Reading an Inverter Datasheet: MPPT Windows, Derating and Efficiency Curves

Every inverter has a **maximum DC input voltage** and an **MPPT voltage window**. The maximum decides the longest string at record-low temperature; the window decides whether the string operates at all on a hot afternoon. Most string-design failures come from checking only the first.

Check Voc at the site's design-low temperature against the maximum DC input with margin, then check Vmp at the site's design-high cell temperature against the bottom of the MPPT window. A string that satisfies the first can still fall out of the window on a 65°C roof.

A datasheet showing 98.5% peak efficiency is telling you the best case at one operating point. What matters is the weighted efficiency across the DC input range your array will actually see - and at low input voltage, many inverters are noticeably worse. For a heavily oversized array, the operating point sits high and weighted efficiency is favourable; for a 1.0 ratio design it can be materially worse than the headline figure.

1. 1

   Compute cold Voc per string

   Module Voc multiplied by the cold-temperature coefficient applied to the site's design-low ambient.

2. 2

   Compute hot Vmp per string

   Check the string still sits inside the MPPT window at the design-high cell temperature.

3. 3

   Check current per MPPT

   Isc at high irradiance against the per-MPPT limit; high-current modules are the common failure.

4. 4

   Read the derating curve

   Confirm the continuous output at the actual mounting location's ambient temperature.

5. 5

   Confirm grid-support capability

   The interconnection may require volt-watt, volt-var or export limiting; verify the model supports the required profile.