A solar panel’s watt rating is measured in a lab under standard test conditions (STC): 1,000 watts of light per square meter, with the cells held at 25 °C (77 °F). Outdoors those conditions rarely line up. Of the 41 panels ranked here, 10 have been measured by independent reviewers in real sun, and the median best result was 87% of the rating.
Why real output is lower
- Sun angle. Output falls as the sun moves off square to the panel. A panel lying flat, or one left facing the same way all day, spends most of the day well off its peak. Re-aiming it every hour or two helps more than anything else.
- Heat. Panels lose output as they warm up, and a dark panel in summer sun runs far hotter than 25 °C. A cold, clear day often beats a hot one.
- Haze, cloud, dust and partial shade. A shadow across part of a panel can cut output far more than the shaded share of its area, because the cells are wired in strings.
- What it’s charging. A power station near full, or one whose input voltage range doesn’t suit the panel, will draw less than the panel can give.
That’s why the rankings score panels on what independent reviewers measured, not just the label. See how we rank.
Efficiency
Efficiency is the share of the sunlight hitting the panel that comes out as electricity. A more efficient panel makes the same watts from a smaller, usually lighter, area. Makers don’t all quote the same figure: some give the efficiency of the bare cells, others of the whole panel including the gaps and frame, which is always lower. The spec tables say which when the maker does. Efficiency doesn’t change how many watts a panel is rated for; it changes how big the panel has to be to make them.
Bifacial panels
7 of the ranked panels are bifacial: they have cells on the back as well, which pick up light reflected off the ground. The rear-side gain depends on what’s under the panel, largest over snow, sand or a light-colored surface and small over grass or dark ground. Ratings and price per watt here use the front side only.
Sources
- Wikipedia, Solar panel: standard test conditions: the STC irradiance and cell temperature.
- Wikipedia, Bifacial solar cells: rear-side gain and ground reflectance (albedo).
- Measured output: each product’s independent tests, linked from its page.