How the power bank calculator works
The two hidden cuts
Voltage conversion: cells store 3.7 V; USB delivers 5 V — capacity in mAh is voltage-specific, so the bank's 20,000 mAh becomes ~14,800 at output. Conversion efficiency: boosting 3.7→5 V wastes 10–20% as heat. Together they turn label capacity into ~60–70% deliverable — the gap every disappointed power bank review is describing.
The Wh figure that matters
Watt-hours (mAh × 3.7 ÷ 1000) is the universal battery currency — airlines cap carry-on banks at 100 Wh (≈27,000 mAh), and laptops measure in Wh. A 20,000 mAh bank is 74 Wh. The calculator shows the Wh line because it is the number that passes through security and the number laptop specs speak.
Matching bank to device
Phones (3,000–5,000 mAh): a 10,000 mAh bank = 2–2.5 real charges — the sweet spot of size and utility. Tablets (7,000–9,000): 20,000 banks. Laptops: measured in Wh (50–100), needing 65–100 W output — capacity alone does not make a bank laptop-compatible. The device input here prices all three honestly.