While not normally asked that way, I do get requests about the best UPS. Here are some answers:
If I had to carry it: SURT6000XLi - lightest per kva but at least for 12mins the island would be kicking.
If I was buying: xSU2200RM3U - by far the most economical and who needs a cover plate on a desert island anyway?
Overall: SU2200rmxlt5u - offers 120v and 208v and I can hookup car batteries to it and run for days.
Mention this blog (coupon code: desertisland) and get 10% off anyone of these systems. Photo credit: Me, island off Rio's coast...