Power bank shopping is confusing on purpose. Capacities are quoted in milliamp-hours (mAh), devices consume watt-hours (Wh), and the conversion between them hides real-world losses. Here is the five-minute version that lets you size any battery correctly.
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The only formula you need
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Watt-hours = (mAh u00d7 3.7 volts) u00f7 1000. A “20,000 mAh” power bank holds about 74 Wh. That 3.7 V is the battery cell voltage u2014 the honest baseline every bank shares regardless of marketing.
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Then subtract the tax
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Converting battery voltage to USB output wastes energy as heat. Plan on keeping roughly 60u201370% of rated capacity as usable power. Our 74 Wh bank realistically delivers 45u201350 Wh.
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What devices actually consume
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A phone charge is 12u201318 Wh depending on the model. A tablet runs 25u201335 Wh. A laptop charge is 50u201370 Wh u2014 which is why a pocket power bank that “charges laptops” often delivers less than one full charge. Headphones, watches and earbuds are rounding errors at 1u20133 Wh each.
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Worked example: a weekend off-grid
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Two phone charges per day (30 Wh), a tablet evening (30 Wh), earbuds and a headlamp (5 Wh) is about 65 Wh per day, 130 Wh for the weekend. Divide by 0.65 for conversion losses and you need roughly 200 Wh of rated capacity u2014 a 27,000 mAh bank plus a spare, or one small power station.
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The airline ceiling
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100 Wh (about 27,000 mAh) is the standard carry-on limit without airline approval. If you fly, two mid-size banks beat one giant one.
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Skip the arithmetic: our free charge calculator does the conversion with your exact devices, and our power bank reviews measure delivered watt-hours u2014 the number that actually matters u2014 for every unit we score.