# mAh and Wh ratings not matching with voltage of device

I was under the impression that
Wh = Ah * V

I have a 20000mAh battery charging device for my phone but am confused about the information provided by it on the side.

It claims to have a 5V output (3.1A max), and a capacity of "20000mAh/74Wh". But following the above equation, 100Wh = 20Ah * 5V. Where does this discrepancy come from?

• The battery voltage is 3.7 V. It has a step up converter to 5 V after it. You are trading current for voltage. Recalculate for 3.7 V instead. On the outside you will see (3.7/5) times less Ah, minus efficiency losses. – winny Jul 13 '16 at 17:42
• this discrepancy is still probably smaller than the "advertised power vs real power" discrepancy – Wesley Lee Jul 13 '16 at 18:12
• As @winny says, it's the 3.7 V Ah capacity and not the 5 V. This is one good reason to use Wh. It's a measure of the energy available. – Transistor Jul 13 '16 at 18:22
• @winny You should make this an answer. – DoxyLover Jul 14 '16 at 5:54
• @DoxyLover It will just be downvoted and/or removed. – winny Jul 14 '16 at 7:45