Battery Life Calculator

Estimate how many hours a battery will power a device from its capacity (mAh), voltage, and the device's power draw, adjusted for real-world efficiency losses.

Quick Facts

Formula
Runtime = Energy (Wh) ÷ Power draw (W)
Energy in watt-hours is capacity (mAh) × voltage (V) ÷ 1000.
Efficiency
Rated capacity is a lab number
Voltage regulation, discharge losses, and battery aging typically leave 75-90% of rated capacity usable.

Your Results

Calculated
Battery energy
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Watt-hours (Wh) stored
Estimated runtime
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Hours of continuous use
Current draw
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Average draw at rated voltage (mA)
Days between charges
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At your daily usage hours

Ready

Enter battery and device specs, then press Calculate.

Understanding Battery Life

This calculator estimates how long a battery will power a device by converting the battery's rated capacity and voltage into stored energy, then dividing by how much power the device draws. It uses the same watt-hour method found on the label of power banks, laptop batteries, and drone battery packs: energy (Wh) divided by power (W) equals hours of runtime.

The formula

A battery's stored energy in watt-hours is Energy (Wh) = Capacity (mAh) × Voltage (V) ÷ 1000. Dividing that energy by the device's power draw gives runtime: Runtime (h) = Energy (Wh) ÷ Power draw (W). Because no battery delivers 100% of its rated capacity in real use, the calculator applies a system efficiency percentage first, so the full formula is:

  • Battery energy: Capacity (mAh) × Voltage (V) ÷ 1000, in watt-hours.
  • Estimated runtime: (Energy × Efficiency ÷ 100) ÷ Power draw (W), in hours.
  • Current draw: Power draw (W) ÷ Voltage (V) × 1000, in milliamps — useful for comparing against the battery's mAh rating.
  • Days between charges: Estimated runtime ÷ your average daily usage hours.

Why the efficiency factor matters

The mAh figure printed on a battery is measured under controlled lab conditions at a fixed discharge rate. Real devices lose some of that energy to voltage-regulation circuitry, internal resistance, temperature, and the fact that most electronics cut off before the cell is fully drained. A system efficiency of 100% assumes none of that loss; 75-90% is a more realistic range for consumer electronics, and an older or heavily cycled battery should use a lower figure still.

Reading your result

Compare the estimated runtime to how long you actually plan to use the device between charges. If the runtime is much shorter than you need, the practical levers are a higher-capacity battery, a lower-power device setting, or a lower duty cycle (using the device less continuously). The current-draw figure is handy for cross-checking against a device's spec sheet, which often lists current in mA rather than power in watts.

Frequently Asked Questions

What is the formula for battery life?
Battery life in hours equals the battery's energy in watt-hours divided by the device's power draw in watts. Energy (Wh) is capacity in mAh multiplied by voltage in volts, divided by 1000. So runtime = (capacity × voltage ÷ 1000 × efficiency) ÷ power draw, where efficiency accounts for real-world losses.
Why include an efficiency percentage?
A battery's rated mAh capacity is measured under ideal lab conditions. In real use, voltage regulation, discharge curve losses, battery aging, and not fully draining the cell all reduce usable energy, typically to about 75-90% of the rated figure. The efficiency input lets you scale the estimate to match real-world behavior.
How do I find my device's power consumption in watts?
Check the device's spec sheet or packaging for a wattage rating, or calculate it by multiplying the operating voltage by the current draw in amps (W = V × A). Many USB devices list current draw in mA at 5V, so a device drawing 500 mA at 5V uses 2.5 W.
What is the difference between battery capacity and battery life?
Capacity (mAh or Ah) is a fixed property of the battery describing how much charge it can store. Battery life (runtime) is how long that stored energy lasts under a specific load, which depends on both the battery's energy content and how much power the connected device draws.

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