Inverters for off-grid use can transform electric energy form batteries (low-voltage direct current, often 12 V) to produce 120 V(AC) for domestic power outlets.
These devices charge batteries from a 120 V (AC) source. They are necessary in an off-grid setup to recharge batteries from a generator.
A more powerful battery charger can charge batteries more rapidly, meaning that the generator can run for a shorter time, and more efficiently.
Combined inverter-chargers are simpler to connect since they only need one battery connection, instead of two.