Plan every charge before you leave home
Stop guessing how many times your power bank can charge your phone, tablet, or laptop. Enter real numbers and get a clear plan.
Capacity Estimator
Assumes 85% charging efficiency. Real results vary with temperature, cable, and battery health.
How this helps on the road
You pack a 20,000 mAh power bank and two devices: a phone and a tablet. The bank label says 20,000 mAh, but that is at 3.7 volts—about 74 watt-hours. Your phone battery holds 12.7 Wh, and the tablet holds 32.4 Wh. After accounting for charging losses, you can expect roughly 5 full phone charges or about 2 tablet charges, not both at full count. Knowing this before a weekend trip means you can decide whether to bring a second bank or ration screen time.
What the numbers mean
Watt-hours measure total energy. A 74 Wh bank can deliver about 63 Wh after typical losses. Divide that by your device watt-hours to get the number of full charges. Charging speed in watts tells you how fast energy moves into the battery. A 12.7 Wh phone charging at 18 W takes roughly 42 minutes from empty to full under ideal conditions.
Common mistakes
- Confusing mAh with Wh. A 10,000 mAh bank at 3.7 V is 37 Wh, but a 10,000 mAh laptop battery at 11.1 V is 111 Wh. Always convert to watt-hours.
- Ignoring efficiency. No charger is 100% efficient. Heat, voltage conversion, and cable resistance eat 10–20% of the energy.
- Forgetting standby drain. Devices and power banks slowly lose charge even when idle. Over a multi-day trip, this adds up.
Quick scenario
You are hiking for three days with a phone (12.7 Wh) and a GPS unit (10 Wh). You bring a 74 Wh bank. You need two phone charges and three GPS top-ups. Total needed: (2 × 12.7) + (3 × 10) = 55.4 Wh. With 85% efficiency, the bank delivers about 63 Wh, so you are covered with a small buffer. If temperatures drop near freezing, expect less and plan for one fewer charge.