Key facts
- Average charging power equals energy added divided by elapsed hours for the same interval and measurement boundary.
- A graph of kW against state of charge does not have energy as the simple area under the curve; a kW-versus-time graph does.
- In the synthetic example, 10–60% adds 40 kWh in 18.4 minutes, averaging about 130.4 kW.
- The same example adds 16 kWh from 60–80% in 16 minutes, averaging 60 kW.
- The cheapest or quickest departure point cannot be determined from the curve alone; route needs, tariffs and charging availability matter.
A charging curve is useful when it tells you how much energy you can add during the part of a stop you actually need. The highest point on the line cannot tell you the average speed from 10% to 60%, or how long another 20 percentage points will take.
The U.S. EPA’s charging guidance explains that DC charging generally slows as the battery approaches full and that temperature, battery characteristics and starting charge affect the session. There is no single curve that represents every vehicle and condition.
First check the axes and what was measured
The vertical axis often shows power in kilowatts, or kW. The horizontal axis may show state of charge, abbreviated SOC, or elapsed time. These are different graphs:
- Power against time: energy is the accumulated power over time. A constant 60 kW for 10 minutes transfers 10 kWh at the measurement point.
- Power against SOC: the horizontal spacing represents battery percentage, not minutes. The area drawn beneath this curve is not directly an energy total.
Check whether the power is measured at the charging station or represents net energy reaching the battery, and whether the timer includes connection and initial ramp-up. Compare curves only with their context: vehicle variant, software, starting SOC, temperature, preconditioning and charger limits. Our peak-power and charging-limit guide covers why a high-rated charger may not produce its headline output.
A synthetic charging curve with explicit assumptions
The following is an invented teaching example, not a test of a real EV. Assume 80 kWh of usable battery capacity, displayed SOC exactly proportional to stored usable energy, and constant net battery-charging power within each interval. We omit connection time, ramp-up, thermal loads and losses. Real vehicle and station readings do not necessarily follow these assumptions.
| SOC interval | Energy added | Assumed power | Calculated time |
|---|---|---|---|
| 10–30% | 80 × 0.20 = 16 kWh | 150 kW | 16 ÷ 150 × 60 = 6.4 minutes |
| 30–60% | 80 × 0.30 = 24 kWh | 120 kW | 24 ÷ 120 × 60 = 12 minutes |
| 60–80% | 80 × 0.20 = 16 kWh | 60 kW | 16 ÷ 60 × 60 = 16 minutes |
The calculation for each constant-power interval is minutes = energy in kWh ÷ power in kW × 60. With a measured, varying curve, use actual interval energy and timestamps where available instead of pretending that its peak lasted for the whole interval.
Compare 10–60% with the later 60–80% interval
The first two rows add 16 + 24 = 40 kWh in 18.4 minutes. Their combined average is:
40 ÷ (18.4 ÷ 60) ≈ 130.4 kW.
The final row adds 16 kWh in 16 minutes, averaging 60 kW. It delivers only 40% as much energy as the earlier 10–60% interval, while taking about 87% as long.
Across the whole 10–80% session, the average is 56 ÷ (34.4 ÷ 60) ≈ 97.7 kW. That is below the 150 kW peak. It is also different from the simple average of the three listed powers, which would be 110 kW. The rows last different amounts of time, so an unweighted average of their power numbers gives the wrong session result.
Choose departure charge from the next leg
Keep the same hypothetical 80 kWh usable pack. Suppose the next leg is 250 km and you budget 18 kWh/100 km, plus a chosen arrival reserve of 8 kWh:
Required departure energy = 250 × 18 ÷ 100 + 8 = 53 kWh.
At 60%, the simplified model has 48 kWh, which is 5 kWh short. Its mathematical departure target is 53 ÷ 80 × 100 = 66.25%. At the assumed 60 kW in that interval, adding the missing 5 kWh takes 5 minutes.
This illustrates why the choice need not be “leave at 60%” or “always stay to 80%.” In a real trip, do not treat 66.25% as a precise operational recommendation: consumption, usable capacity, SOC estimation and charger availability are uncertain. Use the vehicle’s route estimate, a suitable additional margin and a reachable backup.
The U.S. Department of Energy’s winter guidance similarly relates charging targets to conditions and distance needs. A slower part of the curve can still be necessary when the next dependable charger is farther away. Our charging-stop and detour comparison adds driving and stop overhead to that decision.
Translate the extra interval into cost carefully
Use the station’s billed energy for a per-kWh price. Do not automatically multiply a battery-capacity estimate by the tariff. The charging-losses guide explains why station or wall energy can differ from the increase stored in the battery.
For a separate, fictional price example, suppose a receipt reports 18 kWh billed for an interval and the energy rate is 0.50 currency units/kWh. That energy component costs 18 × 0.50 = 9 units. This receipt is independent of the synthetic curve above; it does not establish a battery-to-meter loss percentage.
If a different fictional tariff bills 0.30 units per charging minute, a 16-minute interval costs 4.80 units before other charges. These are arithmetic examples, not current tariffs or a recommendation between networks. Include session, parking, congestion or idle charges where they actually apply. FuelEconomy.gov describes the different energy-, time- and session-based pricing structures.
Build a useful session record
While parked, record the start SOC and time, the time at your chosen intermediate SOC, the end SOC, receipt energy and any charging interruptions. Note preconditioning and weather. Avoid comparing a warm arrival in one test with a cold start in another and attributing the whole difference to the charger.
If you only know whole-session billed energy and total time, you can calculate the whole-session average at that billing boundary. You cannot reconstruct a trustworthy 10–60% average without energy or sufficiently detailed measurements for that interval. Our battery-preconditioning guide covers another important comparison variable.
Use the gross-versus-usable capacity guide to label the energy assumption, then compare time-based prices with the per-minute charging tariff method.
Frequently asked questions
Does a flat charging curve always mean a better car?
No. The height, duration, usable energy added and your required SOC window all matter. A flatter curve at low power may still add energy more slowly.
Should I always stop DC charging at 80%?
No fixed departure percentage solves every route. Use enough energy for the next dependable stop and an appropriate reserve, while following the vehicle’s charging instructions.
Can I compare averages calculated from battery SOC and station kWh?
Label them separately. The time window and measurement boundary must match for a valid comparison; estimated battery energy is not automatically the station’s delivered or billed energy.
Source review: 12 November 2026. Product and authority examples are identified by market; original numerical examples are illustrative calculations, not hands-on test results.
