I did a side by side comparison of the specs between a SW4048 and FX3648.

Beside SW4048 having a higher probablility of giving you a hernia, the one spec that concerned me on the FX3648 was the input frequency range.

At 60Hz +/- 2 Hz I would expect trouble with it following generators particularly if some significant loading changes occurs.

I then read the installation manual and it states that it is not designed to work with generators. This is a significant deficiency if you are using FX3648 for power backup purposes. You would have to totally run the AC power from the inverter or transfer switch to totally on generator and any battery recharging would have to be via an independent charger hooked directly to the generator.

Outbacks work fine with a generator. They have several useful settings just for generators