An amp-hour (Ah) is a unit of electric charge: one amp of current flowing for one hour. It is how small batteries — a car’s 12-volt battery, power tools, e-bikes — state their size. Your EV’s main battery is rated in kilowatt-hours instead, and the reason is voltage: energy equals volts times amp-hours, and because pack voltage varies from car to car, amp-hours alone would tell you only half the story.
Key Takeaways
- An amp-hour is charge: one amp flowing for one hour. It is not, by itself, energy.
- Energy is amp-hours times volts — which is why EV packs are rated in kilowatt-hours.
- Amp-hours still rule where voltage is fixed: 12-volt batteries, single cells and small devices.
- A charger’s amp rating is a flow rate your wiring must handle, not the size of your battery.
Charge Is Not Energy: The Four Units Side by Side
| Unit | What it measures | Where you meet it |
|---|---|---|
| Amp (A) | Current — the rate of flow right now | Charger ratings, such as a 40 A home charger |
| Amp-hour (Ah) | Charge — current multiplied by time | 12-volt batteries, individual cells, tool and e-bike batteries |
| Kilowatt-hour (kWh) | Energy — the work the charge can do | EV battery size, your home electricity bill |
| Kilowatt (kW) | Power — how fast energy moves | Charging speed, from home units to fast chargers |
The bridge between them is one multiplication: amp-hours times volts equals watt-hours. So 100 Ah from a 12-volt battery is 1.2 kWh of energy, while the same 100 Ah at a 400-volt pack voltage is 40 kWh. Same amp-hours, very different batteries — which is exactly why the amp-hour figure on its own cannot size up an EV.
Why EV Spec Sheets Skip Amp-Hours
Most EV packs run at a nominal voltage around 400 volts, and a growing group of cars uses roughly 800 volts. Take two packs with identical amp-hour ratings and the 800-volt one stores about twice the energy — and offers roughly twice the range. Range and cost both follow energy, so the honest headline unit is the kilowatt-hour.
There is a second problem: a pack’s voltage is not fixed. It sits higher when the battery is full and lower as it empties, so a single amp-hour figure for a whole pack would be a moving target. Car makers quote capacity in kWh because that predicts how far the car goes — the basis of how battery health is checked as the pack ages.
Where You Still Meet Amp-Hours
- The 12-volt battery in your EV (and in every petrol car) is rated in amp-hours, because its voltage is always about 12 — at a fixed voltage, amp-hours are enough to compare one with another.
- Individual cells. A single lithium-ion cell is about 3.7 volts, so cell makers quote its capacity in amp-hours; cells wired in parallel add their amp-hours together, which is how packs are built up.
- E-bikes, tools and power stations. Small batteries are sold in Ah or milliamp-hours (mAh) — workable shorthand because the voltage of each product class is understood.
- Old habits. If a listing gives you amp-hours for a car battery, ask for the voltage as well; without it, the number says little, as our comparison of charging levels and their power shows for charging hardware.
Charger Amps Are a Different Number
The amp figure on a charger is a rate, not a store. How much energy lands in the battery depends on the voltage it is delivered at, which is why session screens report kilowatt-hours, not amp-hours. Chargers quote amps because amps are what your home’s wiring and breakers must handle.
The EVDANCE EV-ACS4005T 40A NACS portable charger is adjustable at 16, 24, 32 or 40 A to suit the circuit it plugs into, and the EVDANCE EV-ACS3210T 32A NACS portable charger offers 10–32 A in six steps. Lower the amps and the same battery simply takes longer to fill — the kilowatt-hours it needs do not change.
Frequently Asked Questions
Can I work out my EV battery’s amp-hours?
Roughly: divide its capacity in kilowatt-hours by its pack voltage and multiply by 1,000. Treat the result as an estimate only — pack voltage moves with charge level, which is why manufacturers do not publish an amp-hour figure for the pack.
Is a higher amp-hour battery always the bigger battery?
Only at the same voltage. A 100 Ah battery at 12 volts holds 1.2 kWh; a 50 Ah battery at 400 volts holds 20 kWh. Compare kilowatt-hours when the voltages differ.
Why does my phone battery use milliamp-hours?
A phone runs on a single cell at about 3.7 volts, and its capacity is small, so thousandths of an amp-hour — mAh — are a convenient size of number. Same unit, same rule: multiply by the voltage to get the energy.
Related Reading
- How to Check EV Battery Health
- Does an EV Battery Degrade Over Time?
- Can an EV Battery Be Replaced?
- Does an EV Battery Drain When Parked?
