The most efficient way is to charge your phone with phone turned off. When the device is turned off, no process or apps are running in your device so nothing consumes battery power. If your phone is not turned off, avoid using it while it is being charged.
If your ZenFone 2 is either the model Z550ML or Z551ML, then it supports ASUS BoostMaster technology and we recommend you use the USB cable and adapter provided. The 9V/2A 18W AC adapter can charge your phone at faster speed. Note: This feature only applies to specific models and must be paired with the 18W BoostMaster adapter.
Avoid high ambient temperatures for charging your phone. High temperature can cause damage to battery capacity of your phone'.
Avoid charging your phone by connecting to your computer. The power restored through a computer is often less than your actual power consumption.
Avoid running out of all your battery to 0%. If it shuts off by itself, don't attempt to turn it on again, as it can damage the battery.