Many RE users like to run 120 vac gensets to recharge batteries (during prolongued sunless/windless periods) while doing other 120 vac things simultaneously (such as laundry, vaccuuming, etc.) and this sometimes leads to problems such as:

• Most inverters with battery charge option only charge at a rate of 25 - 50 amperes. Maybe you need or want more current than that.
• Most inverters with battery charge option use transformers. Ac generators are well-known for not having the same 'peak to peak' (when looking at the ac waveform on an oscilloscope) voltage as grid power, which means a significant loss of power transfer capability between the high and low voltage sides of the transformer. It would not be unusual (for example) for any transformer type battery charger (inverter or otherwise) rated at, say, 100 amps, to suffer an output loss of between 25 and 40 amperes when running on generator power. The bottom line is that because of inherent limitations of ac generators, you don't get the 'bang' per fuel 'buck' that you are paying for.

You've worked long and hard to get your home running properly on modified- or sine-wave inverter power.

• Your electrical loads will see either a momentary blackout during the changeover to and from the ac generator, or a nasty surge caused by the bump of an instantaneous changeover (depending on your switching arrangement). This type of 'bump', caused by the inverter and generator not being frequency-synchronized, has the potential to ruin lots of transformer-powered consumer electronics (most audio/video and computer systems). It would be alot simpler just to keep everything running on the inverter(s) and start the generator to charge the batteries directly, with no battery charger in between and with you dialing in the charge rate (via the field control) that you want.