How big a system does your site need?
Tell us what you are running and roughly where you are. This works out the solar, battery and inverter size that would keep it going. It takes about two minutes and you get a figure straight away.
Three days of backup should mean three days. Power is lost every time it changes form on the way from the panels, so a system sized straight off the equipment list comes up short. This works off what the batteries actually have to store.