Most of the powerbanks and UPS power supplies we provide with the clock are 10ah which should least at least 5 hours on a charge.
The longer answer is the clock tends to draws between .7 and 1.4 amps at a given time but keep in mind battery manufacturers will oversell their battery capacity and batteries will of course loose capacity over time. Not to mention a myriad of other factors including temperature and brightness of the clock.
When the feature is ready you will be able to use the gamepad to increase and decrease the brightness of the screen to lengthen the runtime of a single charge if needed. We also may provide you with a backup battery (which can't power the clock while plugged in like the UPS) and this can provide an extra 5+ hours for a 10ah battery. You can also use any standard portable charger as long as it is "fast charge" compatible (2amps or more from a single usb output) it should work.
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