One of the things I was paranoid about, was running out of battery on my camera phone, and being unable to record incredible things (because as we all know, according to Sod's Law, the incredible things will happen subsequent to your camera being out of battery). I once encountered a majestic school of bumphead parrotfish, stately as a herd of mastodons, right after my snorkelling camera's battery died. I was not sure about whether I could easily recharge either phones or power banks at the guesthouses. Long story short, I brought three power banks, not including my flashlight which could also double as a power bank. And a universal adaptor that could take several USB connections, in case I needed to share the socket with other travellers. That was... overkill. In the peak season, it may not be. But in the monsoon there is a lot of vacancy in the guesthouses. In fact, some guesthouses don't open for guests at all in the rainy season. The guesthouses I stayed at also have universal sockets already, so you don't actually need to bring an adaptor.A couple of power banks is enough, for contingencies and convenience. Just one, if you're cool with not really taking pictures.