Every powerbank vendor I've seen gives the battery rating at 3.6/3.7V, so USB implying 5V gives us misleading values. So a 20,000mAh powerbank is 20*3.6V=72Wh,

Every powerbank vendor I've seen gives the battery rating at 3.6/3.7V, so USB implying 5V gives us misleading values. So a 20,000mAh powerbank is 20*3.6V=72Wh, not 20*5=100Wh. I've never seen a vendor that specifies 3.6V, the best you can hope for is that they give mAh and Wh so you can derive it yourself. If we need the Wh rating to keep the mAh rating honest, the mAh becomes redundant before the Wh does.