You're not wrong, but the problem we're hitting is that most mobile tech has divorced product/supply voltage from battery voltage, and the result is a lot of "w

You're not wrong, but the problem we're hitting is that most mobile tech has divorced product/supply voltage from battery voltage, and the result is a lot of "white lies" where the values stated don't add up to a whole. My laptop has a 6000mAh battery and will charge from 5V. But my 10,000mAh powerbank will not fully charge it. The question is how to explain this to my mother. To calculate this properly - you have to do some actual digging to find the nominal battery voltage of the laptop, since it's not any of the USB-PD voltages, and is thus entirely divorced from the laptop's input voltage. And then you have to know that most powerbank vendors tell you the rating of the cells, not the rating of the product. So they will tell you that it's a 10,000mAh battery, and that it outputs 5V - but these two facts are only honest in isolation. It is not a 50Wh battery, it's a 37Wh battery. Or a 7,400mAh product that is powered by 10,000mAh cells. Almost anywhere we use USB-PD, stating Wh is a lot more honest to the consumer. If we tell my mother that her laptop is 70Wh and the powerbank is 37Wh, she can infer it'll give her about half a full charge. If we tell her the laptop is 6000mAh and the powerbank is 10,000mAh, she can be forgiven for inferring it'll give her almost two charges. -- For an example of what I'm calling "white lies": https://www.anker.com/products/a1271 https://www.anker.com/products/a1271 (And to be clear, I'm not blaming Anker for this, I believe it's near-universal. I'm only using Anker as an example because they're a more-trusted brand.) All the way down the bottom of the page, just above the FAQ, they give the capacity as "20100mAh/72.36Wh". 20100mAh is given 19 times - 72.36Wh is given precisely once. From 72.36Wh / 20.1Ah we get a nominal voltage of 3.6V, not 5V. 3.6V is never otherwise stated. I've never seen any vendor tell you that you're seeing the cell rating, not the product rating - but the better vendors will give you enough information to figure it out yourself.