Battery Runtime Calculator
Enter your battery capacity and your device’s current draw or power consumption to estimate the runtime in hours and minutes.
Reviewed by the ToolNestr Editorial Team — July 2026
How battery runtime is estimated
Battery runtime is estimated by dividing the total stored energy by the device power consumption. The formula is: Runtime (hours) = Capacity (Ah) × Voltage (V) × Efficiency / Power (W). The battery capacity in watt-hours (Wh) is calculated as (mAh × V) / 1000 or simply Ah × V.
Real-world runtime is typically 10–30% shorter than theoretical estimates due to the Peukert effect (reduced capacity at high discharge rates), temperature effects, battery age, and system inefficiencies. The efficiency factor accounts for these combined losses.
Worked example
What different battery lifetimes mean
The expected lifetime of a battery depends on its chemistry, capacity and the device it powers. Typical runtimes for common devices vary widely.
Electronics Hobbyist
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EV Owner
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Solar System Designer
Size battery banks correctly for off-grid solar installations by calculating total load requirements and desired backup time.
Product Developer
Design battery-powered consumer products with accurate runtime estimates and optimal battery capacity selection.
| Device | Battery capacity | Typical runtime | Chemistry |
|---|---|---|---|
| Smartphone | 3000 – 5000 mAh | 1 – 2 days | Li-ion |
| Laptop | 40 – 100 Wh | 4 – 10 hours | Li-polymer |
| Power drill | 2 – 6 Ah | 1 – 4 hours | Li-ion |
| Car battery | 45 – 75 Ah | Minutes (starter) | Lead-acid |
| Power bank | 5000 – 30000 mAh | 1 – 10 charges | Li-ion |
| Smartwatch | 200 – 500 mAh | 1 – 3 days | Li-ion |
How to use the Battery Runtime Calculator
Enter battery specs
Input your battery capacity and voltage. Most batteries list these directly on the label or in the datasheet.
Enter device power
Provide either the power draw in watts or current draw in mA. Using power (W) gives more accurate results as it accounts for voltage.
Set efficiency factor
Adjust the system efficiency slider to account for real-world losses. Use 85% as a starting point for most systems.
Tips for accurate runtime estimation
Use watt-hours (Wh) for voltage-independent comparisons
When comparing batteries of different voltages, always convert to Wh. A 12V 10Ah battery (120 Wh) has nearly the same energy as a 24V 5Ah battery (120 Wh). Wh is the true measure of energy capacity.
Account for the Peukert effect in lead-acid batteries
Lead-acid batteries lose capacity at high discharge rates. A battery rated at 100Ah for a 20-hour discharge may only deliver 60Ah if discharged in 1 hour. Lithium batteries are much less affected.
Include inverter losses for AC devices
If you are powering an AC device from a battery through an inverter, add 10-15% to your power draw to account for inverter conversion losses. Pure sine wave inverters are more efficient than modified sine wave.
Consider depth of discharge limits
For lead-acid batteries, never discharge below 50% to maximize cycle life. For lithium, 80-100% DoD is acceptable. Adjust your usable capacity accordingly when sizing a system.
Battery capacity explained: mAh vs Wh
Battery capacity is the total charge or energy a battery can store. It is expressed in milliampere-hours (mAh) or ampere-hours (Ah) for charge, and watt-hours (Wh) for energy. The relationship is Wh = (mAh \u00d7 V) / 1000. A 5000 mAh battery at 3.7 V stores 18.5 Wh of energy. Watt-hours are more useful for comparing batteries of different voltages because they represent total energy regardless of voltage.
Factors affecting runtime
Many factors reduce runtime below theoretical calculations. Device power draw varies during operation \u2014 a laptop uses more power while gaming than while idle. Battery internal resistance causes voltage sag under load, triggering low-voltage cutoffs earlier. Temperature extremes reduce capacity. Battery age and cycle count progressively degrade capacity. Our efficiency factor helps approximate these combined losses.
Efficiency losses in battery systems
Energy is lost as heat in wires, connectors, and the battery\u2019s own internal resistance. Voltage converters (boost/buck regulators) typically achieve 80-95% efficiency. Inverters for AC loads add 10-15% loss. The battery management system (BMS) also consumes a small amount of power. A well-designed DC system can achieve 90%+ overall efficiency.
Series vs parallel battery configurations
Connecting batteries in series (positive to negative) adds voltages while capacity stays the same: two 12V 100Ah in series give 24V 100Ah. Connecting in parallel (positive to positive, negative to negative) adds capacities while voltage stays the same: two 12V 100Ah in parallel give 12V 200Ah. For high-power applications, a series-parallel combination can provide both higher voltage and higher capacity.
Frequently asked questions
How is battery runtime calculated?
Runtime is calculated by dividing the battery capacity by the device current draw. For example, a 5000 mAh battery powering a device drawing 500 mA will last 10 hours. When using power in watts, runtime = battery energy (Wh) ÷ device power (W).
What is the difference between mAh and Wh?
mAh (milliampere-hours) measures charge capacity, while Wh (watt-hours) measures energy capacity. Wh = (mAh × V) ÷ 1000. Wh gives a more accurate representation of total energy regardless of voltage.
Why does my battery run out faster than calculated?
Real-world runtime is shorter due to efficiency losses, Peukert effect (batteries deliver less capacity at high discharge rates), temperature effects, battery age, and device power fluctuations. Our calculator includes an efficiency factor to account for these.
What is the Peukert effect?
The Peukert effect describes how a battery’s usable capacity decreases as the discharge rate increases. A battery rated at 100 Ah over 20 hours may only deliver 70 Ah if discharged in 1 hour. This is significant for lead-acid batteries but less so for lithium-ion.
How does temperature affect battery runtime?
Cold temperatures increase internal resistance and reduce usable capacity. At 0°C, a lithium battery may deliver only 80% of its rated capacity. Extreme heat accelerates chemical degradation and permanent capacity loss.
Does battery age affect runtime?
Yes. Batteries lose capacity with each charge cycle. A lithium-ion battery typically retains about 80% of its original capacity after 500 full cycles. This degradation is caused by chemical changes inside the battery.
What is the difference between series and parallel battery configurations?
Connecting batteries in series increases voltage while keeping capacity the same. Connecting in parallel increases capacity while keeping voltage the same. For example, two 12V 100Ah batteries in series give 24V 100Ah; in parallel they give 12V 200Ah.
How do I estimate runtime for devices with variable power draw?
Use the average power consumption for a rough estimate. For more accuracy, measure the current draw over time with a multimeter or power monitor and calculate the average. Devices like motors and processors have widely varying draw.
What is a battery’s depth of discharge (DoD)?
Depth of discharge is the percentage of battery capacity that has been used. For lead-acid batteries, limiting DoD to 50% significantly extends lifespan. Lithium batteries can handle up to 80-100% DoD with less impact on cycle life.
How accurate are battery runtime calculators?
Calculators provide a theoretical estimate based on ideal conditions. Real-world runtime is typically 10-30% less due to efficiency losses, discharge rate effects, and environmental factors. Use the result as a guideline and add a safety margin.
Sources & references
This tool uses standard formulas and reference values from:
- • NFPA 70 — National Electrical Code (NEC), conduit fill, box fill and conductor ampacity. nfpa.org
- • IPC-2221, Generic Standard on Printed Board Design (trace width / current). ipc.org
- • NIST reference constants and unit definitions; IEEE standards where applicable. nist.gov
For educational and preliminary use. Verify against full engineering calculations and the governing standard before any real design.