I put 12V-powered USB outlets all over the boat. Some things I've found:
- The ones that run off 120VAC will introduce a parasitic load. Not much, but if it's enough to keep the inverter powered up, when otherwise nothing is drawing 120VAC, then you've added the inverter's parasitic load, too. That's a big deal for us, trying to stretch our time on the hook between generator runs.
- Be sure to get the ones that have at least one USB plug with at least a 2.1A capacity. Modern USB fast chargers may need even more. Anything less and you're wasting your money.
- Be sure to use the "right" USB cable for your device. I have some cables which are wired internally to be recognized as "fast" chargers for our Android devices. Otherwise the device may think it's a slow charger. But again, look at the Amp rating on the cable; I've overheated them using a lower-amp cable on a device which will accept higher fast charging.
- Beware of lighted outlets. They can be too bright at night. Ours have a small blue LED which is sometimes too much, but at least the rubber outlet cover blocks most of it when not in use. I've actually had to put a tiny square of electrical tape over the ones in the master berth.
- You can never have enough USB ports in all the places you'll want one. Especially with guests and passengers who almost always show up with a dying battery. And don't forget the cables (your guests will!) There are now at least 4 common USB device formats, with possibly 2-3 more less common but still out there.