Battery Life Calculator
Estimate how long a battery will last for a given load. Enter circuit values to calculate the selected electrical quantity.
Battery & Load
Electronics guide · Electrical power, energy, and wiring
About Battery Life Calculator
The Battery Life Calculator estimates how long a battery may run a device from its capacity and current draw. It can help compare a rough runtime for portable electronics or small projects, but battery labels and real operating conditions mean the result is only an estimate.
Power and energy calculations relate quantities such as voltage, current, resistance, time, and load. AC and DC systems can require different assumptions, and a calculated estimate is not a substitute for checking source capability, wire ratings, protection, efficiency, and equipment specifications.
How to Use This Tool
- 1.Enter battery capacity in mAh and the device's average current draw in mA.
- 2.Set the efficiency or usable-capacity percentage if the tool offers it. Use a lower value when not all rated capacity is available to the load.
- 3.Calculate the estimated hours, then compare it with the device's actual current profile and battery specifications.
Technical Details to Check
1.Check AC or DC assumptions
In AC calculations, voltage and current may refer to RMS values, and phase relationships can affect real power. Do not apply a DC relationship to an AC system unless the tool's stated assumptions fit.
2.Power is not the same as energy
Watts describe a rate of energy use; watt-hours and joules describe an amount of energy over time. A runtime or consumption estimate needs both a load and a duration or duty profile.
3.Real systems lose energy
Inverters, batteries, wiring, motors, and power supplies are not perfectly efficient. Confirm whether the selected calculation includes efficiency, power factor, voltage drop, or only ideal values.
4.Ratings and protection are essential
An estimated current or power value does not determine a safe wire size, fuse, battery connection, or supply. Follow applicable standards, product ratings, and qualified design guidance.
How the Calculation Works
A simple estimate is runtime in hours = capacity in mAh ÷ load current in mA. If a usable-capacity percentage is applied, multiply by that fraction: runtime = (capacity ÷ current) × efficiency. The units cancel to hours when capacity and current use matching prefixes.
Worked Example
A 2,000 mAh battery powering a steady 200 mA load has an ideal estimate of 10 hours. At 80% usable capacity, the estimate becomes 8 hours.
Where This Tool Can Help
Plan approximate runtime for a steady low-voltage load, compare battery choices, or understand how reducing current can extend use.
Estimating a load
Use known voltage and current or power figures to understand a device's approximate demand before checking its label and startup behavior.
Planning energy use
Combine a power estimate with realistic operating hours to compare possible consumption, while accounting separately for standby use and efficiency.
Checking a wiring estimate
Use current or voltage-drop results as an early check, then verify conductor size and protection with the correct installation rules and environment.
Comparing supply options
Compare candidate sources on the same voltage, current, power, and duration basis, and include headroom where the equipment specification requires it.
Tips for More Reliable Results
- Confirm whether voltage and current are RMS, peak, nominal, or measured under load.
- Keep watts, watt-hours, joules, and time units distinct when entering values.
- Include efficiency or power factor only when the tool provides that input and you have a suitable value.
- Check conductor, connector, battery, and supply ratings before using an estimate in a real installation.
Limitations and Assumptions
Actual runtime changes with temperature, battery age, discharge rate, voltage cutoff, regulator losses, and pulsed loads. mAh is charge capacity at a stated condition, not energy by itself; compare voltage and chemistry too.
Electrical power tools estimate values from entered figures and stated assumptions. They do not measure a circuit, model every transient or loss, or certify wiring and equipment as safe. Confirm important designs with manufacturer data and applicable electrical requirements.
Frequently Asked Questions
Why does my device run for less time than the result?
The load may draw more current, and real capacity varies with temperature, age, and cutoff voltage.
Can I compare batteries with different voltages using mAh alone?
No. For energy comparisons, also consider watt-hours or voltage and chemistry.
Is the result guaranteed?
No. It assumes the entered current and usable capacity remain steady. Measure a real setup for a better estimate.
Are watts and watt-hours interchangeable?
No. Watts are a rate; watt-hours measure energy used over time.
Does a calculated value make a circuit safe to build?
No. Safety depends on ratings, protection, installation conditions, and applicable standards as well as the arithmetic.