Which USB power adapter charges a completely dead iPad® more efficiently—the large 12-watt power supply that came with the iPad or the smaller 5-watt power supply that came with the iPhone®? We decided to measure the energy consumed during the charge cycle with a Watts Up Pro.

As you can see from the graph, the larger unit uses more energy per unit of time and completes the charge cycle more quickly. But is it the same total amount of energy? To find out, we integrated under each curve.

Testing iPad power adapters
5-watt Power Supply 12-watt Power Supply
Charge Time 10.6 hours 6.2 hours
Total Energy 63.1 watt hours 66.3 watt hours

The energy required to fully charge the battery is about the same, but there is a small difference. We did not measure temperature in this experiment. From what we have learned about rechargeable batteries in the last few years, we know that charging at a higher current generates more heat, which makes the charging process less efficient. Some people believe that consistently charging the battery at higher current can shorten its overall life. Other aspects to investigate:

  • Temperature
  • Off-brand chargers
  • Charging while the device is on or off
  • Comparing the charge cycle of different phones and tablets
  • Estimating the cost of the energy to charge electronics such as laptops and tablets

Note that for loads lower than about 5 watts, the Watts Up Pro is not nearly as accurate as it is for household appliances such as refrigerators, freezers, and air conditioners.