The description of thermal solar panels correctly states that they produce 150/300/600 kW, however the "Generates heat" interface shows only 150/300/600 W. The units of measurement don't match.

Generates heat isn't part of the description. That's added by the game engine, which doesn't understand the concept of a variable solar heat source. Trust the actual description.

Mods like RateCalculator depends on the actual value and hence calculate heat incorrectly. Taking 150/300/600 W into account. And not 150/300/600 kW

There's nothing that can be done about that unless the RateCalculator mod is updated to understand how Nullius panels work. The auto-generated field isn't just cosmetic. If that field was modified it would produce that much power day and night, which isn't how they're supposed to work. Because the values are chosen to be precisely 1/1000th of the actual value, you can use RateCalculator as a guideline if you know to multiply.