Your panel's spec sheet says "400W." That's a lab number. Here's what actually determines how many of those watts you get, right now, at your location, once angle, clouds, and a few other real-world variables get involved.
🔬 How a Solar Cell Actually Works
A solar cell is a thin wafer of silicon treated so one side has a surplus of electrons and the other a deficit. When a photon of sunlight strikes it with enough energy, it knocks an electron loose. The built-in electric field at the junction between the two layers pushes that electron in one direction, and if there's a circuit to flow through, that push is the current. No moving parts, no combustion, just photons bumping electrons into motion.
📏 What "Full Power" Actually Means
Every panel's wattage rating is measured under Standard Test Conditions (STC), a defined lab benchmark, not "a sunny day":
1000 W/m² of irradiance, roughly full, direct overhead sun at sea level on a clear day.
77°F (25°C) cell temperature: panels actually get less efficient as they heat up, so a hot rooftop in July often runs below its rated output even in bright sun.
AM1.5 spectrum: a standardized amount of atmosphere the light passes through, roughly the sun at a 48° angle, used as a reasonable year-round reference.
💡For context: sunlight at the top of the atmosphere is about 1361 W/m² (the "solar constant"). By the time it reaches sea level through clear air, ~1000 W/m² is what's left; that's already the "after the atmosphere" number STC is built on. Datasheets often also list a NOCT rating (Nominal Operating Cell Temperature, ~113°F / 45°C), a more realistic everyday comparison point.
🧮 Max Watts, Per Cell (Try It)
A cell's peak output is just its area × efficiency × 1000 W/m² (the STC irradiance). Pick a cell size and efficiency to see it live:
🔲
Watts per cell (STC)
--
🔋
Watts for that panel (STC)
--
⚡ What Would You Actually Get, Right Now?
Pick a place, time, and panel setup, and we'll walk the sticker number down to a real-world estimate, step by step.
estimated watts, right now
🤔 Why It's (Almost) Never the Sticker Number
Sun angle: the lower or more off-axis the sun, the less energy hits the panel per square meter.
Clouds & atmosphere: scatter and absorb sunlight before it arrives.
Heat: hotter cells conduct less efficiently. STC's 77°F (25°C) is cooler than most real rooftops in full sun.
Soiling: dust, pollen, and grime block a small but real percentage of light.
Inverter & wiring losses: converting and transporting the power isn't perfectly efficient.