Calculate total output and annual energy production from your solar panel array based on system specifications.
Last updated: March 2026
W
hours (regional avg)
% (typical: 80–90%)
*Output = kW × Peak Sun Hours × System Efficiency
Panel wattage (or power rating) is the maximum electrical power a solar panel can produce under standard test conditions (STC): 1,000 W/m² irradiance, 25°C cell temperature, air mass 1.5. Modern residential panels typically range from 300–450 W, with commercial panels reaching 500–600+ W.
A panel's nominal wattage is its peak power output—the theoretical maximum under ideal conditions. In reality, panels produce less due to weather, temperature, dirt, and wiring losses. A 400 W panel in cool, overcast conditions might produce only 150–250 W.
Total system wattage is calculated by multiplying the number of panels by each panel's wattage. A 10-panel array with 400 W panels = 4,000 W = 4 kW system. This kW rating, combined with your location's peak sun hours and system efficiency, determines annual energy production.
Calculate annual output for a 10-panel, 400W system in Denver with 5.5 peak sun hours/day and 85% efficiency:
This production can offset approximately 6.8 tons of CO₂ annually (using avg US grid emissions of 0.92 lbs CO₂/kWh).
Panel wattage is the rated power of a single panel (e.g., 400 W). System size is total: number of panels × panel wattage, expressed in kW. A 10-panel system of 400 W panels = 4 kW.
Panel ratings are under standard test conditions (1,000 W/m², 25°C). In reality, temperature is often hotter (reducing efficiency ~0.5%/°C), irradiance is lower on cloudy days, and system losses reduce output further.
Peak sun hours is the equivalent number of hours at full 1,000 W/m² irradiance needed to match actual daily insolation. Denver gets ~5.5 hrs/day; Seattle ~3.5 hrs/day. Use NREL Solar Data Viewer or PVWatts for your location.
Typical system losses are 12–20%: inverter (3–5%), wiring (1–2%), temperature derating (5–8%), soiling (1–2%). Total efficiency is commonly 80–86%. Higher-quality systems approach 90%.
Yes. Most panels lose ~0.4–0.5% efficiency per °C above 25°C. A 400 W panel in 45°C (hot day) might produce only 360 W. Cold climates actually have an advantage if irradiance is still strong.
Panels degrade ~0.5%/year. After 25 years, expect 87% of original output. Some systems add 5–10% extra capacity upfront to future-proof against degradation, but this is optional.
Yes. If designing a battery-backup system, use annual average daily output (Annual ÷ 365) to size the battery capacity. Add 20–25% buffer for autonomy days and efficiency losses.
Panels produce DC power. Inverters convert to AC (used by your home appliances and grid). Online calculators typically show AC output (after inverter losses). Always clarify when comparing systems.
| Panel Wattage | Category | Typical Application | Era / Notes |
|---|---|---|---|
| 250 W | Older | Residential rooftops | Pre-2015 |
| 300–330 W | Standard | Common residential | 2015–2018 |
| 350–370 W | Mid-range | Residential & small commercial | 2018–2020 |
| 400–420 W | High-efficiency | Premium residential | 2020–Present |
| 450–470 W | Premium | High-performance systems | 2021–Present |
| 500–520 W | Commercial | Commercial/utility scale | 2022–Present |
| 600+ W | Large commercial | Utility arrays & mega-projects | Emerging |
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