Method
How the estimate works
For a battery labelled in mAh, first calculate Wh = mAh x nominal V / 1,000. Then estimate hours = Wh x usable capacity fraction x output efficiency / average device watts.
Worked example: a 20,000 mAh battery at a stated nominal 3.7 V is 74 Wh. With 90% usable capacity, 85% output efficiency and a steady 10 W draw, the estimate is 5.66 hours.
mAh alone cannot tell you the stored energy. The battery voltage matters. Check that the battery output and cable can actually support the device before relying on the estimate.