Our Methodology
How we calculate charging times, efficiency ratings, heat scores, and compatibility assessments.
Charging Time Formula
Our Charging Time Calculator uses the following formula:
Where:
- Battery: Device battery capacity in milliamp-hours (mAh)
- Wattage: Charger output in watts at the device coil
- Efficiency: Energy transfer efficiency (default: 80% for wireless, 95% for wired)
- Start/End%: The battery percentage range being charged
This formula assumes constant current charging. In reality, charging slows above 80% battery to protect battery health. Our estimates are therefore most accurate for 0–80% ranges.
Efficiency Ratings
Default efficiency values by charging type:
| Charging Type | Default Efficiency | Range |
|---|---|---|
| Qi (standard wireless) | 80% | 72–85% |
| Qi2 (magnetic wireless) | 83% | 78–88% |
| Wired USB-C (reference) | 95% | 90–97% |
| High-wattage proprietary (≥25W) | 78% | 70–83% |
Heat Score
Heat scores are assigned based on measured surface temperature at the charging coil after 30 minutes of charging:
No concern. Optimal charging conditions.
Expected during wireless charging. No action needed.
Above average. Consider removing case or moving to a cooler surface.
Unusually high. Device may throttle charging speed. Enable battery protection mode.
Compatibility Score
Our compatibility score (shown on device pages) reflects the quality and breadth of a device's wireless charging support:
- Standards: Devices supporting Qi2 or MagSafe score higher than Qi-only devices.
- Max wattage: Higher maximum wattage increases the score.
- Reverse wireless: Devices that can also charge other devices via wireless earn bonus points.
- Ecosystem: Support for more standards (Qi + Qi2, or Qi + MagSafe) increases versatility score.
Scores are not comparative rankings between devices. They are independent assessments of how comprehensive each device's wireless charging implementation is.
Limitations and Disclaimers
- Charging times are estimates. Real times vary by software optimisation, temperature, background processes, and charger cable quality.
- Efficiency values are averages. Premium chargers and Qi2 pads typically outperform these estimates.
- Battery capacity figures are manufacturer-stated values. Actual usable capacity may be slightly lower due to thermal management.