Charging at home falls into two categories, and the difference in what they require is substantial.

Level one is an ordinary hundred and twenty volt outlet. Every vehicle comes with a cord that plugs into one. It adds a small number of miles per hour of charging, which is adequate for short commutes and overnight sitting. It requires nothing new electrically, though it is worth confirming the outlet is on its own circuit and in reasonable condition, because it will be under continuous load for many hours at a time.

Level two is two hundred and forty volts, and it is what most people mean by a home charger. It typically adds several times the range per hour. It requires a dedicated circuit, and that circuit is the actual project.

The circuit is sized at a hundred and twenty five percent of the charger's continuous rating, because continuous loads are treated differently from intermittent ones. A forty amp charger therefore wants a fifty amp circuit. That drives conductor size, and conductor size drives cost, particularly over long runs from panel to garage.

Panel capacity is the first question rather than the last. Adding a fifty amp continuous load to a service that is already near its limit is not a matter of finding a free slot. A load calculation determines whether the existing service supports it. On a hundred amp service with electric heat or an electric range, it often does not without either a service upgrade or a load management device.

Load management is worth knowing about because it can avoid an upgrade entirely. These devices monitor total draw and reduce or pause charging when the house is drawing heavily elsewhere. Since a vehicle usually sits for many hours, pausing during the evening peak costs the user nothing in practice.

The choice between a hardwired unit and a plug-in unit on a receptacle affects the protection required and the ease of later replacement.

Permits generally apply. This is a new dedicated circuit for a significant continuous load, which is exactly the category of work inspection exists for.

For more detail, see source