I get that, for a house/solar battery, it sort of makes sense as your typical energy usage would be measured in kWh on your bills. For the smaller devices, though, the chargers are usually rated in watts (especially if it’s USB-C), so why are the batteries specified in amp hours by the manufacturers?
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Measuring it in ampere-hours works fine as long as the batteries (as a complete unit) are roughly of the same voltage, which is the case for most phones, as the vast majority use one single lithium-ion battery cell, and lithium ion based chemistries have very similar voltage levels.
It doesn’t work when you compare batteries of wildly different configurations, such as 100 Li-ion cells in parallel, or 10 sets of 10 cells in parallel which again are connected to form a serial battery that outputs 10x the voltage. Or batteries consisting of cells with entirely different chemistries such as lead acid (2.1V per cell) or NiMH (1.2V per cell).
To compare these, you need to use watt-hours instead to get a useful number.
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