

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.
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.