EV Charger Circuit and Charge Time Calculator
Charge time, circuit size and cost for a home EV charger — including the onboard charger limit that usually decides your speed, not the wall unit.
The wall unit does not set your charging speed. The car does.
Every EV has an onboard AC charger with its own limit, and the actual rate is the lower of the two. Fitting a 48 A unit to a car with a 7.7 kW onboard charger gets you 7.7 kW and a larger installation bill. People buy the biggest EVSE on the shelf and are then puzzled that it charges no faster than the one it replaced. Check the car's specification before the charger's.
- DC fast charging is a different thing entirely and bypasses the onboard charger. That is why a car limited to 7.7 kW at home can still take 150 kW at a motorway stop, and why nothing on this page applies there.
- The circuit is 125% of the charger. NEC 625.41 makes EV supply equipment a continuous load, so a 48 A charger needs a 60 A circuit. Turned around: you may draw only 80% of a breaker's rating continuously. Same arithmetic, and people reliably meet one half and forget the other.
- Above 40 A the unit must be hardwired. Plug-connected EVSE is limited to 40 A — which also means a 48 A unit cannot be unplugged and taken to a new house.
- Charging is not free. About 12% is lost between the wall and the battery, and the meter bills the larger figure. A "75 kWh" full charge draws about 85 kWh.
- If the service cannot spare the circuit, load management is the way out. NEC 625.42 permits a system that lets the charger share capacity with the house and back off when other loads run — usually far cheaper than a service upgrade.
How to use
- Pick the charger and the vehicle — the actual rate is the lower of the two.
- Set the state of charge you are going from and to.
- Read the time, and check whether the car or the charger is the limit.
- Note the circuit size, which is 125% of the charger rating.
Frequently asked questions
Why is my new charger no faster than the old one?
Almost certainly because the car is the limit rather than the charger. Every EV has an onboard AC charger with its own maximum, and the rate you get is the lower of the two figures. A Nissan Leaf accepts 6.6 kilowatts no matter what you plug it into, so a 48 amp wall unit delivering 11.5 kilowatts is spending its extra capacity on nothing. Check the car specification before choosing the charger, not after.
What size circuit does my charger need?
125 percent of its rating, because NEC 625.41 classes EV supply equipment as a continuous load. A 32 amp charger needs a 40 amp circuit, 40 amps needs 50, and 48 amps needs 60. The same rule stated backwards is that you may only draw 80 percent of a breaker rating continuously, which is why a 50 amp circuit tops out at a 40 amp charger. Both statements are the same arithmetic and people reliably satisfy one and forget the other.
Why can my car take 150 kW at a fast charger but only 7 kW at home?
Because DC fast charging bypasses the onboard charger entirely and feeds the battery directly. The onboard unit exists to convert household AC into DC, and it is a small, light, cheap component that manufacturers size for overnight charging. A public DC station does that conversion in its own cabinet, at a scale nobody would fit inside a car. Nothing about home charging limits applies at a motorway stop.
Is Level 1 charging fast enough?
For a great many drivers, yes. A 12 amp Level 1 cable adds roughly four or five miles of range per hour, which is 50 to 60 miles over a night — more than the average daily drive. If you commute under 40 miles and can plug in every night, the supplied cable may be all you ever need, and it requires no installation. Level 2 matters when the daily distance is longer, when the car sits at home for fewer hours, or when you want a full charge in an evening rather than overnight.
How much energy is lost charging?
Around 10 to 15 percent between the wall socket and the battery, in the onboard charger and in battery heating. That means a nominally 75 kilowatt-hour full charge draws about 85 kilowatt-hours through the meter, and the meter is what you pay for. It is also why comparing a car's advertised efficiency against your electricity bill always shows a gap.
Do I need to hardwire the charger?
Above 40 amps, yes. Plug-connected EV supply equipment is limited to 40 amps, so any 48 amp unit must be hardwired. That is worth weighing beyond the code requirement: a plug-in charger can be unplugged and taken to a new house or swapped out easily, while a hardwired one is part of the building. For most cars a 40 amp plug-in unit is the practical sweet spot.
What if my panel cannot take another large circuit?
Load management, before a service upgrade. NEC 625.42 permits an EV supply equipment load management system that shares capacity with the rest of the house and reduces charging current when other big loads run. Since a car typically sits plugged in for far longer than it needs, backing off during the evening peak costs nothing in practice. It is usually thousands cheaper than upsizing the service, and worth asking about explicitly if an installer proposes the upgrade first.
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