I just bought an Anker Astro 5600 mAh backup battery. It powers devices via a standard 5V USB port. However, it does not have 5v X 5600 mAh = 28 Wh of joules. It has 3.7v X 5600 mAh = 20.7 Wh of joules. That's 26% fewer electrons than advertised.
The 3.7v comes from the battery technology used inside, and doesn't even include the efficiency of the DC-DC converter (probably 85-90%) needed to boost 3.7v to 5v.
I looked all over their website, but never found the true Wh, only the mAh of the Li-Poly battery inside. Some manufacturers I've seen do list the true Wh capacity of their products.
If I ever start a business selling electronics for the field, you can be sure I will be more informative as to what you can actually expect from my products.