this simple calculator?
Skip this calculator for serious system design — use NREL's PVWatts (free, web-based) or a paid tool like PVsyst or Aurora Solar for actual production modeling with hourly resolution, weather data, shading analysis, and tilt/orientation optimization. Avoid it for off-grid systems where battery sizing requires hourly load matching, not just daily averages — overnight and weekly variability dominates off-grid design. Do not use it for utility-scale or commercial projects where bankability requires authoritative meteorological modeling (TMY data, long-term solar resource assessment). Skip simple models for partially-shaded systems where the loss is non-linear and depends heavily on inverter topology — microinverters vs string inverters dramatically change the impact of shade. Do not use it for high-temperature applications (rooftops in hot climates where panel temps regularly exceed 70°C) without adjusting efficiency down 10-15% from the 80% default. And do not use it as a verification tool against actual production data — discrepancies between modeled and actual production indicate something is wrong with the system (shading, soiling, equipment failure) and require detailed diagnosis.
How do panel orientation, tilt, and shading affect output?
Substantially. In the Northern Hemisphere, due-south orientation at a tilt angle equal to your latitude produces optimal annual energy. Common deviations and their impacts: due-east or due-west orientation: 15-20% lower annual production but the system generates more in morning or evening, better matching utility time-of-use rates in some markets; due-north: 50-70% lower production (essentially uneconomic in mid-latitudes); flat roof (low tilt): 5-10% lower production but easier installation and self-cleaning by rain; steep tilt (60°+): better winter production, worse summer, generally suboptimal annually except in high-latitude or snow-prone areas. Shading is even more impactful: in many residential cases the optimal layout requires removing or trimming trees because even partial shading dramatically reduces output. Online tools like PVWatts or the satellite-imagery-based Project Sunroof from Google can estimate site-specific shading loss. For sizing, always: (1) get a site assessment with a Solar Pathfinder or similar tool measuring annual shade; (2) use micro-inverters or DC optimizers if any shading is unavoidable; (3) consider whether tree removal or roof modifications are worthwhile for the production gain.