Battery energy, charger losses, and session cost
Battery energy is the amount stored; charger energy includes the assumed losses and determines the energy-based cost. Time uses your average charger-side input power. In time-window mode, charge stops at 100% and unused hours are shown. Total cost adds the entered per-minute and flat-session fees for positive charging.
To compare charging energy with a solar production estimate, use the Solar Energy Calculator for the same period.
The formula
A state-of-charge change describes a fraction of usable battery capacity. Dividing by wall-to-battery efficiency adds conversion and charging losses. Since power is entered at the charger side, time must use charger energy rather than battery energy. Minutes are actual modelled charging hours multiplied by 60, even if the available window is longer. A flat fee is applied only if positive energy is added. Cents/paise prices divide by 100; changing currency changes labels without an exchange-rate conversion.
Worked example
A 60 kWh usable battery moving from 20% to 80% stores 36 kWh. At 90% efficiency it draws 40 kWh. With an average input of 7.2 kW, that takes about 5.556 hours and costs 8 at a tariff of 0.20 per kWh. With only 2 hours at the same average power, it stores 12.96 kWh and ends at 41.6%; charger energy is 14.4 kWh and energy cost is 2.88 USD. In the original 20%-to-80% plan, a 0.02 USD/minute fee adds 6.667 USD and a 1 USD session fee gives a total of about 15.667 USD.
How to use this calculator
- Choose Reach a target charge or Charge for available time.
- Enter usable battery capacity, starting charge and the target percentage or available hours.
- Enter session-average charger-side power and wall-to-battery efficiency. Peak charger ratings may overstate the average.
- Enter your energy tariff, price unit and currency. Open advanced settings for any additional per-minute or flat-session fee.
- Review live energy, duration, ending charge and total cost. The comparison table assumes the same average power for its charge targets.
Rated power is not a charging curve
Vehicle limits, charger limits, battery temperature, and charge level affect power. Fast charging often slows as the battery fills. Entering a charger’s peak rating as the average can therefore underestimate duration. The target table deliberately holds your average-power assumption constant; it is not a vehicle-specific prediction of tapering.
Energy tariffs and other session fees
Enter the tariff for charger-side kWh. Additional per-minute prices use actual modelled charging time, and a flat session fee is added when positive charging occurs. Zero-charge plans return zero fees here. Idle/overstay charges, parking, subscriptions and time-varying tariffs are not modelled. Align the provider’s metering basis with the power and efficiency assumptions before comparing with a bill.
Assumptions & limitations
What this calculation assumes
- Usable capacity, average input power, and efficiency remain fixed for the estimate.
What to keep in mind
- No live vehicle charging curve, battery-temperature model, provider pricing feed, idle/parking fees, subscriptions or time-varying tariffs.
Common questions
Why does charging draw more than the battery gains?
Some energy is lost during conversion and battery charging, or used by vehicle systems. The efficiency field represents the combined effect assumed here.
Can the target equal the starting charge?
Yes. With no requested charge increase, this model returns zero energy, charging time and fees, including the optional flat session fee. A provider may use different minimum-charge rules.
What if my available time is longer than the time to 100%?
The estimate stops at 100%. Charging time and per-minute fees cover only the time needed to reach that level; the rest appears as unused window time. It does not predict real full-charge tapering.
Sources & further reading
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