Battery capacity is quoted in two ways. Amp-hours (Ah or mAh) tell you how much charge the battery holds – how many amps it can supply for how many hours. Watt-hours (Wh) tell you how much energy it holds, which also depends on voltage. A 12 V, 100 Ah lead-acid battery and a 3.7 V, 324 Ah bank of phone cells both store 1,200 Wh, even though their amp-hour figures look very different.
Watt-hours are the fair way to compare batteries of different voltages, and airlines use them: power banks above 100 Wh need approval to fly. Amp-hours are handy when you know a device’s current draw. The conversion is simply energy = voltage × capacity.
The C-rate describes how hard a battery is charged or discharged relative to its capacity. At 1C a 2.2 Ah battery delivers 2.2 A and is empty in an hour; at 0.5C it delivers 1.1 A for two hours; at 2C it delivers 4.4 A for 30 minutes. Datasheets limit the C-rate, and real batteries deliver somewhat less at high rates.
How to use the Battery Capacity Calculator
- Calculate
- Battery voltage
- Stored energy
- Battery capacity
- Charge or discharge rate
- C-rate — 1C empties the battery in 1 hour, 0.5C in 2 hours, 2C in 30 minutes.
- Charge or discharge current
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose what to calculate and enter the other two quantities. Then give the C-rate or the current. The calculator multiplies or divides by voltage to convert between Ah and Wh, multiplies capacity by C-rate for the current and takes 1 ÷ C-rate for the run time.
Worked example
A 12 V battery stores 26.4 Wh. Its capacity is 26.4 ÷ 12 = 2.2 Ah (7,920 coulombs). Discharged at 1C it supplies 2.2 A, or 26.4 W, and is empty after 60 minutes; at 0.5C it supplies 1.1 A for 2 hours.
Typical battery capacities
| Battery | Voltage | Capacity | Energy |
|---|---|---|---|
| AA alkaline | 1.5 V | 2.5 Ah | 3.75 Wh |
| 18650 Li-ion cell | 3.6 V | 3 Ah | 10.8 Wh |
| Smartphone | 3.85 V | 5 Ah | 19.25 Wh |
| Laptop | 11.4 V | 5 Ah | 57 Wh |
| Car starter battery | 12 V | 60 Ah | 720 Wh |
Inputs explained
| Field | What to enter |
|---|---|
| Calculate | Choose one: Capacity (Ah) from energy and voltage, Energy (Wh) from capacity and voltage, Voltage from energy and capacity |
| Battery voltage | Units: V, mV |
| Stored energy | Units: Wh, kWh, mWh, kJ, J |
| Battery capacity | Units: Ah, mAh |
| Charge or discharge rate | Choose one: I know the C-rate, I know the current |
| C-rate | In C. 1C empties the battery in 1 hour, 0.5C in 2 hours, 2C in 30 minutes. |
| Charge or discharge current | Units: A, mA, µA |
What each result means
- Electric charge = [Battery capacity]×3600
- Charge or discharge current = [C-rate]×[Battery capacity]
- Power at that current = [C-rate]×[Battery capacity]×[Voltage]
- Time to fully charge or discharge = 1÷[C-rate]
- Time in minutes = 60÷[C-rate]
Formula
E (Wh) = V × Q (Ah) · Q = E ÷ V · I = C-rate × Q · t (h) = 1 ÷ C-rate · charge (coulombs) = Q × 3600
Worked example
Inputs — Calculate: Capacity (Ah) from energy and voltage; Battery voltage: 12 V; Stored energy: 26.4 Wh; Charge or discharge rate: I know the C-rate; C-rate: 1 C.
Results — Battery capacity = 2.2 Ah; Stored energy = 26.4 Wh; Voltage = 12 V; Electric charge = 7,920 coulombs; C-rate = 1 C; Charge or discharge current = 2.2 A; Power at that current = 26.4 W; Time to fully charge or discharge = 1 h 0 min; Time in minutes = 60 min.
Key terms
- Amp-hour (Ah)
- Charge delivered by 1 ampere flowing for 1 hour (3,600 coulombs).
- Watt-hour (Wh)
- Energy delivered by 1 watt for 1 hour (3,600 joules).
- C-rate
- Charge or discharge current divided by the rated capacity.
- Nominal voltage
- The average voltage of a cell during discharge, e.g. 3.6–3.7 V for lithium-ion.
Frequently asked questions
How do I convert mAh to Wh?
Wh = mAh × V ÷ 1000. A 5,000 mAh phone battery at 3.85 V holds 19.25 Wh.
How do I convert Wh to Ah?
Ah = Wh ÷ V. A 1,200 Wh battery at 12 V is 100 Ah.
What does C-rate mean?
Current relative to capacity: 1C on a 2 Ah battery is 2 A, which empties it in about 1 hour.
How long does a 200 Ah battery take to charge?
At 20 A (0.1C) about 10 hours plus 10–20% for charging losses and the slower final stage.
Why do I get less capacity than rated?
High discharge rates, cold temperatures, ageing and the cut-off voltage of your device all reduce usable capacity.
Can I take a power bank on a plane?
Most airlines allow power banks up to 100 Wh in hand luggage; 100–160 Wh needs approval.