I was originally running off a 15A just to keep the batteries topped off when I realized I could easily add a 30A service on my outside garage wall.
I built a new garage 5 years ago and I had them wire a 30A 220V circuit in case I ever needed it.
The box just had a blank cover on it.
So I took the cover off and drilled a hole through the center of the box and through the wall.
I then just tapped into one of the hot wires for 110V and ran it and the neutral and ground through the wall.
I then installed a 30A service box with its own breaker on the outer wall.
Worked out great and it was right wear I park it.