As above, hard to comment if we dont know your setup, 500w from the site hookup is trivial, some sites will trip at 2kw, but a typical unassisted battery won't give 500w for much more than half an hour, then its flat. It all depends on your installation - solar? B2B? static for the weekend or some driving? Whats the hookup charger? Leisure battery capacity?
Cheers
Phil