The Mudita Harmony has a built-in rechargeable 2600mAh lithium-ion battery. The clock is charged via the USB-C port on the back of the device.
The Harmony should last for about 10-14 days of moderate use on a full battery. More intense use can reduce this time significantly.
The most energy-intensive parts of the Harmony are the speaker and the backlight. Frequent use of these features will cause the clock to run out of energy sooner. The brighter the backlight setting and the louder the volume, the more energy is required. For example, leaving the clock with its speaker working constantly and on the highest volume would make it discharge in a matter of hours.