You should be able to calculate the watt hours based on the voltage of the internal batteries. Its likely a 3.7v lithium, so this anker bank would be 3.7v x 24,000mAh = 88.8Wh
The issue is comparable to mileage numbers... I dont trust that at all.
Usually they just multiply the number of cells with their "rated" mAh-value, but who knows what their cutoff voltage is. Also you need to integrate over the voltage (which keeps falling during discharge) and the discharge graph also depends on how much power you draw. Then the 4.2V=>5V (or 9..19V for USB PD) conversion losses are also not counted.
