"My battery charger
requires 535 watts input" ...means about 4-5 amps which the Yamaha is capable of. The question is what is the OUTPUT of your battery charger with that input. At 100% efficiency 535 AC watts = about 45 DC amps which probably means you'll get something like 30 AMPS. This may be enough for your battery bank...kinda depends on your dailiy use on a 21 footer. For most cruisers, with refrigeration and autopilots etc. this would not be enough.
For something utilitarian that can come in handy in a lot of situations...it looks great. I have a friend with a Pacific Seacraft 37 that bought the honda1000 and is really unhappy with it since the charging output is quite restricted and he wishes he had spent the $$ for the 2000W unit and the higher amperage for his much larger battery bank...so...it all depends on your need. I really liked the DC output feature on the Yamaha but i wonder why they were so restrictive on the output... guess it would have raised the price too much to beef that up.