Why 20,000 mAh does not mean four phone charges
At an example cell voltage of 3.7 V, 20,000 mAh represents 74 Wh. An assumed 80% delivery efficiency gives 59.2 Wh before any further charging losses inside the receiving device. Dividing that by a phone's battery energy is only an estimate. Battery age, temperature and using the phone while charging all affect the result.
Source: Anker support: why rated capacity differs from expected capacity ↗Laptop charging needs power as well as energy
Wh describes the size of the energy store. Watts describes how quickly energy can be supplied. A high-capacity pack can still be unsuitable for a laptop if its single-port output or USB Power Delivery profiles do not match. Check the laptop, cable and chosen port together. Multi-port total wattage does not promise that same wattage from one port.
Treat efficiency as an assumption until measured
Anker explains that voltage conversion and other losses reduce usable capacity. Our efficiency field is deliberately editable: the default is a worked example, not a measured value for a listed product. For comparisons, prefer independent delivered-energy tests made at similar output voltages and loads. For travel, read the printed Wh marking and your carrier's current rules separately.
Source: Anker support: why rated capacity differs from expected capacity ↗Your pre-purchase checklist
- Match mAh to its nominal cell voltage.
- Check single-port output and supported charging profiles.
- Use delivered-energy tests when comparing two packs.
Sources and scope
Original editorial guidance with linked source material. Manufacturer guidance describes operation and features; it is not independent testing. Calculator results use your inputs and stated assumptions.
- Anker support: why rated capacity differs from expected capacity ↗Manufacturer explanation