Solar Panel Wattage Calculator — Panel Output Power & Energy
Calculate the actual power output of a single solar panel at a given irradiance and operating temperature — not just the nameplate STC rating. Enter the panel area, efficiency, solar irradiance at your site, and the panel operating temperature to get the temperature-corrected wattage, daily kWh output, and annual energy production.
m²
%
W/m²
h/day
%/°C
°C
Realistic panel power output at the operating temperature entered
- 1
STC rated power
1.7 × 0.2 × 1,000 = 340 - 2
Temperature delta
45 − 25 = 20 - 3
Correction factor
1 + (-0.35 ÷ 100) × 20 = 0.93Below 1 means output falls above the 25°C STC reference. - 4
Temperature-corrected output
340 × 0.93 = 316.2
How does this calculator work?
P_STC (W) = area (m²) × efficiency × 1 000 W/m². Temperature-corrected: P = P_STC × [1 + (Tcoeff/100) × (T − 25)]. At −0.4 %/°C and 45°C, output drops about 8% from STC. Daily kWh = P_actual × peak sun hours ÷ 1 000. Annual kWh = daily × 365. STC is always the nameplate figure; real output is lower in hot conditions.
Formula
How this is calculated
A solar panel's nameplate wattage is measured at Standard Test Conditions (STC): 1 000 W/m² irradiance and 25°C cell temperature. Real-world output differs from the nameplate for two main reasons.
First, irradiance varies with time of day, season, and weather. The calculator lets you set any irradiance value: 1 000 W/m² replicates STC; a clear summer day might reach 900–1 100 W/m²; cloudy conditions can drop to 100–300 W/m². Output scales linearly with irradiance — at 800 W/m² a panel produces 80% of its STC rating.
Second, and often overlooked, is temperature de-rating. Silicon solar cells lose efficiency as they heat up. The power temperature coefficient (typically −0.35 to −0.45 %/°C for monocrystalline silicon) describes how much output falls per degree above the STC reference of 25°C. On a hot summer day the panel surface can reach 50–60°C, meaning a drop of 8–12% below the STC rating. The Nominal Operating Cell Temperature (NOCT) — usually around 45–48°C — is the standard test point for realistic outdoor conditions. Enter your expected operating temperature to see the corrected output.
Daily energy is the temperature-corrected wattage multiplied by the location's peak sun hours. Annual energy multiplies daily energy by 365. This calculator gives per-panel figures; multiply by the number of panels for a whole-system estimate.
Frequently asked questions
Nameplate wattage is measured at STC (25°C, 1 000 W/m² irradiance). Real panels typically run hotter — often 40–55°C on a sunny day — and silicon loses about 0.35–0.45% of output per °C above 25°C, so a 400 W panel might only produce 360–380 W at operating temperature. Additional losses come from inverter efficiency, wiring resistance, soiling, and shading.
Standard monocrystalline silicon panels achieve 18–22% efficiency; polycrystalline panels are 15–17%. Premium panels using PERC, TOPCon, or HJT technologies reach 22–24%. Multi-junction concentrator cells used in space applications exceed 40%, but these are not used in residential rooftop systems. For most residential installations, 20%+ efficiency is considered high-end.
STC (Standard Test Conditions): 1 000 W/m² irradiance, 25°C cell temperature, AM1.5 spectrum — a lab standard for rating and comparing panels. NOCT (Nominal Operating Cell Temperature): the cell temperature a panel reaches at 800 W/m² irradiance, 20°C ambient temperature, and 1 m/s wind speed. NOCT is a more realistic outdoor reference point; NOCT power output is typically 10–15% lower than STC.
Also known as
TG we-Calculate Editorial Team. (2026). Solar Panel Wattage Calculator — Panel Output Power & Energy [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/solar-panel-wattage-calculator
TG we-Calculate Editorial Team. "Solar Panel Wattage Calculator — Panel Output Power & Energy." TG we-Calculate. 2026. https://we-calculate.com/calculator/solar-panel-wattage-calculator.
TG we-Calculate Editorial Team, "Solar Panel Wattage Calculator — Panel Output Power & Energy," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/solar-panel-wattage-calculator
@misc{wecalculate_solar_panel_wattage_calculator, title = {Solar Panel Wattage Calculator — Panel Output Power & Energy}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/solar-panel-wattage-calculator}}, year = {2026}, note = {TG we-Calculate} }
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