Battery Capacity Calculator

About the Battery Capacity Calculator

The Battery Capacity Calculator helps you determine battery runtime, convert between capacity units (mAh, Ah, Wh), and size batteries for your specific power needs. Whether you're designing a portable electronics project, choosing a power bank, or sizing a solar battery system, understanding battery capacity is essential.

Battery capacity is typically rated in milliamp-hours (mAh) or watt-hours (Wh). While mAh tells you the charge stored, Wh accounts for voltage and gives a more accurate picture of total energy. This calculator converts between all common units and estimates actual runtime based on your device's power consumption, discharge rate, and efficiency losses.

Real-world battery performance differs from rated specifications due to temperature, discharge rate (Peukert effect), age, and conversion efficiency. This calculator includes adjustable efficiency and discharge factors so you can get realistic estimates rather than theoretical maximums. Use the preset device profiles or enter custom values for your specific application. That makes it easier to compare a battery pack against the actual load you plan to run.

When This Page Helps

Use this calculator when you want a realistic battery runtime estimate instead of a nameplate-only number. It is useful for portable devices, power banks, and small battery systems where voltage, load, and efficiency all matter. That helps you size capacity before the battery is already in the build and avoid a pack that comes up short in practice.

How to Use the Inputs

1. Enter the battery capacity in mAh or Ah
2. Enter the battery nominal voltage (e.g., 3.7V for Li-ion, 1.5V for alkaline)
3. Enter your device's power draw in milliamps (mA) or watts (W)
4. Adjust the efficiency factor to account for real-world losses (typically 80-90%)
5. Select a common device preset or enter custom values
6. Review runtime estimate, energy in Wh, and other conversion results
7. Use the comparison table to see runtime across different load levels

Example Calculation

Tips & Best Practices

• Always use nominal voltage, not fully-charged voltage, for capacity calculations
• Li-ion batteries should not be discharged below 20% for longevity — factor this into runtime
• Power bank efficiency is typically 75-85% due to voltage conversion losses
• For solar systems, size batteries for 2-3 days of autonomy as a rule of thumb
• Higher discharge rates reduce effective capacity — use conservative estimates for high-drain devices
• Battery capacity decreases with age; expect 80% capacity after 300-500 charge cycles for Li-ion

Understanding Battery Capacity Units

Battery capacity can be expressed in several units, and understanding the differences is crucial for proper sizing. Milliamp-hours (mAh) is the most common rating for small batteries and tells you how many milliamps a battery can deliver for one hour. However, mAh doesn't account for voltage, so it's not ideal for comparing batteries of different chemistries.

Watt-hours (Wh) is a more universal energy unit that factors in voltage. Airlines use Wh to regulate lithium batteries (100 Wh carry-on limit). Kilowatt-hours (kWh) is used for larger battery systems like electric vehicles and home energy storage.

Battery Chemistry Comparison

Different battery chemistries have different characteristics. Lithium-ion batteries offer high energy density (150-250 Wh/kg) and low self-discharge, making them ideal for portable electronics. Lead-acid batteries are cheaper but heavier (30-50 Wh/kg) and suffer more from the Peukert effect. NiMH batteries offer moderate performance with better environmental characteristics.

When sizing a battery system, consider not just capacity but also maximum discharge rate (C-rating), operating temperature range, cycle life, and self-discharge rate. A battery rated at 2000 mAh with a 1C maximum discharge can only deliver 2A continuous current.

Practical Battery Sizing Guidelines

For portable projects, calculate your worst-case power draw and multiply by desired runtime with a 20-30% safety margin. For solar off-grid systems, size your battery bank for 3 days of autonomy at 50% depth of discharge. For electric vehicles, consider both total energy (range) and peak power delivery (acceleration).

Frequently Asked Questions

• What's the difference between mAh and Wh?

  mAh (milliamp-hours) measures charge capacity without considering voltage. Wh (watt-hours) measures total energy by accounting for voltage: Wh = Ah × V. Wh is more useful for comparing batteries at different voltages.

• Why is my battery's actual runtime less than calculated?

  Real-world factors reduce runtime: voltage conversion losses, temperature effects, battery age/degradation, and the Peukert effect (capacity decreases at higher discharge rates). Use 80-90% efficiency for realistic estimates.

• What is the Peukert effect?

  The Peukert effect describes how battery capacity decreases at higher discharge rates. A battery rated at 10Ah at C/20 might only deliver 8Ah at C/5. Lead-acid batteries are most affected; lithium batteries are less so.

• What voltage should I use for my battery?

  Use the nominal voltage: 3.7V for Li-ion/LiPo, 1.2V for NiMH, 1.5V for alkaline, 2V per cell for lead-acid, 3.2V for LiFePO4. For battery packs, multiply by the number of series cells.

• How do I calculate battery capacity needed for my device?

  Multiply your device's power draw (mA) by desired runtime (hours) and divide by efficiency: Required mAh = (Load mA × Hours) / Efficiency. Add 20% margin for safety.

• Can I compare batteries with different voltages?

  Yes, use Wh (watt-hours) instead of mAh for comparison. A 3000 mAh, 3.7V battery (11.1 Wh) stores more energy than a 5000 mAh, 1.5V battery (7.5 Wh).