In the past all we had to choose from was a basic 12 volt cigarette lighter or power outlet. I installed one of these in a blog post a while back. While on Amazon.com one day a few new 12 volt outlet options popped up during a search. One had an actual voltmeter and a USB charger. There is another panel mounted device with all three options of a voltmeter, USB charger and a 12 volt outlet. You can purchase any of them on my Amazon Store.
These are easy to add to just about any boat either to the main control panel or in sleeping quarters or in handy areas near the helm.
I decided to add the voltmeter and the USB outlets to my main fuse panel down below. This would allow me to easily see the state of charge of my house battery and charge my mobile devices.
The inserts for the voltmeter and the USB charger rotate within the mounting plate so you can mount them vertically or horizontally.
In my case the vertical mounting of the outlets was preferred. Two 1 1/4 inch holes were drilled into the panel to allow the devices to stick through and enable the wiring connections behind the panel.
The far right, MISC, fuse and switch were used to power the meter and USB outlets. The USB outlet only draws about 2.1 amps so a 5 amp fuse was used to make sure it would power the outlet without burning out. 16 gauge wire was used to connect the devices to the switch and fuse.