Battery Capacity (Ah) Calculator
Enter any two of current I, time t and capacity Ah and the third is solved. Then add voltage, depth of discharge and efficiency to get the battery bank you actually need, in Ah, Wh and number of batteries.
Ah = I × tWh = Ah × VBank = Ah / (DoD × η)Step-by-step solution
Battery bank summary
Battery diagram
The shaded part of the battery is the usable fraction (depth of discharge). The load drains it at the chosen current for the chosen time.
Runtime versus current
Reference data
| Chemistry | Usable DoD | Round-trip efficiency | Cycle life (approx.) |
|---|---|---|---|
| Flooded lead-acid | 50% | 75 to 85% | 300 to 700 |
| AGM / Gel | 50 to 60% | 80 to 85% | 400 to 1,000 |
| Li-ion (NMC) | 80% | 90 to 95% | 1,000 to 2,000 |
| LiFePO₄ | 80 to 100% | 92 to 98% | 2,000 to 6,000 |
| Load | Power (W) | Current (A) | Ah for 5 h | Ah for 24 h |
|---|
Appliance powers are typical figures and vary by model. Check the nameplate for the real wattage.
FAQ
What is an amp-hour?
One amp-hour is a current of 1 A flowing for 1 hour (3,600 coulombs). A 100 Ah battery can in theory supply 5 A for 20 hours.
Why can I not use the full rated capacity?
Lead-acid batteries age quickly if discharged below about 50%. Lithium chemistries can use 80% or more. Divide the load Ah by the depth of discharge to get the nominal battery size.
Why do I need efficiency?
Inverters, chargers and the battery itself lose some energy. Dividing by η (for example 90% with an inverter) covers that loss.
Ah or Wh?
Ah depends on voltage. Wh = Ah × V compares batteries at different voltages. A 100 Ah 12 V battery stores 1,200 Wh, the same as a 50 Ah 24 V battery.
What about high-current drain (Peukert)?
Lead-acid batteries deliver less than their rated Ah at high discharge rates, and capacity falls in the cold. This tool assumes rated capacity at the stated current, so add margin for heavy or cold-weather loads.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| Ah | Battery capacity (ampere-hours) | Ah |
| I | Discharge current (constant) | A |
| t | Discharge duration | h |
What it means
Battery capacity in ampere‑hours (Ah) is the product of the discharge current I and the time t (hours) for which it can supply that current: Ah = I × t. It indicates the total charge a battery can deliver. For example, a battery rated at 100Ah can supply 5A for 20 hours (5×20=100). However, the actual capacity depends on the discharge rate; higher currents reduce the effective capacity (Peukert effect). This rating is used for sizing batteries for backup power, electric vehicles, and renewable energy systems. Example: A 12V, 100Ah battery stores 1200Wh of energy (12×100), but the usable amount depends on the depth of discharge.
Worked example
Battery Capacity (Ah) – Practical Example
Real‑World| Parameter | Value |
|---|---|
| I | 5 A |
| t | 20 h |
| Formula | Ah = I × t |
Common mistakes
Watch for unit mismatches (W vs kW, single- vs three-phase) and remember to include power factor or efficiency where the formula requires it.Applications
Battery capacity in amp‑hours Ah = I × t indicates the total charge a battery can deliver. This is used to size batteries for various applications, from consumer electronics to backup power. Engineers use it to select batteries, to calculate runtime, and to design energy storage systems.
- Battery selection for portable and stationary applications
- Backup power and UPS sizing
- EV and hybrid vehicle battery sizing
- Energy storage for solar and wind systems
- Educational understanding of battery capacity
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