Because the rating is measured under standard test conditions — a defined light intensity, a defined cell temperature and a defined spectrum — which almost never occur outdoors together. Heat, angle, haze and shading each take a share of the difference.
Peak watts are a laboratory figure, published so that two panels can be compared to each other. They are not a forecast.
What standard test conditions are
The rating is taken at a fixed irradiance, a fixed cell temperature and a defined light spectrum. Those conditions are chosen because they are repeatable, not because they resemble a roof in August or a balcony in February.
What the sky takes away
Light arriving at an angle spreads over more panel for the same energy. Cloud and haze cut intensity. A panel lying flat gathers very differently from one tilted towards the sun, and the difference across a day is larger than most specification differences between panels.
What heat takes away
Cells lose output as they warm, and a panel in still air runs far hotter than the air around it. The temperature coefficient on the datasheet is the number that quantifies this, and it is one of the few published figures that predicts real-world behaviour rather than describing a test.
What the controller takes away — or gives back
Between the panel and the battery sits a controller. A maximum power point tracker continuously searches for the panel’s best operating point; a simpler controller ties the panel to the battery voltage instead and discards the difference. Which one is fitted changes the daily yield noticeably, independently of the panel itself.
Where this page and the manufacturer’s documentation differ, the documentation is the one to follow. Every capacity, output, input window, current limit, cycle count and temperature threshold on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And for any panel rating: peak watts are measured under standard test conditions — a defined irradiance, a defined cell temperature and a defined spectrum. Those conditions exist so that two panels can be compared, not so that a yield can be predicted. This page reports the maker’s figure as a test result and derives no daily energy from it.
What people ask before they order
Does a hot panel produce less?
Yes. Output falls as cell temperature rises, at a rate the maker publishes as a temperature coefficient. A bright hot afternoon can yield less than a bright cool morning.
Does one shaded cell matter?
Far more than its area suggests. Cells are wired in series, so a shadow across one corner can throttle the whole string — which is why bypass diodes and panel placement get the attention they do.
Last reviewed 16 September 2026