Four things can limit a charge, and only the smallest one matters
Charging power passes through a chain, and the chain is only as strong as its narrowest link. First is the branch circuit: a 240 V circuit on a 60 A breaker can supply 48 A continuously, giving 11.52 kW. Second is the charging station, which may be set below the circuit's capability by a DIP switch or a configuration menu. Third is the vehicle's onboard charger, the rectifier inside the car that converts AC to the DC the battery needs — commonly rated 7.4, 11 or 11.5 kW, and often far below what a good home circuit can supply. Fourth is the battery itself, which sets a temperature-dependent and state-of-charge-dependent acceptance limit that dominates on DC fast charging.
On AC, the onboard charger is very often the binding constraint, which is why installing a 100 A circuit for a car with a 7.4 kW onboard charger buys nothing. On DC, the onboard charger is bypassed entirely — the station supplies DC directly to the pack — so the limits become the station's rating, the vehicle's peak DC acceptance and the pack's own taper.
Efficiency then reduces whatever survives. Some of the supplied power becomes heat rather than stored energy, so the power that actually raises the state of charge is the supply power multiplied by the efficiency. That is why this calculator asks for it: at 90% efficiency an 11.5 kW feed raises the state of charge as though it were 10.35 kW.
The National Electrical Code adds a constraint at the top of the chain. NFPA 70 Article 625, in the 2023 edition, treats electric-vehicle supply equipment as a continuous load, which means the branch circuit must be rated at least 125% of the charging current. Turn that around and the charging current must not exceed 80% of the breaker rating — the reason a 60 A circuit gives 48 A, not 60 A.
The formula, and why DC charging needs a second one
Below the taper, charging time is a straightforward division. Energy needed is C × ΔSOC / 100. Power at the battery is the supply power times efficiency. Time is one divided by the other. A 75 kWh pack going from 20% to 80% needs 45 kWh; at 10.35 kW into the battery that is 4.35 hours.
DC fast charging cannot use a single power figure, because the pack will not accept its peak rate all the way to full. As lithium-ion cells fill, the voltage difference driving the charge shrinks and the risk of lithium plating rises, so the battery management system reduces current. The reduction is usually gentle up to somewhere around 80% state of charge and steep above it.
This calculator models that with a deliberately simple curve: full rated power up to 80%, then a linear fall to one fifth of the rated power at 100%. Writing u for percentage points above 80, power is P(u) = P·(1 − 0.04u). The time to move through a slice of state of charge is the slice's energy divided by the power there, so the total is an integral, and it evaluates in closed form to 25·C/(100·P·η) · ln((1 − 0.04u₀)/(1 − 0.04u₁)). Charging the last 20% of a 75 kWh pack at a nominal 150 kW therefore takes about 12 minutes — the same length of time as the preceding 40%.
Two honest caveats about that model. Real taper curves are vehicle-specific, published by manufacturers only sometimes, and shaped as much by pack temperature as by state of charge — a cold pack may never reach its rated peak at all, and a preconditioned one holds high power longer. Treat the region above 80% as an estimate whose shape is right and whose exact numbers depend on your car.
Cost follows time on any network that bills by the minute; on networks that bill by energy the two are independent. Price your session with the EV charging cost calculator once you know how long it will take.
Worked example: 75 kWh from 20% to 80% on a 48 A Level 2 circuit
A 75 kWh usable pack, 20% to 80%, on a home charging station set to 48 A on a 240 V, 60 A circuit. The car's onboard charger is rated 11.5 kW. Charging efficiency is 90% and the car averages 3.5 mi/kWh.
- Energy needed. 80 − 20 = 60% of 75 kWh = 45.0 kWh.
- Supply power. 240 × 48 ÷ 1000 = 11.52 kW.
- Apply the onboard limit. min(11.52, 11.5) = 11.5 kW. The car is very marginally the constraint.
- Power at the battery. 11.5 × 0.90 = 10.35 kW.
- Time. 45.0 ÷ 10.35 = 4.348 hours, or 4 hours 21 minutes.
- Energy drawn from the supply. 45.0 ÷ 0.90 = 50.0 kWh. The 5 kWh difference is loss.
- Range added. 45.0 × 3.5 = 157.5 miles, at 10.35 × 3.5 = 36.2 miles per hour of charging.
- Code check. 48 A is exactly 80% of the 60 A breaker, so it complies with the NEC continuous-load rule.
Now consider the same car on a 150 kW DC charger. Below 80% there is no taper, so at 90% efficiency the battery sees 135 kW and 45 kWh takes 45 ÷ 135 = 0.333 hours — 20 minutes. Continuing to 100% adds another 18.75 kWh but, with power tapering to a fifth of rated by the top, takes a further 22 minutes. Stopping at 80% is not superstition; it is where the time cost per mile added roughly triples.
How to read the result
Read the limiting-factor message first. If the onboard charger is binding, no amount of money spent on the circuit or the station will make the charge faster — you would need a different car. If the supply is binding, a higher-current circuit will help, up to the onboard limit and no further. This one line usually decides whether an electrical upgrade is worth paying for.
Miles per hour of charging is the number to plan around at home. It converts an abstract kilowatt figure into something you can compare with your commute. About 36 miles of range per hour of charging, as in the example, means an eight-hour overnight window adds roughly 290 miles — comfortably more than almost anyone drives in a day, which is why most home charging never needs to be fast.
Add margin in the cold. Below freezing, a lithium pack accepts less power and may spend part of the session being warmed rather than filled. Both effects lengthen the charge, and neither is captured by a single efficiency figure. On DC fast charging, preconditioning the pack — which most navigation systems do automatically when you route to a charger — is worth several minutes.
Do not confuse the station's rating with what you will get. A 350 kW station delivers 350 kW only to a vehicle that can accept it, at a state of charge and pack temperature where it is allowed. Enter the lower of the station's rating and your car's published peak DC rate, and expect the average over the session to be well below the peak because of the taper.
For journey planning, combine this with distance: how far you must go decides how many stops you need, and the road trip fuel cost calculator gives the equivalent arithmetic for a combustion vehicle if you are comparing the two.
Charging levels, circuit requirements and time to add 45 kWh
| Level | Volts | Amps | Minimum breaker | Power | Hours for 45 kWh |
|---|---|---|---|---|---|
| Level 1, household outlet | 120 | 12 | 15 A | 1.44 kW | 34.7 |
| Level 2, small circuit | 240 | 16 | 20 A | 3.84 kW | 13.0 |
| Level 2, dryer-style circuit | 240 | 24 | 30 A | 5.76 kW | 8.7 |
| Level 2, common home install | 240 | 32 | 40 A | 7.68 kW | 6.5 |
| Level 2, 50 A circuit | 240 | 40 | 50 A | 9.60 kW | 5.2 |
| Level 2, 60 A circuit | 240 | 48 | 60 A | 11.52 kW | 4.3 |
| Level 2, maximum single-phase | 240 | 80 | 100 A | 19.20 kW | 2.6 |
The last row is achievable only by vehicles with an unusually large onboard charger; most cars cap out between 7.4 and 11.5 kW on AC, so the 48 A and 80 A rows will normally deliver the same time as one another. Check your car's onboard charger rating before paying for a larger circuit.
Assumptions and what the model leaves out
- Constant power on AC. True in practice for almost all of an AC session, though most vehicles taper slightly in the last few percent even on AC.
- A generic DC taper. The linear fall from full power at 80% to one fifth at 100% is a modelling choice, not a manufacturer's curve. Vehicles differ substantially, and some begin tapering well below 80%.
- No temperature effects. A cold pack accepts less power and may divert energy to heating. A hot pack after a long fast run can also be limited. Neither appears here.
- No preconditioning energy. Energy spent warming or cooling the pack during a session is drawn from the supply but never raises state of charge.
- A single power figure for the station. Shared DC cabinets split power between stalls, so a 350 kW cabinet with two cars connected may deliver far less to each.
- Usable capacity, not gross. Enter the usable figure. Using gross pack size overstates the energy needed and therefore the time, by the size of the reserved buffer.
Choosing between Level 1, Level 2 and DC
The three levels solve different problems and the right answer is usually a combination. Level 1 — a standard household outlet at 120 V and 12 A — adds roughly 4 to 5 miles of range per hour. That sounds useless until you notice it is around 50 miles over a 12-hour overnight period, which covers the majority of daily driving for many households with no installation cost at all.
Level 2 at 240 V is the practical home standard. A 40 A circuit delivering 7.68 kW adds around 25 miles per hour of charging for a typical car, filling any realistic overnight need. Going beyond that is worth paying for only if your onboard charger can use it and you routinely need to refill a large pack in a short window.
DC fast charging is a travel tool rather than a daily one. It is the most expensive energy you can buy, it is the hardest on the pack when used constantly, and it delivers its value in a narrow band of state of charge. The discipline of arriving low and leaving at 80% is what makes an electric road trip work, because that is where the power curve is flat and high.
One planning subtlety worth knowing: because charging power is roughly constant below 80% but range added per kilowatt-hour depends on how efficiently you drive, the miles-per-hour figure is not a property of the charger alone. A vehicle doing 4.5 mi/kWh gains range 29% faster than one doing 3.5 mi/kWh on exactly the same charger. Efficiency buys charging speed as well as running cost.
Finally, consider the whole ownership picture rather than the charging line alone. Charging hardware is a one-off cost, electricity is a modest recurring one, and depreciation is usually the largest number of all — size it with the vehicle depreciation calculator, and compare per-mile running costs against a combustion vehicle with the fuel economy (MPG) calculator.