What the standard defines
Standard Test Conditions fix the three variables that decide a photovoltaic module’s output so that two modules can be compared: an irradiance of 1 000 W/m², a cell temperature of 25 °C, and an air mass of 1.5, which describes the spectrum of the light. Every peak wattage, every open-circuit voltage and every short-circuit current on a panel’s label is measured under those conditions.
The same series of standards defines how the measurement is made and what tolerance the maker declares around it — often a percentage band rather than a single figure.
What it does not promise
It does not describe a roof. Cell temperature on a sunlit panel commonly runs thirty degrees above ambient, and the temperature coefficient published in the datasheet — a percentage of power lost per degree above 25 °C — is what turns that into watts.
Nor does it describe a winter morning, when the opposite happens: a cold panel’s open-circuit voltage rises above its rated figure, which is why a controller’s maximum input voltage must be checked against the cold case rather than the label.
What it works with
STC figures are the input to every sizing decision on this site: matching a panel to a charge controller’s voltage window, to a station’s solar input, or to a micro-inverter’s per-input limit. They are read as laboratory figures with their conditions attached, never as a forecast of what a particular installation will deliver.
What people ask before they order
Why does my 400 W panel never produce 400 W?
Because the figure is measured at STC: full reference irradiance, 25 °C cell temperature and a defined spectrum. A sunlit roof runs much hotter than 25 °C, and the temperature coefficient the maker publishes says how much that costs.
What is NOCT?
Nominal Operating Cell Temperature: a second set of conditions closer to a real installation — lower irradiance, moving air — under which some makers also publish output. Where a maker gives both, this site reports both.
Last reviewed 16 September 2026