EV charger specifications are full of numbers — 40A, 9.6 kW, 240V — and it is not always obvious what any of them mean in practical terms. How fast will this actually charge my car? What is the difference between amps and kilowatts? Why does my 40A charger sometimes deliver less than 40A? This guide answers all of it, without requiring an engineering background.

The Three Numbers That Matter

These three units are related by a simple formula that explains everything about charging speed:

At 240 volts (Level 2) and 40 amps: 240 × 40 ÷ 1,000 = 9.6 kW. At 240 volts and 16 amps: 240 × 16 ÷ 1,000 = 3.84 kW. At 120 volts (Level 1) and 12 amps: 120 × 12 ÷ 1,000 = 1.44 kW.

Kilowatts are what determine real-world charging speed. More kilowatts means more energy stored per hour, which translates directly to more miles of range per hour of charging.

Volts: Fixed by Your Electrical System

Voltage is determined by your outlet or circuit — not by the charger. A standard household outlet in North America is 120 volts. A dedicated EV charging circuit, dryer outlet, or range outlet is 240 volts. You cannot change the voltage a charger operates at without changing the electrical circuit it connects to.

This is why the Level 1 versus Level 2 distinction matters so much. Doubling the voltage from 120V to 240V doubles the power delivered at the same amperage — before even considering the typically higher amperage of Level 2 chargers. A 16A circuit at 240V delivers more than five times the power of a 12A circuit at 120V.

Amps: Set by the Charger (and Limited by the Circuit)

Amperage is where charger specifications vary. An EV charger draws a specific amount of current — measured in amps — from the circuit it is connected to. The charger’s rated amperage is its maximum draw, which is limited by two factors:

1. The charger’s design. A 16A charger can never draw more than 16A. A 40A charger draws up to 40A, but can be set lower through adjustable output controls.
2. The circuit’s capacity. A 50-amp circuit can safely supply up to 40A continuous draw to a charger (the 80-percent rule). Connecting a 48A charger to a 50-amp circuit would exceed safe continuous draw limits — which is why 48A chargers require a 60-amp dedicated circuit.

Smart chargers like the ShockFlo G1 allow you to adjust output amperage — from 16A to 40A — through the app. This lets you reduce draw when other appliances are running and maximize it overnight when the circuit is otherwise idle.

Kilowatts: What Actually Determines Charging Speed

Kilowatts measure the rate at which energy is transferred to your vehicle. The more kilowatts delivered, the more range added per hour. But there is a critical ceiling: your vehicle’s onboard AC charger sets the maximum kilowatts it will accept, regardless of what the external charger supplies.

If your vehicle accepts a maximum of 7.2 kW and you connect a 40A Level 2 charger (capable of 9.6 kW), the vehicle’s onboard charger limits the actual power to 7.2 kW. The external charger reduces its output automatically through the J1772 pilot signal negotiation.

The Full Picture: Charger Output vs Vehicle Acceptance Rate

Highlighted cells show where the charger output and vehicle acceptance rate are matched — maximum efficiency. In all other cells, one side is the limiting factor. This is why knowing your vehicle’s onboard AC charging rate before buying a charger matters: it tells you exactly how much charger output you can actually use.

kWh: The Energy Unit You See on Charging Estimates

Kilowatt-hours (kWh) measure total energy — not rate. One kilowatt-hour is the energy delivered by one kilowatt of power running for one hour. Your EV’s battery is measured in kWh (a 75 kWh battery, for example). Your charging sessions consume kWh, and your electricity bill charges you per kWh.

• A 9.6 kW charger running for one hour delivers 9.6 kWh of energy
• At an electricity rate of $0.15/kWh, that session costs $1.44
• At 3 miles of range per kWh (typical EV efficiency), 9.6 kWh adds approximately 29 miles of range

The ShockFlo app tracks cumulative kWh per session and per month, making it straightforward to calculate your actual charging cost over time.

Why Does My Charger Sometimes Deliver Less Than Rated?

Several factors can cause a charger to deliver below its rated maximum:

• Vehicle limit: The most common cause. Your car accepts less than the charger offers.
• Temperature: In cold weather, EVs limit charging current while the battery warms up. In extreme heat, thermal management may also reduce charging rate.
• Battery state: Near 100% charge, most EVs reduce the charge rate to protect the battery — a process called taper charging. This is normal and expected behavior.
• Charger setting: If the charger’s adjustable output is set below its maximum, it delivers the configured rate, not its maximum capacity.

Conclusion

The three numbers in every EV charger spec — volts, amps, and kilowatts — combine to determine real-world charging speed. Volts are fixed by your circuit. Amps are set by the charger. Kilowatts are the result, and your vehicle’s onboard charger sets the ceiling on how many kilowatts it will accept. For most EV owners, the ShockFlo G1 at 40A and 9.6 kW covers the onboard charging capacity of every popular EV — delivering overnight results without the cost of a higher-amperage hardwired installation. See our Level 2 EV charger guide for full model comparisons.