I usually find that most folks are quite cool about battery technology but there is a frequent theme coming up which is my pet hate of the ignorance of knowing Volts, Amps, Watts, Watt-hours.
The batteries inside our devices are usually around 3.7V and contain a product-dependent amount of Amp-hours in the region of 2400mah to 3400mah. If you were to crack open an Anker Powercore (there are videos) you’d typically see such batteries inside and look them up and see their volts and mah.
Volts is a measure of the energy, the vigor or “speed” of the electrons (they move at the same speed roughly) and Amps is the measure of how many are moving, so the total energy is the speed x quantity, or Volts x Amps and the energy is called Watts. Watts = Volts x Amps.
Now you cannot create or destroy energy but you can waste it in non-useful forms, such as electronic energy wasted as heat. You can change its shape so say you change from many “slow” electrons into fewer “fast” electrons. So for example a 3.7V 0.3A output battery = 0.111 Watts and it can sustain that output for say 4 hours = 0.444 Watt-hours.
You can do this alterning of “speed” vs quantity with your arm and a lever. If you want to make yourself appear stronger you can use a long stick and move one end a long distance and the other end a short distance and the force at the end is increased, but the other end moves less distance. Total energy, as force x distance is the same but you altered the distance downwards so it moved the force upwards. This is how pliers or or nut crackers work.
If you were to step up the voltage (make them “faster”) then there are less of them (Amps) so your total energy is the same, so a 3.7V 0.3A output can be made into 5V 0.222A output as they are the same 1.11 Watts.
So your USB socket is 5V to make that 5V from 3.7V steps up the Voltage so steps down the current and so say your 10000mah of 3.7V becomes 7400mah at 5V. 7400/10000 = 74%. That does NOT mean you lost 26% of the energy, no it just changed its shape (like you used a long stick lever to change the force vs distance). You then have the efficiency of the electronics which does the step-up of the voltage, so say a 95% efficient electronics would take the example 7400mah and actually deliver in electronic energy 7030mah. So extremely good products would give about 7000mah from the 10000mah you see on the packaging. That does NOT mean it is a bad product at all.
I read Amazon reviews where they basically divide the mah of the battery by the mah of the gadget and say “bad product” which is totally unfair. The mah of the product is not at the same voltage as the output.
This is further magnified by power loss through cables and the gadget itself.
I usually use the rough number of half of the quoted mah to get a decently safe idea of what I’m going to get so say a 3000mah phone internal battery, say a 10000mah external USB battery I’d expect about just under 2 full charges, probably nearer to 3 full charges but for a safe lower number I’d use 2. That has nothing to do with any products being bad, indeed Anker could make a perfect product but it would never get more than about 70%, so allowing for cable loss and gadget inefficiencies, a 50% number is a safe method.