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Panel power ratings use a reference sunlight level. What irradiance is that reference in NREL's PVWatts?

1000 W/m2 at a cell temperature of 25 C.

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  1. 200 W per square meter
  2. 3000 W per square meter
  3. 1000 W per square meter ✔

Why: Panel ratings refer to a standard reference: strong sunlight and a 25 C cell. Real roofs are often hotter, so output is lower.

Huh, didn't know that: PVWatts is NREL's free calculator that estimates the production of grid-connected PV systems anywhere in the world.

The dad joke
Panels at 25 C feel great. On a hot roof, they get a little sluggish.
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References

Answer: 1000 W per square meter. Panel ratings refer to a standard reference: strong sunlight and a 25 C cell. Real roofs are often hotter, so output is lower.

  1. NREL: PVWatts Version 5 Manual www.nrel.gov/docs/fy14osti/62641.pdf
    The reference cell temperature Tref is 25◦C, and reference irradiance is 1000 W/m2
    Checked 2026-10-10.
  2. NREL PVWatts Calculator pvwatts.nrel.gov
    Estimates the energy production of grid-connected photovoltaic (PV) energy systems throughout the world.
    Checked 2026-10-10.

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