Battery Runtime Estimator
Estimate how long a battery can power your device using capacity, voltage, and load. Enter either power draw (W) or current draw (mA).
What this battery life calculator does
This calculator estimates runtime by converting battery specifications into usable energy, then dividing by your device’s average power demand. It is useful for phones, tablets, power banks, IoT devices, drones, LED lighting, portable monitors, and custom electronics projects.
Instead of relying on rough guesses, you can quickly answer practical questions like:
- Will this battery last through a work shift?
- How many days can my sensor node run between charges?
- How much runtime do I lose because of conversion inefficiency?
How the math works
1) Convert battery capacity to watt-hours (Wh)
Battery energy (Wh) = (mAh ÷ 1000) × Voltage
mAh tells you charge; voltage tells you how much energy each unit of charge carries. Together they give total stored energy.
2) Estimate usable energy
Usable Wh = Battery Wh × (Usable Capacity %) × (System Efficiency %)
Real systems cannot use 100% of rated energy. Voltage regulators, cables, thermal effects, and battery protection cut into the theoretical maximum.
3) Divide by load power
Runtime (hours) = Usable Wh ÷ Load Power (W)
If you enter current instead of power, this page computes power using P = V × I.
How to improve estimate accuracy
- Use average, not peak power: spikes can be dramatic, but long-term runtime depends on average load.
- Measure real usage: use a USB power meter or multimeter data logger when possible.
- Account for temperature: cold environments can reduce effective capacity.
- Adjust usable percent: if your device shuts off early due to cutoff voltage, reduce this value.
- Include conversion losses: boost converters and voltage regulators often run between 80% and 95% efficiency.
Example scenarios
Portable camera rig
A 10,000 mAh battery at 3.7 V stores about 37 Wh. If your camera setup averages 6 W and your practical usable/efficiency factor is 0.8, runtime is about 4.9 hours.
IoT sensor gateway
If your gateway averages 1.2 W and the battery provides 18 Wh usable, expected runtime is roughly 15 hours. With duty-cycling and sleep modes, average power may drop dramatically, extending runtime into days.
Common mistakes to avoid
- Comparing mAh values across different voltages without converting to Wh.
- Ignoring inverter/adapter losses for AC-powered accessories.
- Using marketing capacity numbers with no safety margin.
- Assuming constant load when your device has active/idle cycles.
Battery life optimization checklist
- Lower screen brightness and radio usage (for mobile devices).
- Use efficient DC-DC converters matched to your load range.
- Reduce always-on background tasks.
- Enable sleep states and wake-on-event behavior.
- Choose battery chemistry and cell count based on actual power profile.
Final note
Battery life estimates are only as good as the assumptions behind them. Start with conservative values for efficiency and usable capacity, test in real conditions, then refine inputs. That process gives you planning numbers you can trust.