About EV Charging Calculator (Time, Cost & Breaker Size)
The EV charging calculator answers the three questions every new EV owner and every electrician planning a charger circuit asks. How long will a charge take? Enter the battery capacity, the current and target state of charge, and the charger power — a Level 1 cord at 1.4 kW or a Level 2 unit from 3.3 to 19.2 kW — and the tool computes the energy added and the hours and minutes on the plug, using a representative 90% onboard-charger efficiency (real chargers run roughly 85–95%).
What will it cost? The wall meter pays for the losses too, so the tool divides the added energy by the charger efficiency to get the wall kWh, multiplies by your electricity rate, and — given your vehicle efficiency in mi/kWh — reports the cost per mile next to what the same mile costs in a gas car. Which breaker does the circuit need? EV charging is a continuous load, so NEC 625.41 requires the overcurrent device to be at least 125% of the maximum EVSE current; the tool matches that against the NEC 240.6(A) standard breaker ladder and also inverts the rule to show the largest EVSE a breaker you already have can legally serve.
How It Works
- Pick a mode: charge time, charging cost, or circuit / breaker size.
- Time: enter the usable battery capacity (kWh), the current and target charge percentages, and the charger power — choose a preset (L1 1.4 kW, L2 3.3 / 7.2 / 9.6 / 11.5 / 19.2 kW) or type a custom AC value up to 22 kW. The added kWh is battery x SoC span; hours = added kWh / (charger kW x 0.90).
- Cost: the same charge inputs plus your electricity rate ($/kWh). The wall energy is added kWh / 0.90 because losses are metered but never reach the battery. Optionally add the vehicle efficiency (mi/kWh) for the per-mile cost, and a gas car (mpg and $/gal) for a side-by-side per-mile comparison.
- Circuit: enter the continuous EVSE output current in amps, or the charger kW to convert at 240 V (Level 2 only; amps = kW x 1000 / 240). The required rating is 1.25 x that current (NEC 625.41), matched to the first standard breaker size in NEC 240.6(A) — an exact fit takes that size, so a 32 A EVSE needs exactly a 40 A breaker.
- The circuit result also shows the inverse 80% rule — the largest continuous EVSE current the chosen breaker supports (0.8 x its rating) — and points to a full load calculation when the required rating exceeds 100 A.
Worked Example
A 75 kWh EV arrives home at 20% and should reach 80% overnight on a 7.2 kW (30 A) Level 2 charger. The battery needs 75 x (80 − 20)/100 = 45 kWh. At 90% charger efficiency the battery sees 6.48 kW, so the session takes 45 / 6.48 = 6.94 h — displayed as 6 h 57 min. The wall meter supplies 45 / 0.90 = 50 kWh; at $0.15/kWh the charge costs $7.50. At 3.5 mi/kWh the session adds 157.5 miles, $0.0476 per mile — a 30 mpg gas car at $3.50/gal pays $0.1167 per mile, so the EV saves about $0.069 every mile. The circuit for the 30 A EVSE must be sized at 1.25 x 30 = 37.5 A, and the first NEC 240.6(A) standard size at or above that is a 40 A breaker.
Formulas
- Energy added and charge time
E_added = C_batt x (SoC_target - SoC_current) / 100; t = E_added / (P_charger x eta)- Wall energy and session cost
E_wall = E_added / eta; cost = E_wall x rate- Per-mile cost and gas comparison
miles = E_added x eff; cost_mi = cost / miles; gas_mi = price_gal / mpg- Branch-circuit sizing (NEC 625.41)
I_required = 1.25 x I_EVSE; breaker = first NEC 240.6(A) size >= I_required- Inverse 80% rule
I_EVSE_max = 0.8 x breaker rating
Standards & References
- NEC 625.41 -- overcurrent protection for EVSE sized at 125% of maximum load (continuous duty)
- NEC 210.20(A) -- general rule: OCPD >= noncontinuous load + 125% of continuous load
- NEC 240.6(A) -- standard ampere ratings for fuses and inverse-time breakers
Frequently Asked Questions
Why does the breaker need to be 125% of the charger current?
EV charging is a continuous load — it runs at full current for three hours or more — and sustained current heats the breaker and the conductors around it. NEC 625.41 therefore requires the overcurrent device to be rated at least 125% of the maximum EVSE load (the same 125% continuous-load rule as the general branch-circuit requirement in 210.20(A)). Flipped around, a breaker may only carry a continuous load up to 80% of its rating: a 50 A breaker legally serves a 40 A EVSE, never a 48 A one.
Does this calculator apply to DC fast charging?
No — the circuit mode and the efficiency model are for AC charging only. A DC fast charger (CCS, NACS DC, CHAdeMO) bypasses the vehicle's onboard charger entirely and feeds the battery directly, so the 240 V amps conversion, the 90% onboard-charger efficiency, and residential branch-circuit sizing do not apply. DCFC speed is set by the station's power cabinet and the battery's charge curve, and it tapers heavily above roughly 80% state of charge.
Where does the 90% charger efficiency come from?
The onboard charger converts AC from the wall to DC for the battery and loses some energy as heat in the process; typical measured efficiencies run about 85–95% depending on the charger design, the charging power, and even the cable. This tool uses a fixed representative 90% in both the time and cost modes — the losses make a charge take about 11% longer and cost about 11% more than the battery-side energy alone would suggest, because the meter bills the wall side.
What wire gauge do I need for my EV charger circuit?
The breaker size from this tool sets the minimum, but the conductor must have an ampacity that matches or exceeds it after ambient-temperature and bundling corrections, and long runs to a detached garage may need to be upsized for voltage drop. Use the conductor-ampacity calculator for the NEC 310.16 gauge selection and the voltage-drop calculator for the run length — typical pairings are 8 AWG copper on a 40 A breaker and 6 AWG copper on a 50 A breaker, but verify for your conditions.
How can I charge for less money?
Most of the levers are on the rate side. Many utilities offer time-of-use (TOU) or dedicated EV rates where overnight electricity costs a fraction of the peak price — scheduling the charge window in the car or the EVSE to start off-peak often halves the cost computed here. Charging at home is also almost always cheaper per kWh than public Level 2 or DC fast charging. This calculator prices the whole session at one flat rate, so run it once per rate tier if your plan has several.
Is a bigger home charger always better?
Only up to what the car accepts and the panel supports. The vehicle's onboard charger caps AC power — many EVs take 7.2–11.5 kW, so a 19.2 kW (80 A) EVSE charges them no faster. Bigger circuits also escalate cost quickly: an 80 A continuous load needs a 100 A breaker and heavy conductors, and may not fit an existing service without a load calculation (see the electrical-load tool). For overnight charging, a 7.2–9.6 kW unit refills most EVs from 20% to 80% in well under 8 hours.