A solar panel is sold on one number: its wattage. A "200W" folding panel sounds like it will deliver 200 watts to your power station, and on most days it will not. That is not usually because the panel is faulty or the listing is dishonest. It is because the headline figure is measured under laboratory conditions that real roofs, campsites and balconies rarely match, and because the panel has to suit the device it plugs into. This guide walks through the lines of a typical spec sheet, what each one tells you, and which ones decide whether a panel works with the equipment you already own.
Rated power and Standard Test Conditions
The wattage on the label, usually written as Pmax or "rated power", is measured under Standard Test Conditions (STC): a cell temperature of 25°C, irradiance of 1,000 watts per square metre and a light spectrum called AM1.5. As Sinovoltaics explains, those conditions roughly represent the sun at noon around the equinoxes with the panel aimed straight at it, and "these conditions are rarely encountered in the real-world."
STC is still useful. It is the same yardstick for every manufacturer, so it lets you compare one panel with another. What it does not tell you is how much power you will collect on an ordinary day. Treat the rated wattage as a ceiling for comparison, not as a promise of output.
NOCT: the figure closer to real life
Many datasheets add a second set of numbers at NOCT, the Nominal Operating Cell Temperature. The simulation software maker PVsyst defines the conditions as 800 W/m² of irradiance, 20°C ambient air, a wind speed of 1 m/s and an open-circuit module, with the exact rules set out in the IEC/TS 61836 standard.
Two things follow. First, the power listed at NOCT is lower than the STC figure, and it is a fairer guide to a sunny but not perfect day. Second, the NOCT temperature itself tells you how hot the cells run in those conditions: a lower NOCT means a cooler-running panel. If two panels have the same rated power, compare their NOCT power figures too.
Temperature coefficient: why hot days cost power
Solar cells lose output as they warm up. The US Department of Energy puts it this way: "Higher temperatures cause the semiconductor properties to shift, resulting in a slight increase in current, but a much larger decrease in voltage." The spec sheet captures this as the temperature coefficient of Pmax, a negative percentage per degree Celsius.
Australian buyer's site SolarQuotes notes that the figure is based on "the panel's temperature, not the air temperature," and that dark panels in the sun can run much hotter than the air around them. It describes anything around "-0.3% per °C or lower" as good, with budget or older designs nearer -0.35% to -0.38%/°C.
The arithmetic is simple. A panel with a coefficient of -0.35%/°C whose cells reach 55°C is 30 degrees above the 25°C test point, so it loses about 30 × 0.35 = 10.5% of its rated power from heat alone, before shade, dust or angle are counted. If you will use panels in hot weather, a lower coefficient is worth paying attention to. Portable panels laid flat on warm ground or a car roof, with little air behind them, are a case where this matters.
Voltage and current: Voc, Vmp, Isc and Imp
These four lines are the ones most buyers skip and the ones that most often explain disappointing results.
- Voc (open-circuit voltage) is the voltage with nothing connected. It is the highest voltage the panel produces, and the one that must never exceed what your charger or power station accepts.
- Vmp (voltage at maximum power) is the voltage at which the panel delivers its rated power under STC.
- Isc (short-circuit current) is the most current the panel can push.
- Imp (current at maximum power) is the current at the rated power point. Vmp multiplied by Imp gives the rated wattage.
Here is why they matter. A power station's solar input is not just a wattage limit; it is a voltage window with a current cap. Anker's published specification for the SOLIX C1000 Gen 2, for example, lists its solar input as 11 to 28 V at up to 8.2 A, or 29 to 60 V at up to 14.5 A, with a 600 W maximum. At low voltage the current cap limits what you can draw, however many panels you add.
The reviewer behind The Technology Man showed what this means in practice: because the lower range is capped at 8.2 A, panels rated around 24.5 V would not deliver much more than about 200 W even with three wired in parallel, and two 200 W panels in parallel gave him around 170 W. Only when he supplied 40 V and above did the unit reach its 600 W solar maximum. The lesson applies to any brand: read the input window of your power station or charge controller first, then choose panels whose Vmp lands in the part of that window that allows the most current.
Series, parallel and the cold-weather trap
Wiring panels in series adds their voltages; wiring them in parallel adds their currents, as Victron Energy's guide to matching panels with controllers sets out. Series is how you climb into the higher, more useful part of an input window. It is also how people damage equipment.
Voc rises as panels get colder. Victron's guidance is blunt: "The colder it is, the higher the open circuit voltage on a PV array will be." Its charge controller manual advises that for arrays in cold climates, or where night temperatures approach or fall below 10°C, owners should allow for output above the rated Voc and, "as a rule of thumb, maintain an additional 10% safety margin." It warns that overvoltage damage "is not covered by warranty." A series string that sits comfortably under the limit on a mild afternoon can exceed it on a cold, bright morning. Add up the Voc of every panel in series, add the margin, and compare that total with the maximum input voltage of your device.
Efficiency: what it does and does not tell you
The Department of Energy defines conversion efficiency as "the percentage of the solar energy shining on a PV device that is converted into usable electricity." On a spec sheet it mainly tells you how much area you need for a given wattage. A more efficient panel is smaller or lighter for the same rated power, which matters for a balcony rail, a van roof or a backpack. It does not make a 200 W panel produce more than 200 W. Check whether a percentage on a listing refers to the module as a whole or to an individual cell, because the two are not the same measurement and only the module figure describes what you are buying.
What to expect from portable panels
Folding panels sold alongside power stations face every one of the conditions above at once: they are often propped at an imperfect angle, they heat up, and clouds come and go. The Technology Man's test of a folding panel is a fair illustration of the gap between label and output, recording around 20 W on one overcast day and around 70 W on another. That is not a fault; it is what a lab rating looks like under a real sky. Plan around the output you will get on ordinary days, not the figure printed on the bag.
Durability and safety marks
Two international standards come up on panel datasheets. TÜV Rheinland's certification listing refers to IEC 61215 as the design qualification and type approval standard for PV modules, and describes IEC 61730, the safety standard, as the requirements for "safe electrical and mechanical operation throughout their expected lifetime," with tests judged on whether they would result in electric shock, fire or injury. A listing that names neither is not necessarily unsafe, but it gives you less to go on. For portable panels, also look for an ingress protection (IP) rating, and check whether the listing says it applies to the whole product, including any built-in USB ports and connectors, or only to part of it.
A short checklist
- Find your device's solar input window: the voltage ranges, the current cap in each range and the maximum wattage.
- Choose panels whose Vmp, alone or in series, falls in the range that allows the most current.
- Add up Voc for any series string, allow a margin for cold weather, and stay under the device's maximum input voltage.
- Compare panels on power at NOCT as well as at STC.
- Prefer a lower (closer to zero) temperature coefficient if you will use the panels in heat.
- Use efficiency to judge size and weight, not output.
- Look for IEC 61215 and IEC 61730, and a stated IP rating for portable use.
Read this way, a spec sheet stops being a list of jargon and becomes a compatibility check. The wattage tells you which panels are comparable; the voltage, current and temperature lines tell you which of them will actually work well with what you own.
