How it works
Four steps produce the result. Energy to add is battery capacity × (target percent − current percent) ÷ 100. Grid energy is that figure divided by (1 − losses ÷ 100), because charging always burns some energy as heat in AC-to-DC conversion and cell balancing. Cost is grid energy × your price per kWh, so you pay for the losses. Charging time is the energy divided by the charger's kW rating. If you supply a miles-per-kWh figure, it also derives cost per 100 miles.
Charger power sets the time and nothing else: Level 1 (120V, 1.4 kW), Level 2 Home (7.4 kW), Level 2 Public (11 kW), DC Fast (50 kW), DC Ultra-Fast (150 kW), or Custom for anything else. Losses default to 12 percent, with the field's own guidance putting typical AC charging at 10 to 15. On the defaults — a 60 kWh pack from 20 to 80 percent at $0.16/kWh on Level 2 Home — that is 36 kWh added, 40.91 kWh drawn, $6.55 and 4h 52m.
The time figure assumes constant power for the whole session, which is where it drifts furthest from reality. Real DC fast charging tapers sharply as the pack fills, so a 20 to 80 percent stop on a 150 kW charger takes noticeably longer than energy divided by 150 suggests. The time is also derived from the energy reaching the battery rather than the higher grid draw. Idle fees, per-session fees, network subscriptions, time-of-use tariffs and cold-weather losses are all outside the model.
Frequently asked questions
What does it cost to charge a 60 kWh EV from 20 to 80 percent? +
At $0.16 per kWh with 12 percent losses, $6.55. The pack takes 36 kWh but the meter sees 40.91 kWh. At 3.5 miles per kWh that works out to about $5.19 per 100 miles, which the tool shows as a separate row.
How long does charging take on a normal 120V household outlet? +
Pick Level 1 (120V, 1.4 kW). The same 36 kWh top-up that takes under five hours on a 7.4 kW home charger stretches to roughly 25 hours and 43 minutes, which is why Level 1 only really works for short daily commutes and overnight trickle charging.
Why is the grid energy higher than the energy added to the battery? +
Because charging is not perfectly efficient. The onboard charger converts AC to DC and the battery management system balances cells, both of which shed heat. The default 12 percent loss means a 36 kWh top-up pulls 40.91 kWh from the wall, and your meter bills all of it.
What charging loss percentage should I actually enter? +
Twelve percent is the default and sits in the middle of the 10 to 15 percent range typical of home and public Level 2 AC charging. DC fast charging runs higher, since the station handles conversion and the pack's thermal management works harder at high current.
How do I get cost per mile out of this? +
Fill in the optional vehicle efficiency field in miles per kWh; 3.5 is a reasonable default for a mid-size EV. The tool then divides the total cost by the miles that charge adds and reports cost per 100 miles. Leave it blank and that row disappears.
Charger power sets the time, not the cost
The charger options — 1.4 kW from a domestic socket, 7.4 kW single-phase home charging, 11 kW three-phase, 50 kW rapid — determine how long a charge takes and barely affect the energy required. What changes the cost per kWh is the tariff, and rapid charging is usually on a much more expensive one.
The 20% to 80% default range is deliberate: charging speed falls sharply above roughly 80% state of charge as the pack taper protects the cells, so the last 20% takes disproportionately long on a rapid charger and is usually not worth waiting for.
Losses, and why the meter reads more than the battery gains
Energy is lost between the wall and the battery in the onboard charger and to thermal management — commonly 10–20% on AC charging, worse in cold weather when the pack must be warmed before it will accept charge. The kWh you are billed for is therefore more than the kWh you can drive on.
That gap is larger on slow AC charging than on rapid DC, because the fixed overhead of running the vehicle's systems is spread over a longer session.
Tariff structure is the other thing a single rate cannot capture. On time-of-use pricing the same charge can differ threefold depending on when it happens, which is the single biggest lever on running cost and the reason scheduled charging exists.
