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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Short answer is, small devices all tend to have the same battery voltage, while bigger batteries don’t.
In phones, using mAh is mostly acceptable as *most phones* are using a battery that’s around the same voltage, usually 3.7-3.8 volts. That’s the standard voltage of a single lithium battery “cell”, just as AA or AAA batteries are always around 1.5 volts. If the voltage of the batteries is the same, you can compare their mAh ratings directly. There’s nothing stopping you from defining this in Wh, but we don’t typically do that.
With bigger batteries, there’s a lot more variation in how the individual cells inside the battery pack are configured, so voltage can be anything from the single cell voltage of 3.8ish volts, to an 800v pack you’d find in a new electric car, or maybe even higher. When you can’t compare packs of the same voltage, you can multiply it by the Ah rating to get the Wh rating, and that will let you compare their *capacities* more directly.
A 6 cell pack where each cell is 2.5Ah will always have around ~57Wh of capacity, but depending on how those cells are connected, you could have a lot of combinations, 22.8V @ 2.5Ah and 11.4V @ 5Ah being just 2 of them.
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