Energy
Estimate the charging power and circuit size needed for your daily driving.
Enter your numbers to see the answer.
Home EV charger sizing guide
Most home charging happens overnight, so the question is not how fast a charger can go but how much power it needs to refill a day’s driving in the hours the car is parked. Enter your daily miles, efficiency in miles per kWh, the hours the car is plugged in each night, and an electrical headroom percentage. The calculator finds the energy a day costs, divides it by the charging window to get the average power needed, then divides by the headroom factor so the charger is not sized to run at its limit. A second row multiplies the result by 1.25, the continuous-load factor electricians use when sizing the circuit. For a US driver at 40 miles a day the answer is small—well under 2 kW—which is why a 120 V Level 1 cord (about 1.4 kW) can technically keep up, though it leaves no slack for a long day or a cold snap. A 240 V Level 2 unit at 7–11 kW refills the same 40 miles in 1–2 hours and copes with 150-mile days.
Daily energy (kWh) = daily miles ÷ efficiency (mi/kWh). Average power (kW) = daily energy ÷ charging hours. Minimum charger (kW) = average power ÷ (headroom % ÷ 100). Circuit sizing (kW) = minimum charger × 1.25.
Worked example: 40 miles a day at 3.5 mi/kWh, 10 hours plugged in, and 80% headroom. Daily energy = 40 ÷ 3.5 = 11.4 kWh. Average power = 11.43 ÷ 10 = 1.14 kW. Minimum charger = 1.143 ÷ 0.80 = 1.4 kW, the figure the calculator displays. Circuit headroom = 1.43 × 1.25 = 1.8 kW equivalent. A 1.4 kW Level 1 cord (120 V, 12 A) is exactly at the limit; allowing 12% charging losses the real average draw is 11.43 × 1.12 ÷ 10 = 1.28 kW, so it just about works. Change the inputs to 100 miles a day with 8 hours and the minimum jumps to 100 ÷ 3.5 ÷ 8 ÷ 0.8 = 4.5 kW, squarely Level 2 territory.
Daily miles has a kilometre option (converted at 1.609344 km per mile). Efficiency is miles per kWh: convert kWh/100 km with 62.137 ÷ (kWh/100 km), so 20 kWh/100 km is 3.1 mi/kWh, and Wh/mi with 1,000 ÷ Wh/mi. Charging hours accepts minutes or days from its unit menu, but hours is the natural choice. Headroom is the percentage of charger capacity you are willing to use on an average night; 80% is conservative, 100% assumes the charger runs flat out.
This is a modelled estimate, not a meter reading. Real consumption moves with weather, occupancy, appliance age, standing charges, and tariff structure, and equipment rarely runs at its nameplate rating. Use the figure to compare options against each other, then confirm against your own bill and the manufacturer's specification before committing to a purchase. On this page the figure rests entirely on miles driven per day, ev efficiency, available charging hours, electrical headroom, market / jurisdiction, effective date, data source, update owner and review date, so start there if the home ev charger sizing returns something you did not expect.
Good to know: sizing to the average day is a trap if your driving is lumpy—size to the longest day you need to recover in one night, or accept a Level 2 charger that finishes early most nights. Charging efficiency is not included: Level 1 loses 15–20% between wall and battery and Level 2 about 10%, so real energy drawn is higher than the daily kWh shown. The 1.25 circuit row is a minimum: a 7.7 kW (32 A) charger sits on a 40 A circuit and an 11.5 kW (48 A) charger on 60 A. Cold weather can add 30% to daily energy, so a charger sized with zero slack falls behind in January.
Energy results depend on tariffs, equipment behavior, region, weather, and installation assumptions. Verify rates and electrical decisions with the utility, manufacturer, or qualified professional.
Sources
How it works
Estimate the charging power and circuit size needed for your daily driving. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With Miles driven per day = 40 mi · EV efficiency = 3.5 mi / kWh · Available charging hours = 10 hours · Electrical headroom = 80 % → 1.4 kW (minimum charger output for your routine). Change an input above and this example updates with your numbers.
Common questions
For 40 miles a day at 3.5 mi/kWh with 10 hours available, the minimum is 1.4 kW—Level 1 is enough on paper. Most households choose a 7–11 kW Level 2 unit anyway, which restores 40 miles in about 1.5 hours, absorbs 100–150 mile days, and allows charging inside a short off-peak window.
At about 1.4 kW (120 V, 12 A) a Level 1 cord adds roughly 4–5 miles of range per hour, so 10 hours gives 40–50 miles minus 15–20% charging losses. If you drive under 35 miles a day and park 10+ hours, it works; beyond that, or in winter, you will slowly fall behind.
Divide charger kW by 0.24 for the current at 240 V, then multiply by 1.25 for the circuit: a 7.2 kW charger draws 30 A and needs a 40 A circuit; 9.6 kW draws 40 A and needs 50 A; 11.5 kW draws 48 A and needs 60 A. The calculator’s circuit row gives the kW equivalent of that 125% rule.
Daily miles divided by miles per kWh: 40 miles at 3.5 mi/kWh is 11.4 kWh, roughly 40% of a typical US household’s 30 kWh daily use. Add 10–15% for charging losses, giving about 12.5–13 kWh drawn from the meter for that 40 miles.
Enter 4 hours as the window. For 40 miles a day the minimum rises to 11.43 ÷ 4 ÷ 0.8 = 3.6 kW, still modest; for 80 miles it is 7.1 kW, which is why a 7.2–7.7 kW (32 A) charger is the sweet spot for time-of-use tariffs with a short cheap window.