Unfortunately these 12V batteries have generally just one third of the declared capacity (less than 7Ah for this 20000mAh one).
The reason: http://howiem.com/wordpress/index.php/2013/11/01/chinese-12v-6800mah-li-po-battery-mini-review/
Fortunately this does not apply for common RC battery packs, where the capacities are declared correctly at the nominal working voltage.