By EV Charge Cost Editorial Team. Published September 28, 2026, updated October 26, 2026.
A wall charger for home use is not really a charger. It is a smart, protected switch that supplies AC power and tells the car how much current it may take. The actual conversion from AC to the DC your battery stores happens inside the car, in the onboard charger. That component has a maximum power, and it sets the ceiling on every AC session.
The rule: the smaller number wins
Effective AC charging power = the lower of (a) what the circuit and wall unit can supply and (b) what the onboard charger accepts. Nothing you do at the wall raises (b).
Onboard ratings vary by model, trim and year. The 7.2, 11 and 19.2 kW columns below are common example ratings for illustration, not the spec of any particular car. Check your owner's manual for yours. Some EPA listings even name the onboard charger in the trim, such as trims of the Cadillac Lyriq, Cadillac Optiq and Cadillac Vistiq.
| Wall supply | Supply power | 7.2 kW onboard | 11.0 kW onboard | 19.2 kW onboard |
|---|---|---|---|---|
| 120 V, 12 A | 1.44 kW | 1.44 kW · 4 mi/h | 1.44 kW · 4 mi/h | 1.44 kW · 4 mi/h |
| 240 V, 16 A | 3.84 kW | 3.84 kW · 11 mi/h | 3.84 kW · 11 mi/h | 3.84 kW · 11 mi/h |
| 240 V, 32 A | 7.68 kW | 7.20 kW · 21 mi/h | 7.68 kW · 23 mi/h | 7.68 kW · 23 mi/h |
| 240 V, 40 A | 9.60 kW | 7.20 kW · 21 mi/h | 9.60 kW · 29 mi/h | 9.60 kW · 29 mi/h |
| 240 V, 48 A | 11.52 kW | 7.20 kW · 21 mi/h | 11.00 kW · 33 mi/h | 11.52 kW · 34 mi/h |
| 240 V, 80 A (100 A circuit) | 19.20 kW | 7.20 kW · 21 mi/h | 11.00 kW · 33 mi/h | 19.20 kW · 57 mi/h |
Read across a row and the speed stops rising once the wall supply passes the onboard rating. A car with a 7.2 kW onboard charger gets the same result from a 40 A, 48 A or 80 A unit. The last row needs a 100 A circuit, which only makes sense for the few vehicles with onboard chargers large enough to use it.
Other reasons a session runs slow
- The car's current setting. Many cars let you cap AC current in the charging menu, and some reduce it automatically at certain locations. Check it before blaming the charger.
- The charger's setting. A charger configured for a smaller circuit (for example 24 A on a 30 A breaker, 5.76 kW) will stay there.
- Lower voltage. Some apartment and commercial buildings supply 208 V rather than 240 V. The same 32 A then gives 6.66 kW instead of 7.68 kW.
- A nearly full battery. Charging tapers near 100%, so the last few percent come in slower.
- Cold battery. In cold weather some energy goes to warming the pack, which reduces the rate at which range is added.
- Power sharing. Two chargers splitting one circuit each get part of the total.
DC fast charging is different
DC fast chargers do the AC-to-DC conversion inside the station and feed the battery directly, bypassing the onboard charger. That is why a car limited to 7.2 kW at home can charge many times faster at a highway station, where the battery's own charging curve sets the limit instead. See the DC fast charging curve.
How to use this when buying a charger
- Find your car's maximum AC charging power in the manual or spec sheet.
- Size the circuit so the continuous current (80% of the breaker) roughly matches it: power ÷ 240 V gives the amps.
- If you expect a future car with a larger onboard charger, ask your electrician whether wiring for more amps now is sensible, and set the charger lower until then.
- Have a licensed electrician do the installation under a local permit and follow the manufacturers' instructions.
Sizing help is in how many amps you need; for session times, try the charging time calculator.