What is system efficiency in this context?

System efficiency accounts for energy losses between the battery and your load. These include DC-DC converter losses (typically 5–20%), wiring resistance losses, and battery internal resistance losses. A 90% efficiency means only 90% of stored charge reaches the load; you must store 100/90 ≈ 11% more to compensate.

Why add a safety margin?

Battery capacity degrades over charge cycles and at temperature extremes. A 20–30% safety margin ensures your device still meets its runtime target when the battery has aged or when operating in cold conditions. New batteries used at room temperature can use a smaller margin (10–15%).

What is the formula used?

Required capacity (mAh) = (Load current (mA) × Runtime (h)) / (Efficiency / 100). The recommended capacity then adds the safety margin: Recommended = Required × (1 + Safety margin / 100).

Should I enter peak or average current?

Use the average current draw over the full runtime. If your device has sleep/active cycles, calculate the duty-cycle-weighted average: (I_active × t_active + I_sleep × t_sleep) / total_time. Peak current affects component ratings but not total energy storage.

How does this differ from the Battery Life Calculator?

The Battery Life Calculator tells you how long a given battery will last. This tool works in reverse — you specify the desired runtime and it tells you how large a battery you need. Use this tool during the design phase; use the Battery Life Calculator to verify an existing battery.

What efficiency value should I use for a Li-ion direct load?

If your load connects directly to the battery terminals with no converter, use 95–98% to account only for internal resistance and wiring losses. If you use a boost or buck converter, use the converter efficiency rating, typically 85–92% for common modules.

Battery Capacity Calculator

Enter your device current draw, desired runtime, and efficiency to calculate the minimum battery capacity (mAh) required.

Device & Runtime Parameters

Load Current (mA)

Average current drawn by the device over its operating cycle.

Desired Runtime (hours)

How long the device must run on a single charge. Use decimals for minutes (1.5 = 90 min).

System Efficiency (%) (70–98%)

Account for converter, wiring, and internal resistance losses. 90% is a good default.

Safety Margin (%) (0–100%)

Extra headroom for aging, temperature, and tolerance. 20% is recommended for most designs.

Recommended Battery Capacity

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mAh

Formula: Minimum (mAh) = (I × t) / η
Recommended (mAh) = Minimum × (1 + margin / 100)
Where I = current (mA), t = runtime (h), η = efficiency (0–1).

Summary

This tool calculates the minimum battery capacity (in mAh) required to run a device for a specified duration. It accounts for system efficiency losses — such as converter losses, self-discharge, and temperature derating — so you can size your battery pack correctly from the start. The result includes both a theoretical minimum and a recommended capacity with a safety margin.

How it works

Enter the average current draw of your device in milliamps (mA).
Enter the desired runtime in hours (decimal values are accepted, e.g. 1.5 for 90 minutes).
Enter the system efficiency as a percentage (typical range: 70–95%).
Optionally set a safety margin (%) to add headroom for aging and worst-case conditions.
The calculator divides the required charge by efficiency and adds the safety margin.
Results show the theoretical minimum mAh and the recommended capacity to purchase.

Use cases

Size a battery pack for an IoT sensor or wireless node.
Determine the battery needed for a portable electronics prototype.
Plan power storage for a remote monitoring system with a defined service interval.
Estimate battery requirements for drones, rovers, or RC vehicles.
Calculate backup power capacity for embedded systems during outages.
Validate that an existing battery meets runtime requirements before deployment.
Compare different battery chemistries by adjusting efficiency values.
Plan battery replacement cycles for field-deployed equipment.

Frequently Asked Questions

Last updated: 2026-05-28 · Reviewed by Nham Vu

Load Current
Desired Runtime
Raw Charge Needed
System Efficiency
Theoretical Minimum
Safety Margin
Recommended Capacity