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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Both kWh and mAh measure energy capacity. They are used in this way because of the units used in typical applications.
Small battery devices typically specify the current they draw.
Appliances typically specify the wattage they use.
If you know the current draw, mAh gives you the approx capacity in hours.
Hours = mAh/mA
If you know the wattage, kWh gives you approx capacity in hours.
Hours = kWh/kW
This is also related to convention. Power is sold in kWh, so large batteries that connect to grids use the same units for convention.
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