Calculate battery capacity, runtime, and energy in mAh, Wh, and Ah. Estimate how long a battery will power your device with load and efficiency factors.
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 tool 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.
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.
Runtime (hours) = (Battery Capacity in mAh × Efficiency) / Load in mA. Energy (Wh) = Capacity (Ah) × Voltage (V). Capacity (Ah) = Capacity (mAh) / 1000. Adjusted for Peukert effect: Effective Capacity = Rated Capacity × (Rated Discharge Rate / Actual Discharge Rate)^(k-1), where k is the Peukert exponent.
Result: 17.0 hours
A 10,000 mAh battery at 3.7V with 85% efficiency powering a 500 mA load: (10000 × 0.85) / 500 = 17.0 hours runtime. Energy = 10 Ah × 3.7V = 37 Wh.
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.
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.
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).
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.
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.
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.
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.
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.
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).