EV Charging Calculator
Electric vehicle charging brings its own specific version of the kW-to-kWh question: chargers are rated in kW (their power output), but what you actually care about is how many kWh will end up in the battery, how long that takes, and roughly what it'll cost. This calculator handles all three, accounting for the real-world charging losses that a simple kW × time calculation misses.
The Formula (With Efficiency Accounted For)
Onboard AC-to-DC conversion (the process that happens inside the car for Level 1 and Level 2 charging) isn't lossless — typical efficiency runs 88-92%, meaning 8-12% of the energy from the wall never makes it into the battery, dissipated mostly as heat.
DC fast charging skips the car's onboard converter (the conversion happens in the charging station instead), so efficiency is usually a bit higher, often 92-95%.
Worked Example — Home Level 2 Charging
A 7.2 kW home charger, 90% efficiency, charging for 6 hours overnight:
For an EV with roughly 3-4 miles of range per kWh, that's a meaningful daily commute's worth of range replenished overnight.
Worked Example — DC Fast Charging
A 150 kW DC fast charger, 93% efficiency, for 25 minutes (0.417 h):
Note that real-world fast charging rarely sustains the peak rated kW for the whole session, since charging speed tapers as the battery fills (typically slowing significantly past 80% state of charge), so this is a best-case estimate for the early part of a charging session.
Estimating Cost
Once you know the kWh delivered, multiply by your electricity rate (for home charging) or the charging network's per-kWh rate (for public charging) to get a real cost figure:
A 38.88 kWh home charge at $0.16/kWh costs roughly $6.22 — worth comparing against a public DC fast charging session, which typically charges a premium rate per kWh for the speed and convenience.
Common EV Charger Power Levels
| Charger Type | Typical Power | Approx. Range Added per Hour* |
|---|---|---|
| Level 1 (standard outlet) | 1.4–1.9 kW | 3–5 miles |
| Level 2 (home/public) | 3.3–11.5 kW | 12–40 miles |
| DC Fast Charging | 50–350 kW | 100+ miles in minutes (tapers with battery %) |
*Range estimates vary significantly by vehicle efficiency — use your specific EV's rated miles-per-kWh for an accurate figure.
Frequently Asked
Why did my charging session use more kWh from my meter than what shows on my car's dashboard?
That gap is exactly the conversion loss described above — your home meter measures energy drawn from the grid, while your car's display typically shows energy actually stored in the battery, which is lower due to the ~8-12% conversion loss.
Does charging speed stay constant the whole session?
No — most EVs (especially on DC fast charging) taper their charging rate as the battery fills, often slowing noticeably past 80% state of charge to protect battery health. A charger's rated kW is closer to accurate in the 10-70% charge range.
How do I estimate monthly EV charging cost?
Multiply your typical daily kWh added (using this calculator) by your electricity rate, then by 30 for a monthly estimate — or track it directly if your charger/utility provides usage data.
For sizing a home solar array to offset EV charging costs, see the solar kW to kWh calculator.
Founder & Lead Renewable Energy Specialist
Abdul Rehman is a Renewable Energy Consultant and Solar Systems Specialist. He built kwtokwh.com to provide transparent, accessible calculation tools that help homeowners, electricians, and facility engineers accurately convert electrical power (kW) into billed energy consumption (kWh) calibrated against NIST and IEC metrology standards.