Comment by giobox

KOReader is widely known to be less energy efficient than Nickel (the native reader on Kobo devices). It's a commonly observed issue.

It still lasts a long time! It's just not as efficient as Nickel on the same hardware.

Personally I prefer Calibre + Nickel over KOReader on Kobo devices (its trivial to convert DRM-free eBooks to formats Nickel will open, Calibre will do it automatically if you plug a Kobo in), although on Kindle devices I would still jailbreak and use KOReader.