It's a permanent double conversion UPS; it's always inverting. You could activate the "high efficiency" bypass mode to directly connect the input to the output in the presence of mains AC input, but this would also pass through disturbances like fluctuations, harmonics, and surges, so I don't have it enabled. This wastes about 80W in my setup but whatever. I'm not worried about the inverter temperature is the point I'm trying to make; but I was considering this when I did it, which is why they're wired to run at full speed all the time.