Phone batteries are advertised in milliamp-hours, power banks shout five-digit mAh figures, yet laptop batteries and airline rules talk about watt-hours. Both describe battery capacity, but they are not interchangeable — and the mAh number, on its own, is quietly incomplete. Comparing two batteries by mAh alone is like comparing two water tanks by the width of their outlet pipes while ignoring how tall the tanks are.
The missing ingredient is voltage. Once you put it back into the equation, battery comparisons become honest.
Charge versus energy
Milliamp-hours measure electric charge: how many electrons the battery can push out over time. Watt-hours measure energy: charge multiplied by the voltage pushing it. The formula is simple — watt-hours equal amp-hours times volts. A 5,000 mAh battery at 3.7 volts holds about 18.5 Wh; the same 5,000 mAh at 7.4 volts holds twice the energy. Since energy is what actually runs your screen and processor, Wh is the fairer yardstick whenever voltages differ.
The power bank illusion
Most lithium cells operate near 3.7 volts, and manufacturers quote mAh at that cell voltage. But your devices charge over USB at 5 volts or more, and boosting the voltage costs capacity plus conversion losses. A 20,000 mAh power bank holds about 74 Wh; delivered at 5 volts with realistic efficiency, that yields perhaps 13,000 to 15,000 mAh of usable output. This is why a 20,000 mAh bank charges a 5,000 mAh phone closer to three times than four — nobody necessarily lied, but the two mAh figures were never measured at the same voltage.
Where Wh already rules
Watt-hours dominate wherever mixed voltages make mAh useless. Laptop batteries are labeled in Wh because their packs use different cell arrangements. Aviation rules cap carry-on batteries by Wh — commonly 100 Wh without approval — because energy, not charge, determines fire risk. Electric vehicles scale up to kilowatt-hours for the same reason. Tellingly, the mAh convention survives mainly in phones and power banks, where marketing prefers the bigger-sounding number: 5,000 mAh simply sells better than 18.5 Wh.
To compare any two batteries fairly, convert both to watt-hours by multiplying amp-hours by voltage — it takes ten seconds and cuts straight through the marketing.
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