How this instrument works
The energy your electricity meter registers for an EV charging session is always somewhat more than the energy that actually ends up stored in the battery, because AC charging involves converting alternating current from the grid into direct current the battery can store, and that conversion loses some energy as heat. This calculator separates those two figures: energy added (what the battery actually gains) and energy drawn from the grid (what you're actually billed for), which is always the larger number.
Energy added is simple: battery capacity multiplied by the percentage-point gain, from your starting charge to your target charge. Energy from the grid divides that by charging efficiency — a lower efficiency means more energy is lost in conversion, so more has to be drawn from the grid to deliver the same amount into the battery. Cost follows directly from the grid-energy figure, since that's the number your utility actually charges for.
Charging efficiency for home Level 2 AC charging commonly falls in roughly the 84-93% range according to published studies (a range reported by the U.S. Department of Energy's fueleconomy.gov, citing Idaho National Laboratory testing), rather than one universal fixed number — it varies by charger hardware, cable length, ambient temperature, and vehicle. This calculator defaults to 90%, a commonly cited typical figure, but leaves it fully adjustable rather than asserting one fixed proprietary number for every setup.
- Enter your battery's total capacity in kWh — check your vehicle's specification sheet.
- Enter your starting charge percentage and target charge percentage.
- Enter your electricity rate per kWh from a recent utility bill.
- Enter charging efficiency — 90% is a reasonable default for home Level 2 AC charging, adjustable if you have a more specific figure for your setup.
- Read Charging cost — what that session actually costs, based on energy drawn from the grid, not just energy added to the battery.
Worked example — a 75 kWh battery charged 20% to 80%
A 75 kWh battery (a typical mid-size EV pack) charged from 20% to 80% — a 60-percentage-point gain: energy added = 75 × 0.60 = 45 kWh exactly. At 90% charging efficiency, energy drawn from the grid = 45 ÷ 0.90 = 50 kWh — 5 kWh more than actually lands in the battery, lost to conversion.
At $0.15 per kWh, the cost is 50 × $0.15 = $7.50 for that charging session. Notice the bill is based on the 50 kWh grid figure, not the 45 kWh that actually charges the battery — the 10% efficiency loss adds a real, if modest, cost on top of the energy that's actually stored and usable.
Questions
Why is my electricity bill higher than battery capacity × electricity rate would suggest?
Because AC charging isn't 100% efficient — converting grid alternating current into the direct current a battery stores loses some energy as heat, so the energy your meter registers (and you're billed for) is always somewhat more than the energy that actually lands in the battery. A charging efficiency of 90%, for example, means you're billed for about 11% more energy than the battery actually gains.
What charging efficiency should I actually use?
Home Level 2 AC charging efficiency commonly falls in roughly the 84-93% range depending on charger hardware, cable, and conditions, based on published testing (U.S. Department of Energy / Idaho National Laboratory data). This calculator defaults to a commonly cited 90%, but if you have a smart charger that reports its own efficiency, or you can compare your utility meter reading against your vehicle's reported charge added, use that more specific figure instead.
Does DC fast charging have the same efficiency as home AC charging?
Not necessarily — this calculator is scoped to home Level 2 AC charging, the most common home-charging setup. DC fast charging converts AC to DC at the charging station rather than in the vehicle, which can shift the efficiency profile somewhat; if you're specifically costing out fast-charging sessions, treat the 84-93% home-charging range as a rough reference point rather than a confirmed match.
Why does starting and ending at the same percentage give $0 cost?
Because no charge was actually added — if starting and target percentage are equal, energy added is 0 kWh, so energy drawn from the grid and cost are both 0 as well. It's the trivial identity case: a charging session that adds no energy costs nothing, exactly as the formula predicts.