This can be used to provide an idea of how long a device will last, given a constant (or average) power draw rate. You can learn about Amp Hour or Ampere Hour. For instance, a 3000 mAh battery could power a device drawing 100 mA (milliamp) for 30 hours. A device drawing 200 mA would last only 15 hours. +
It’s 6.67 A. Now we have both numbers; we have a 200 Ah battery and we know the AC has a 6.67 A draw. How long will a 200 Ah battery last if it has to power this AC? Let’s calculate: 200 Ah Battery Life = 200 Ah / 6.67 A = 30 hours. In short, a 200 Ah battery will be able to power an 800 W 120 V air conditioner for about 30 hours.

Perhaps the most common term used to describe power banks, mAh is the main differentiator of these devices. You’ll have 5,000 mAh power banks, 10,000 mAh, or even 20,000 mAh. mAh is short for milliamp-hours, which is a unit of charge that refers to the amount of “charge” that a battery or power bank can hold.

Based on your battery pack's capacity of 20000 mAh, that would come out to around 40 hours. Note that the Pi doesn't do well with voltage drops though, so towards the end, it might shut off hours early, and I'm probably forgetting some other power somewhere. Here's a video of a Pi A+ running on 8 AA batteries for 45 hours.
The Dizaul 5000mAh Portable Solar Power Bank comes equipped with a 5000mAh lithium-ion battery and a single solar panel rated at 5.5V/1.2W. The manufacturer lists the power output at 5V and a maximum of 2.4A, for a smartphone charging speed of two hours.

To keep your power bank in good shape when not in use, store it in a cool and dry place. Avoid exposing it to extreme temperatures or humidity as it can affect the battery’s performance. It is also a good idea to store your power bank in a protective case or pouch to prevent any physical damage. 5.

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how long does a 5000mah power bank last