- Phone batteries use 3.7 V DC
- USB chargers output 5 V DC
- 230 V AC → 5 V DC conversion efficiency ≈ 0.8 (0.75 – 0.9) 1 2
- 5 V → 3.7 V conversion efficiency ≈ 0.9 (0.88 – 0.95) 3
- Every 1 000 mAh (= 1 Ah) of a phone battery stores 1 Ah × 3.7 V = 3.7 Wh of energy
Depending on the efficiency of the charger, charging 1 000 mAh takes:
- Ideal: 3.7 Wh / 0.9 / 0.95 ≈ 4.33 Wh
- Excellent: 3.7 Wh / 0.85 / 0.92 ≈ 4.73 Wh
- Good: 3.7 Wh / 0.8 / 0.9 ≈ 5.14 Wh
- Decent: 3.7 Wh / 0.75 / 0.88 ≈ 5.6 Wh
- Bad: 3.7 Wh / 0.7 / 0.85 ≈ 6.22 Wh
- Horrible: 3.7 Wh / 0.6 / 0.75 ≈ 8.22 Wh
So, fully charging a phone with a 4 000 mAh battery every day for a month with a decent charger would take 5.6 Wh × 4 × 30 = 672 Wh = 0.672 kWh