Understanding Amps, Volts, and Charging Speed
TL;DR
Approximate single-phase AC charging power = Volts × Amps ÷ 1,000. A standard 120V/15A household circuit delivers up to 12A continuous (~1.4 kW); a 240V/50A circuit with a 32A portable EVSE delivers 32A continuous (~7.7 kW). The actual charging rate is whichever is lowest among the branch-circuit continuous allowance, the EVSE's rated maximum, the vehicle's onboard AC charger input, and the battery's accept rate.
Volts, amps, kilowatts, and kilowatt-hours — the four terms
Before the examples, four definitions you'll see repeatedly:
| Term | Meaning |
|---|---|
| Volts (V) | Electrical potential difference. The "push" behind the current. |
| Amps (A) | Electrical current. The rate at which charge flows. |
| Kilowatts (kW) | Charging power. 1 kW = 1,000 watts. Higher kW = faster charging. |
| Kilowatt-hours (kWh) | Amount of energy. 1 kWh = charging at 1 kW for 1 hour. Battery capacity is measured in kWh. |
The equation at the center of AC charging
Watts ≈ Volts × Amps for single-phase AC input power. This approximation estimates kW well enough to compare charging setups.
Two worked examples:
- Standard 120V/15A circuit, EVSE operating at 12A continuous: 120 × 12 = 1,440 watts (~1.4 kW).
- 240V NEMA 14-50 outlet on a 50A circuit, 32A portable EVSE: 240 × 32 = 7,680 watts (~7.7 kW). The same circuit paired with a 40A-rated EVSE would deliver 240 × 40 = 9,600 watts (~9.6 kW). The EVSE's rating is the ceiling in each case, not the outlet.
DC fast charging is different: it bypasses the car's onboard AC-to-DC converter. More on that below.
Why 120V is "Level 1" and 240V is "Level 2"
Charging levels are defined by the input voltage, per the SAE J1772 standard. Level 1 is 120V AC (standard North American household); Level 2 is 240V AC (dedicated residential circuit) or 208V AC (common at commercial properties on three-phase service, per AFDC). A 240V EVSE operating at 16A is still Level 2 — same category as a 240V/48A wall unit, just slower.
- Level 1 (120V AC): plugs into a standard NEMA 5-15 or 5-20. On a dedicated 20A circuit, an appropriately-rated EVSE can deliver up to 16A continuous.
- Level 2 (240V or 208V AC): plugs into a 240V outlet (NEMA 14-50, 14-30, 6-50, 6-20) or a hardwired circuit; commercial 208V installations are also Level 2. Deliverable power ranges from ~3.8 kW (16A on 240V) to about 19.2 kW (80A on 240V), depending on the EVSE and the vehicle.
- DC fast charging: delivers direct current straight to the battery through the J1772 CCS, CHAdeMO, or NACS DC connector family. Station ratings vary widely; see the DC section below.
The 80 percent continuous rule and circuit sizing
EV charging is treated as a continuous load for circuit sizing under the U.S. National Electrical Code. The result is the practical rule that EV charging is limited to 80% of the circuit's breaker rating: a 50A circuit allows 40A continuous, a 15A circuit allows 12A continuous. Branch-circuit sizing combines provisions from Articles 210.19 (branch-circuit ampacity) and 625.41/625.42 (EV supply equipment sizing and maximum current).
In practice: when you see an EVSE rated at "32A continuous" installed on a 50A circuit, the 32A is the equipment's own rating and the 50A circuit is sized larger than strictly required. The 80% rule is a sizing rule for the installation — the EVSE can be any current rating up to that limit.
Common outlets and their maximum nominal AC charging power
The "maximum nominal AC power" column assumes an EVSE rated to use the full continuous current the circuit allows. If the EVSE is rated lower (e.g. a 32A portable EVSE on a 50A circuit), the EVSE's rating is the ceiling.
| Outlet / circuit | Voltage | Breaker | Continuous current (80%) | Maximum nominal AC power under these assumptions |
|---|---|---|---|---|
| NEMA 5-15 (standard household) | 120V | 15A | 12A | ~1.4 kW |
| NEMA 5-20 (dedicated 20A branch) | 120V | 20A | 16A | ~1.9 kW |
| NEMA 14-30 (newer dryer) | 240V | 30A | 24A | ~5.8 kW |
| NEMA 14-50 (RV / welder family) with 40A EVSE | 240V | 50A | 40A | ~9.6 kW |
| NEMA 14-50 with 32A portable EVSE (common) | 240V | 50A | 32A (EVSE-limited) | ~7.7 kW |
| Hardwired 60A circuit with 48A EVSE | 240V | 60A | 48A | ~11.5 kW |
Tesla's own documentation illustrates this distinction for their 32A Mobile Connector: on a 50A circuit, the Mobile Connector still delivers 32A, not 40A — the EVSE's internal current limit applies even when the circuit could support more (Tesla Mobile Connector support page). Any portable EVSE works the same way: verify the device's rated maximum before assuming a circuit upgrade will increase your charging rate.
Your car has its own limit: the onboard charger
Household electricity is alternating current (AC). An EV battery stores energy chemically and charges using direct current (DC). On Level 1 and Level 2 charging that conversion happens inside the car, in a component called the onboard charger (OBC). The OBC has a maximum AC input rating. If your circuit and EVSE can together deliver 11.5 kW but your OBC input caps at 7.7 kW, you charge at 7.7 kW — the lowest-rated link in the chain always wins.
Three examples with direct manufacturer sources (the Tesla, Ford, and Chevy pages are the easiest to verify; AC input ratings for other vehicles should be read from your vehicle's own documentation):
| EV (model year) | AC input max | Source |
|---|---|---|
| 2024 Tesla Model 3 Long Range | 48A / 11.5 kW on 240V (Model 3 RWD is rated at 32A — different trims within one model differ) | Tesla onboard-charger support page |
| 2024 Ford Mustang Mach-E (all U.S. trims) | 48A AC charging capability on 240V | 2024 Mustang Mach-E Order Guide (PDF) |
| 2023 Chevy Bolt EUV | 11.5 kW with separate 48A Level 2 EVSE. The bundled Chevrolet dual-level charge cord supplies up to 32A at 240V — reaching 48A requires separate equipment. | 2023 Bolt EV/EUV Get-To-Know guide, p. 17 (PDF) |
Other EVs: Hyundai, Kia, Rivian, VW, Ford Lightning, Nissan Leaf, BMW, Porsche, and others each publish their own AC input rating. Read the number from your vehicle's owner's manual or window sticker; don't infer it from a brand name or a kW figure quoted elsewhere.
Takeaway for the 32A-on-14-50 case: a 32A portable EVSE plugged into a 50A circuit delivers 32A because the EVSE's own rating limits the current. The 14-50 outlet can supply 40A continuous, and the vehicle may accept more — but the slowest link (the 32A EVSE) sets the actual rate at ~7.7 kW. Upgrading only the EVSE to 40A would deliver ~9.6 kW on the same circuit; upgrading only the circuit changes nothing if the EVSE and vehicle still cap below.
Why DC fast charging is different
DC fast charging bypasses the onboard AC-to-DC converter. The DC fast station contains its own high-power rectifier and feeds direct current straight into the battery through the vehicle's DC charging contactors. The onboard AC charger isn't in the loop.
That's why DC fast rates can reach well above typical Level 2 rates even though a vehicle's AC onboard charger is capped at 11.5 kW.
The actual delivered rate at a DC fast station is a function of the station's rated maximum, the vehicle's rated DC acceptance, and the battery's current state-of-charge and temperature. AFDC's station overview identifies these equipment, vehicle, and state-of-charge factors as the primary determinants. Published 0-80% times (which some manufacturers quote) depend on a specific vehicle at a specific station under specific conditions.
Worked example: how long to add 100 miles at AC
Methodology note: charging time depends on which energy figure you start from. EPA's labeled "kWh/100mi" figure measures energy drawn from the wall (already includes AC-to-DC conversion losses on L1/L2). A "miles per kWh" figure typically measures battery-side energy — so to get outlet-drawn energy from a battery-side figure, divide by charging efficiency. The example below uses a battery-side assumption of 3.5 mi/kWh and applies ~90% efficiency as an illustrative figure (actual efficiency varies by vehicle, charger, and conditions).
- Battery energy for 100 miles at 3.5 mi/kWh: 100 ÷ 3.5 ≈ 28.6 kWh
- Outlet-drawn energy at ~90% charging efficiency: 28.6 ÷ 0.9 ≈ 31.7 kWh
Time to add 100 miles using the outlet-drawn figure:
- Level 1 (1.4 kW): 31.7 ÷ 1.4 ≈ 22-23 hours. An overnight 10-hour charge adds roughly 45 miles under these assumptions.
- Level 2, 16A/3.8 kW: 31.7 ÷ 3.8 ≈ 8.3 hours.
- Level 2, 32A/7.7 kW: 31.7 ÷ 7.7 ≈ 4.1 hours.
- Level 2, 48A/11.5 kW: 31.7 ÷ 11.5 ≈ 2.8 hours.
DC fast charging times are typically published as 10-80% figures (not 100 miles added), because the delivered rate changes significantly across the state-of-charge range. Check your vehicle's documented 10-80% time at a specific station rating.
For a precise cost estimate on your specific vehicle, state electricity rate, and charger choice, see our EV charging cost calculator.
Picking the right charger based on your actual constraints
Four honest scenarios for choosing a home charger:
You have a standard 120V outlet, your daily miles are low, and you have long charging windows: a Level 1 portable charger on 120V may cover your daily energy need. Confirm the EVSE's rated continuous current and compatible outlet before purchase, and check your real daily energy need against your vehicle's actual kWh/100mi — consumption varies with climate, driving speed, and load.
You have a dedicated NEMA 14-50 outlet (or can install one), your car's AC input accepts 32A or more, and you want a plug-and-play portable setup: a 32A portable Level 2 EVSE uses the full 32A continuous rate the EVSE is rated for. Our WenStorm Level 2 Portable Charger plugs into an existing 14-50 outlet and delivers up to 32A/~7.7 kW on compatible vehicles.
You want faster home charging than 7.7 kW: consider a 48A hardwired Level 2 on a 60A circuit, if your vehicle's AC input accepts 48A. Hardwired installation requires an electrician.
You want the maximum residential AC rate (around 19.2 kW): this requires a vehicle rated for 80A AC input. Only certain configurations exist and the specific trim or package matters — consult your vehicle's own documentation and the installer before buying 100A-circuit hardware.
Frequently asked questions
What does kW actually mean for EV charging?
Kilowatt (kW) is a unit of power — the rate at which energy flows. 1 kW = 1,000 watts. For EV charging: higher kW means faster. A 1.4 kW Level 1 charger adds energy about 5× slower than a 7.7 kW Level 2 charger. The amount of energy stored is measured in kilowatt-hours (kWh); the rate at which you add it is kW.
Why does my charger pull 12A when the outlet is rated 15A?
EV charging is treated as a continuous load under the U.S. National Electrical Code (branch-circuit sizing per Articles 210.19/215.2 combined with EV-specific sizing in Article 625.41/625.42). The resulting rule limits EV charging to 80% of the circuit's breaker rating: 15A × 0.8 = 12A. This is a sizing provision for the circuit and equipment — a correctly-sized circuit and a correctly-matched EVSE are required for compliant installation.
Why does my 32A charger deliver only 7.7 kW on a 50A outlet?
Because the EVSE's own rating is 32A, not 40A. A 50A circuit allows up to 40A continuous, but the EVSE's internal electronics cap the current at the device's rated maximum. 240V × 32A = ~7.7 kW. To get the full 40A/~9.6 kW the circuit could allow, you'd need an EVSE rated for 40A continuous. See Tesla's Mobile Connector support page for the same example.
Can I charge faster by using a higher-amp charger?
Only if the slower link in your chain is the charger itself. If your circuit, charger, and vehicle AC input all currently cap at 32A, upgrading just the charger to 48A delivers no speed gain. The bottleneck is wherever the lowest rating sits.
What's the difference between amps and amperage?
They're the same thing. "Amps" is the unit abbreviation (symbol A), "amperage" is the quantity being measured. Both refer to electric current. A 32A-rated EVSE permits charging at up to 32A; actual current may be lower if the vehicle or circuit limits it.
Does a 240V outlet always charge faster than 120V?
At the same amperage, yes — 240V delivers roughly twice the power of 120V. A 240V/16A circuit delivers 3.8 kW vs 120V/16A at 1.9 kW. In practice this almost always means faster charging, because 240V circuits are typically higher-amperage (20A, 30A, 50A), compounding the speed difference.
Can I plug my EV into any outlet?
Only if the outlet type and amperage match what your EVSE's plug and electronics are rated for. A Level 1 EVSE is designed for a NEMA 5-15 or 5-20 (120V, 15A or 20A). A Level 2 EVSE needs a 240V residential outlet (commonly NEMA 14-50 for portable use, or hardwired) or a 208V commercial outlet with compatible equipment. Plugging equipment designed for one voltage into a different-voltage outlet is a safety hazard, even if a physical adapter exists.
What's the fastest I can realistically charge at home?
For a 2024 EV with a 48A AC input (most Tesla models, Rivian, Ford Mach-E, Chevy Bolt EUV): about 11.5 kW on a 60A circuit with a 48A hardwired EVSE. Higher rates (around 19.2 kW) require a vehicle rated for 80A AC input, a 100A circuit, and matching equipment — check your vehicle's exact configuration.
About this guide
Written by Marcus Chen, WenStorm's EV cost & practicality lead. Charging-level definitions follow SAE J1772. Commercial 208V Level 2 verified via AFDC's station overview. Continuous-load sizing follows the U.S. National Electrical Code — branch-circuit provisions (Articles 210.19/215.2) combined with EV-specific sizing (Article 625.41 for overcurrent sizing, Article 625.42 for maximum current). The 32A-on-50A example references Tesla's Mobile Connector support page. Vehicle AC input ratings cite each manufacturer's own documentation inline. DC fast charging characteristics referenced from AFDC.
