I have two Solis S6 Pro 8KW inverters, at two different houses. The first one was installed in March 2025, and the second one in June 2026.
The newer (2026) unit seems to have a higher threshold before the fans switch on (somewhere around 3kw, vs 2kw for my 2025 unit), which makes me wonder if the fan speed is controllable via the firmware.

But my real question is this - are the fans physically capable of variable speed, or are they fixed speed? Because on cold days (eg ambient 20C), surely the fans can spin slower when the load is low (eg 2.5KW). I'm curious about the engineering/design decision to make the fans either on/off versus variable speed, depending on ambient temperature and load?

interested

for curiosity , what is the ambient temperature in your house ?

On 2026/08/02 at 9:46 AM, Ben Harper said:

I have two Solis S6 Pro 8KW inverters, at two different houses. The first one was installed in March 2025, and the second one in June 2026.
The newer (2026) unit seems to have a higher threshold before the fans switch on (somewhere around 3kw, vs 2kw for my 2025 unit), which makes me wonder if the fan speed is controllable via the firmware.

But my real question is this - are the fans physically capable of variable speed, or are they fixed speed? Because on cold days (eg ambient 20C), surely the fans can spin slower when the load is low (eg 2.5KW). I'm curious about the engineering/design decision to make the fans either on/off versus variable speed, depending on ambient temperature and load?

The fans are temperature load controlled intelligent cooling, not pure fixed on and off.

Your older unit kicking in at 2 kW is normal for earlier firmware.

Newer units have a higher threshold 3 kW, which is an improvement.

On a cold day 20 °C at only 2.5 kW they ideally shouldn’t need to run hard if they do, it’s still more about internal component temps than ambient.

Solis fans are generally quieter and better managed than other brands.

So look at getting the firmware updated.