PhysicsCalcs
Electricity

Battery Capacity Calculator

Calculate required battery capacity, convert between mAh and Wh, estimate runtime, and size batteries with efficiency and depth of discharge.

Interactive calculator

Battery Capacity Calculator

Calculate required battery capacity, convert between Ah and Wh, estimate runtime, and size batteries for your application.

Average current draw

Desired runtime in hours

e.g. 80 for 80%

e.g. 90 for 90%

Your result will appear here.

Choose a calculation mode, fill in the known values, and click Calculate.

Quick Guide

  • Choose: required capacity, energy, Ah\u2194Wh conversion, or runtime.
  • Enter current/power, runtime, voltage as needed.
  • Adjust DoD and efficiency for realistic sizing.

Table of Contents

Key Takeaways

  • Battery capacity is measured in Ah (amp-hours) or mAh (milliamp-hours).
  • Energy (Wh) = Voltage × Capacity (Ah). Wh is better for cross-voltage comparisons.
  • Required capacity = (current × runtime) / (DoD × efficiency).
  • Always derate for depth of discharge and system efficiency.
  • A 3000 mAh battery at 3.7 V stores 11.1 Wh of energy.

What Is Battery Capacity?

Battery capacity is the total amount of electrical charge a battery can store and deliver. It\u2019s measured in amp-hours (Ah) or milliamp-hours (mAh). Combined with voltage, capacity determines the total energy (Wh) available to power your device.

Battery Capacity Infographic

Capacity Formulas

Ah=I×tAh = I \times t

Capacity from current and time

Wh=V×AhWh = V \times Ah

Energy from voltage and capacity

mAh vs Wh: When to Use Which

FeaturemAh (Capacity)Wh (Energy)
MeasuresCharge (current \u00D7 time)Energy (power \u00D7 time)
Depends on voltage?NoYes (Wh = V \u00D7 Ah)
Best forSame-voltage comparisonsCross-voltage comparisons
Example3000 mAh phone battery50 Wh laptop battery

How to Use

  1. 1Select a calculation mode.
  2. 2Enter known values (capacity, current, voltage, runtime).
  3. 3Set DoD and efficiency for sizing modes.
  4. 4Click Calculate.
  5. 5Review results and apply safety margins for real designs.

Examples

Required Capacity

200 mA for 24 hours, 80% DoD, 90% eff. Ah = (0.2 × 24) / (0.8 × 0.9) = 6.67 Ah.

mAh to Wh

3000 mAh at 3.7 V. Wh = 3.7 × 3 = 11.1 Wh.

Applications

Battery selection for electronics, UPS sizing, solar battery system design, EV range calculation, portable device design, IoT sensor power budgets, and backup power planning.

Common Mistakes

  • Comparing mAh across different voltages without converting to Wh.
  • Not accounting for DoD and efficiency in sizing calculations.
  • Assuming rated capacity is fully usable.
  • Confusing mAh (capacity) with mA (current).

Accuracy and Limitations

This calculator uses ideal formulas. Real battery capacity varies with discharge rate, temperature, age, and chemistry. Always use manufacturer datasheets and add safety margins for critical applications.

Frequently Asked Questions

What is battery capacity?›

Battery capacity is the total charge a battery can deliver, measured in amp-hours (Ah) or milliamp-hours (mAh). A 2000 mAh battery can theoretically deliver 2000 mA for 1 hour, or 200 mA for 10 hours.

What is the difference between mAh and Wh?›

mAh measures charge capacity at the battery voltage. Wh measures energy and accounts for voltage: Wh = V × Ah. Use Wh when comparing batteries at different voltages.

How do I size a battery?›

Determine your average current draw and desired runtime. Calculate: Required Ah = (I × hours) / (DoD × efficiency). Add a safety margin of 10–20%.

Why is actual capacity less than rated?›

Rated capacity assumes ideal conditions. Real capacity is reduced by high discharge rates, cold temperatures, battery age, and internal resistance losses.

What is a good DoD to use?›

For Li-ion: 80–90% DoD. For lead-acid: 50–60% DoD. Lower DoD extends cycle life significantly.

Sources / References

  1. Battery University: Understanding Battery Capacity
  2. NIST: SI Units
Manish Kumar

Author & technical reviewer

Manish Kumar

PhysicsCalcs tools are reviewed with an educational focus: clear formulas, transparent assumptions, and practical context for students and science learners.

Learn more about Manish