Battery Amp Hour Calculator
Use this tool to estimate required battery size (Ah), expected runtime, or battery energy in watt-hours.
What is an amp hour (Ah) in a battery?
An amp hour (Ah) is a measure of battery capacity. It tells you how much current a battery can deliver over time. For example, a 100Ah battery can theoretically supply 100 amps for 1 hour, 10 amps for 10 hours, or 5 amps for 20 hours (ignoring losses and real-world limitations).
In practical battery planning, Ah helps you answer two common questions:
- How large should my battery be? (capacity sizing)
- How long will my battery last? (runtime estimation)
Core formulas used in this calculator
1) Required battery capacity (Ah)
Required Ah = (Load Watts × Runtime Hours) ÷ (Battery Voltage × Efficiency × Usable DoD)
Where efficiency and depth of discharge (DoD) are entered as decimals in calculation (for example, 90% = 0.90).
2) Estimated runtime (hours)
Runtime = (Battery Ah × Battery Voltage × Efficiency × Usable DoD) ÷ Load Watts
3) Battery energy
Watt-hours (Wh) = Battery Ah × Battery Voltage
This is useful when comparing batteries with different voltages and capacities.
Why Ah alone is not enough
Many people compare batteries using only Ah, but that can be misleading. A 100Ah battery at 12V stores about 1,200Wh, while a 100Ah battery at 24V stores about 2,400Wh. Same Ah, different energy.
For accurate planning, always consider:
- Battery voltage (12V, 24V, 48V, etc.)
- Load in watts
- Inverter or system efficiency losses
- Allowed depth of discharge (battery chemistry dependent)
- Safety margin for aging, cold weather, and load spikes
How to size a battery bank step by step
Step 1: Add up your load in watts
Estimate the power draw of all devices that run at the same time. If you have mixed usage throughout the day, break it into separate scenarios and total the energy needed.
Step 2: Decide your backup duration
How long do you want the battery to run your system with no charging source? Common targets are 2 hours, 8 hours, overnight, or 24 hours.
Step 3: Choose realistic efficiency and DoD
- Inverter/system efficiency: often 85% to 95%
- Lead-acid usable DoD: often around 50%
- LiFePO4 usable DoD: often 80% to 90%
Step 4: Add a safety margin
A 10% to 25% margin helps protect against battery aging, temperature effects, and unexpected extra usage.
Example battery sizing
Suppose your load is 150W, target runtime is 6 hours, battery voltage is 12V, system efficiency is 90%, and usable DoD is 80%.
- Energy needed = 150 × 6 = 900Wh
- Effective usable fraction = 0.90 × 0.80 = 0.72
- Required Ah = 900 ÷ (12 × 0.72) = 104.17Ah
With a 20% margin, recommended battery size is about 125Ah. In practice, you might select a 12V 150Ah battery or a battery bank close to that rating.
Lead-acid vs lithium in Ah planning
Lead-acid batteries
- Usually lower upfront cost
- Best life when not deeply discharged
- Often designed around ~50% usable DoD
Lithium (LiFePO4) batteries
- Higher upfront cost
- Higher usable DoD (commonly 80%+)
- Flatter voltage curve and typically longer cycle life
Common mistakes to avoid
- Ignoring inverter losses
- Using total Ah without considering voltage
- Assuming 100% of rated battery capacity is safely usable
- Forgetting startup surge loads (fridges, pumps, compressors)
- Not adding margin for winter temperatures or battery aging
Quick FAQ
How many amp hours is a 12V 100Ah battery?
It is 100Ah by rating and stores about 1,200Wh of nominal energy (12 × 100).
How long will a 100Ah battery run a 100W load?
Ideal math gives about 12 hours at 12V, but real runtime is less after efficiency losses and DoD limits. A realistic estimate is often closer to 7 to 10 hours depending on battery type and settings.
Can I directly compare Ah across different voltages?
Not accurately. Convert to watt-hours (Wh) first, then compare.
Bottom line
An amp hour calculator is the fastest way to size a battery correctly and avoid underpowered systems. Use realistic assumptions for efficiency and depth of discharge, then add a healthy safety margin. If your setup powers critical loads, round up to the next standard battery size.