Understanding Battery Capacity
Battery capacity is measured in Amp-hours (Ah) or Watt-hours (Wh). Ah indicates how much current a battery can provide for one hour. Wh = Ah × Voltage gives total energy storage.
Battery Calculator
Our battery calculator helps you size batteries for solar systems, UPS backup, electric vehicles, and portable electronics. Calculate how long a battery will last, how many batteries you need, charging requirements, and energy storage capacity in Ah or Wh.
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Updated: September 2025
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Battery Calculator calculator
80%
Ah
V
W
hours
A
Results
19.2
hours runtime
Total Energy
1200 Wh
Usable Energy
960 Wh
C-Rate
0.042C
DoD
80%
battery_charging_full Battery Chemistry
Lead Acid
2.0V/cell
NiMH
1.2V/cell
Li-ion
3.7V/cell
LiFePO4
3.2V/cell
speed C-Rating Guide
- 🔋 1C = Full discharge in 1 hour
- ⚡ 2C = Full discharge in 30 min
- 🚗 EV batteries: 1-3C typical
How to Use the Battery Calculator
1
Select Mode
Choose to calculate runtime, required capacity, or charging time
2
Enter Battery Specs
Input capacity (Ah), voltage, and battery type
3
Add Load/Charger
Enter power consumption or charger current
4
View Results
See calculated runtime, energy, or charging time
The Formula
Battery capacity in Wh = Ah × Voltage. Runtime depends on load power and usable capacity (DoD). Lead-acid batteries typically use 50% DoD, lithium can use 80-90%.
Runtime (hours) = Battery Capacity (Wh) ÷ Load Power (W)
lightbulb Variables Explained
- Wh Watt-hours (energy capacity)
- Ah Amp-hours (charge capacity)
- V Battery voltage
- DoD Depth of Discharge (usable capacity %)
tips_and_updates Pro Tips
1
Lithium batteries can discharge to 80-90% DoD safely
2
Lead-acid batteries should only discharge to 50% for longevity
3
Wh = Ah × V (100Ah at 12V = 1200Wh)
4
For solar systems, size for 2-3 days of autonomy
5
Account for inverter efficiency (85-95%) in calculations
6
Cold temperatures reduce effective battery capacity
7
C-rate indicates charge/discharge speed (1C = full charge in 1 hour)
Battery capacity, measured in milliamp-hours (mAh) or watt-hours (Wh), determines how long a device can operate before needing a recharge — but translating raw capacity into actual runtime requires accounting for voltage conversion efficiency, discharge rates, temperature effects, and the device's varying power demands. A 10,000 mAh power bank does not deliver 10,000 mAh to your phone because voltage conversion from the battery's 3.7V to the USB output's 5V loses 20-30% to heat and conversion inefficiency, yielding approximately 6,500-7,000 mAh of usable charge. Our battery calculator computes runtime from capacity and load current, determines required battery size for a target runtime, calculates charging time from charger specifications, and handles series and parallel battery configurations. It supports common battery chemistries (lithium-ion, lithium-polymer, NiMH, lead-acid, alkaline) with their characteristic voltage curves, and accounts for real-world efficiency losses that make theoretical calculations optimistic by 15-35%.
Battery Sizing for Solar Systems
For off-grid solar, size batteries for 2-3 days autonomy. Consider depth of discharge (DoD) to protect battery lifespan. Lithium batteries offer higher DoD than lead-acid.
Frequently Asked Questions
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Data sourced from trusted institutions
All formulas verified against official standards.