Kobo states that in standby mode it will hold it's charge for more than two months.
In other words, put it in standby, put it in a drawer, take it out after two months and it will still resume instantly.
Standby hardly uses any power. Keeping the memory powered uses only 1.5 volts at less than 1 mWh.
Rebooting on the other hand costs loads of power because the CPU will run at full throttle as it checks hardware and reads the OS into memory.
So putting your Kobo to sleep will gain you some time before charging.