Wireless Charging Efficiency Explained
Wireless charging is 75-85% efficient, meaning 15-25% of energy is lost as heat. Here is what efficiency means and how it affects your charging time.
Wireless charging efficiency is typically 75-85%. A 15W charger delivers approximately 12W to the battery after efficiency losses. Higher efficiency means less heat and faster effective charging.
What Is Wireless Charging Efficiency?
Wireless charging efficiency is the percentage of electrical energy drawn from the power outlet that actually reaches your phone battery.
Formula: Efficiency = (Energy to battery / Energy from wall) × 100
A 15W wireless charger operating at 80% efficiency draws 18.75W from the wall to deliver 15W to the charging system, of which only 12W reaches the battery (15W × 80% = 12W effective).
Why Wireless Charging Loses Energy
Inductive energy transfer involves several conversion steps, each losing a small percentage:
AC to DC conversion — The power adapter converts mains AC to DC. Loss: ~5-8%
Power regulation — The charger regulates voltage and current for the coil. Loss: ~3-5%
Coil resistance heating — Current flowing through the transmitter coil heats the wire. Loss: ~5-10%
Magnetic field losses — Not all magnetic field energy is captured by the receiver coil. Loss: ~5-15% (depends heavily on coil alignment)
Receiver coil heating — The receiver coil in the phone also loses energy to heat. Loss: ~3-5%
Total losses typically sum to 15-25%, giving 75-85% efficiency.
How Alignment Affects Efficiency
The largest variable in wireless charging efficiency is coil alignment:
| Alignment | Efficiency |
| Perfect (Qi2 magnetic) | 82-86% |
| Good (carefully centred) | 78-82% |
| Moderate (~1cm offset) | 68-75% |
| Poor (~2-3cm offset) | 50-65% |
This is why Qi2 magnetic alignment provides a real, measurable improvement over standard Qi — it eliminates alignment variability.
Using Efficiency in the Calculator
The Charging Time Calculator includes an efficiency slider. Recommended values:
| Charger Type | Recommended Efficiency |
| Cheap/uncertified Qi pad | 70-75% |
| Standard Qi certified | 78-82% |
| Qi2 certified charger | 82-86% |
| Wired USB-C PD | 92-95% |
Setting a more accurate efficiency gives you a more accurate time estimate.