How much power loss is included when charging a cell phone via inverter?
You can't go by the wattage – that's just a peak wattage, and that isn't relevant to efficiency. 

Since efficiency of power supplies, no matter whether step-up (your "inverter") or step-down (your phone charger) is not a constant but depends on the load, you either need a very good model of your electronics (which can't exist for a phone, because you can't know a priori in which state the battery is, and what the phone does during charging), or you'll need to measure.

So, measure.