18W Iphone charger - too much?

18W should be no issue, BUT make sure when you plug your (car version of the charger, I assume?) charger into a 12V port in your Prius that you only charge in READY mode (ie, when driving or ready to drive the car), and not in any accessory mode, lest you run down your 12V battery.

 

18 watts is a very slight load in the life of a car; the hybrid system can be shuffling tens of thousands of watts around while you're driving.

It's good advice to plug in to one of the car's 'power outlets' that are off when the car is, so you don't inadvertently draw down the small battery in back. At 18 watts even that would take a while (the battery holds about 540 watt hours, but remember the rest of the car is using some of that too, so you won't get 30 hours of phone charging; you could forget and go 15 and maybe still be able to start the car).

With a policy of just having the car in READY whenever you're using electrical loads, you avoid having to fuss about that.

 

USB port charge is limited to 500mA @ 5v, which is 2.5W. The 12v "cigarette lighter port" is limited to 12v @ 10A generally so 120W. You could charge with efficiency losses a good phone like with Xiaomi's "Super Charge Turbo" at 100W without an issue. My phones charge at 45W all day long, no issue. I use an adapter with 2 45W ports for myself and passenger.