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A solar panel's rated wattage is measured in a laboratory with the cell held at 25 degrees. A Perth roof in February is not a laboratory. NOCT power density — watts per square metre with the panel's nominal operating cell temperature applied — is the datasheet figure that survives that gap, and you can work it out in about a minute from numbers the manufacturer already prints.
NOCT comparison basics
- Power density is just watts per square metre. At standard test conditions it is efficiency restated: a 23.8 per cent module is 238 W/m². The NOCT version redoes the sum in warmer, dimmer conditions.
- You need five figures from the datasheet: rated watts, the NOCT (or NMOT) temperature, the temperature coefficient of Pmax, and the module's length and width in millimetres.
- Worked on the AIKO Neostar 3S+ 475 in our range, 475 W at 238 W/m² becomes 360 W at 180 W/m².
- All three 475 W panels we currently sell measure 1762 by 1134 mm. This metric separates panels across the market. It will not separate those three.
- NOCT assumes 20 degree air and a one metre per second breeze. February in Perth is neither, so read NOCT as fairer than the rated figure, not as the truth.
Power per square metre
Panels stopped being a standard size around 2018, and comparing two of them on rated watts alone quietly compares two different amounts of roof. A 500 W panel that is physically larger than a 475 W panel may be doing nothing clever at all. Dividing the output by the module's area removes the trick: watts per square metre is what your roof gets, whatever shape the manufacturer chose to sell it in.
At standard test conditions that division is not very interesting, because it is efficiency wearing different units. A module rated 19.9 per cent efficient is 199 W/m². A 21 per cent module is 210 W/m². Both are measured at 1000 W/m² of light with the cell held at 25 degrees, which is a condition your roof reaches for approximately no minutes of the year.
NOCT is the more useful test point. Nominal operating cell temperature is defined at 800 W/m² of irradiance, 20 degree ambient air and a one metre per second breeze — a mild day, not a hot one, but at least a day. Apply the module's own temperature coefficient at that cell temperature and divide by the module's area and you have a number that compares panels the way a roof does.
Five datasheet inputs
- Rated power at STC, in watts. Front page of every datasheet.
- NOCT, or NMOT, in degrees. Usually in the temperature ratings block, written as something like 44 °C ± 2 °C. Enter the middle figure and ignore the tolerance.
- Temperature coefficient of Pmax, as a negative percentage per degree. Same block.
- Module length and module width, in millimetres, from the mechanical data.
Doing the sum
Two steps. The first converts the rated power to what the panel makes at NOCT, because most datasheets that print the temperature also print the result and some do not:
Pmax at NOCT = rated watts × (1 + (NOCT − 25) × temperature coefficient ÷ 100) × 0.8
That trailing 0.8 is the light, not the heat. NOCT is defined at 800 W/m² rather than the 1000 W/m² of the rated test, so four fifths of the irradiance carries four fifths of the output. It is worth knowing which part of the drop is which.
NOCT power density = Pmax at NOCT ÷ (length in mm × width in mm ÷ 1,000,000)
Run it on the Risen RSM96-11-475BNDG, which prints an NMOT of 44 °C and a Pmax coefficient of −0.29 per cent per degree. Rated 475 W, so 475 × (1 − 19 × 0.0029) × 0.8 = 359 W. The module is 1762 by 1134 mm, which is 1.998 square metres. That gives 180 W/m².
Now check it against a panel that publishes the answer. The AIKO Neostar 3S+ 475 prints 360 W at NOCT on the datasheet itself, in the same 1762 by 1134 mm frame — also 180 W/m². Two different cell architectures, two different temperature coefficients, and a difference you could not see from the street.
Panel output under different conditions
| AIKO Neostar 3S+ 475, three conditions | Pmax |
|---|---|
| Rated at STC: 1000 W/m², cell held at 25 °C | 475 W |
| Perth roof: full sun, cell at 65 °C (calculated from the datasheet coefficient) | 426 W |
| Rated at NOCT: 800 W/m², 20 °C air (datasheet figure) | 360 W |
The middle row is the one people misread. A panel on a Perth roof on a February afternoon runs at a cell temperature around 60 to 70 degrees, and at −0.26 per cent per degree that costs the AIKO about 49 W against its rating. It is not down at 360 W, because the sun on that afternoon is not the 800 W/m² that NOCT assumes. The NOCT number is lower than the hot-roof number, and the reason is the light, not the heat.
Comparing module formats
Not between the three panels we sell. Risen, Jinko and AIKO all land within a couple of watts per square metre of each other, in the same 1762 by 1134 mm frame, at the same 475 W. On a 65 degree roof the AIKO's better coefficient is worth about six watts a panel over the other two — real, measurable, and not a reason to choose anything. If a salesperson builds a case on that gap, the case is the gap.
Where it earns its keep is the comparison nobody sets up for you: a 700 W module against a 475 W one. The bigger number is mostly a bigger panel, and per square metre of roof the two can be within a whisker. It is also worth knowing that the large-format modules quoted in those comparisons are built for commercial and utility racking, and generally will not lay out sensibly on a Perth tile roof with a hip and a couple of vents in the way.
If you want the temperature side of this rather than the area side, the TOPCon explainer covers why n-type cells hold up better in heat. The current panel range lists all three modules with their datasheets attached, and if you would rather we did the arithmetic against your roof and your switchboard, ask for a written price.