I’ve written about power-banks before but more importantly without an adaptor, there won’t be any power. When travelling to Japan I usually carry my Fujifilm International Adaptor, 2 UK to Japan Adaptors and a spare USB cable. Having a travel adaptor (an international one) is very convenient and a basic necessity when you’re abroad to recharge your gadgets (phone / camera battery / power-banks typically), it does not however convert voltage or frequency.
Japan operates on a different voltage compared to the rest of the world, 100 Volts. It’s common to find yourself with a plug type A with 2 flat parallel pins. Sometimes, but rarely you’d come across something similar that has 2 flat parallel pins plus a grounding pin, type B. Japan’s usage of frequency is 50 Hz in the East (Kanto, Tohoku and Hokkaido) and it’s 60 Hz in the West (Chubu, Kansai, Chugoku, Shikoku, Kyushu and Okinawa) I haven’t had any problems so far and would recommend doing some research first.