A problem is that some sites that don’t need cookie banners use them anyway due to a poor understanding of the law and excess of caution.

Well… sort of.

Batteries perform differently under load. A battery that delivers 10Wh under a 1W load will probably deliver less (and get warmer) under a 10W load. Power supplies also perform differently under load, and DC-DC switching power supplies perform differently based on the output voltage. Generally, a larger voltage conversion and/or a higher load is less efficient. There’s also going to be some base power consumption in the circuit, so the most output power is probably achieved at some sort of medium load.

To make things more fun, batteries are usually tested under constant current, not constant power. The increasing current as the battery drains of a constant power load will result in less total power, and constant output power often means increasing input power as the battery drains.

In short, the real world is complicated. Giving best and worst case Watt-hours could be a reasonable approach.

I’d be a little concerned about safety. A manufacturer or distributor that’s willing to lie so blatantly about capacity might also be willing to sell cells that failed QC.

Powerbanks are where it’s most problematic. They’re usually reporting the capacity of the battery cells in mAh. Those cells will be at 2.8-4.2V during operation, but the powerbank outputs 5V, or in modern powerbanks some higher number. 5000 mAh at the 3.6V average of the cells during discharge is certainly not 5000 mAh at the 9V it’s giving to my phone.

It’s not going to give my phone 2000 mAh @ 9V or 18 Wh as the math would suggest either because it’s well below 100% efficient. I’m not sure what’s reasonable to demand in terms of advertising here since efficiency will vary with output voltage and output wattage.

I guess they could still lie about watt-hours

Your battery illustrates that; 9.9Ah * 3.7V is, indeed 36.63Wh. Buying anything without trust in the seller or verification by a third party is a crapshoot.

Some people have privacy expectations that are not realistic in an unencrypted, federated, heterogeneous environment run by hobbyist volunteers in their spare time.

It you have something private and sensitive to share with a small audience, make a group chat on Signal. Don’t invite any reporters.

Why do you have an app for that?