Battery Life Calculator
Estimate battery runtime, required battery size, supported load power, or supported load current for power banks, 12V batteries, solar batteries, UPS systems, electronics, and IoT devices.
Calculator is for informational purposes only. Terms and Conditions
Choose what to solve for
Pick the output, battery preset, and load input type.
Enter the known values
The calculator converts everything to watt-hours and watts internally.
Visual Check
See the relationship between battery energy, usable energy, load, and runtime.
Solution
Live result, quick checks, warnings, and full solution steps.
Quick checks
- Check—
Show solution steps See the equation, substitutions, assumptions, and result path
- Enter values to see the full solution steps and checks.
Source, Standards, and Assumptions
Calculation basis, constants, assumptions, and limitations.
Source/standard information updates after a valid calculation.
- Assumptions will appear after a valid calculation.
On this page
Calculator Guide
How to Use the Battery Life Calculator
The Battery Life Calculator above estimates how long a battery will run a device, or how much battery capacity is needed for a target runtime. Enter battery capacity, voltage, and load current or power, then use the advanced options to account for usable capacity, converter efficiency, reserve margin, and self-discharge.
Battery life is usually a comparison between usable stored energy and effective load. A simple calculator can use \(mAh \div mA\), but the most reliable estimate uses watt-hours, average power, and realistic derating for battery chemistry, age, temperature, and voltage cutoff.
Quick Answer
For an ideal current-based estimate, battery life in hours is \( \text{capacity} \div \text{current draw} \). For a power-based estimate, convert battery capacity to watt-hours and divide by load watts. If the device sleeps, cycles, or uses a voltage converter, calculate the average load first and apply realistic efficiency and usable-capacity factors.
Do not rely on the ideal result when…
Do not treat the simplified runtime as a guaranteed battery life for medical devices, safety systems, emergency backup, field equipment, solar storage, code-related electrical work, or any application where failure has safety, financial, or operational consequences. Confirm the design with manufacturer data, discharge curves, temperature limits, voltage cutoff requirements, and qualified engineering judgment.
Inputs and Outputs Used by the Calculator
A battery life calculator needs battery capacity, battery voltage, and the load demand. The load can be entered as current, power, or an active/sleep profile depending on what information you have.
| Type | Value | What It Means | Common Unit |
|---|---|---|---|
| Input | Battery capacity | The rated charge or energy stored in the battery before derating. | mAh, Ah, Wh |
| Input | Battery voltage | Nominal voltage used to convert charge capacity into stored energy. | V |
| Input | Current draw | The average current the device pulls from the battery. | mA, A |
| Input | Power draw | The average load power when the device rating is given in watts instead of current. | mW, W |
| Input | Active/sleep profile | A duty-cycle method for devices that alternate between high-current active mode and low-current sleep mode. | mA and time |
| Output | Battery life | The estimated runtime after the selected assumptions are applied. | hours, days, weeks, months |
| Output | Required capacity | The battery size needed to reach a target runtime. | mAh, Ah, Wh |
How the calculator modes work
In Battery Life mode, enter the battery capacity and the calculator estimates runtime. In Required Battery Capacity mode, enter the target runtime and the calculator estimates the battery size needed. Use Current Draw mode when the load is known in \(mA\) or \(A\), Power Draw mode when the load is known in \(mW\) or \(W\), and Active / Sleep Profile mode when the device cycles between high-current and low-current states.
Practical insight
If you know current draw and battery capacity at the same voltage, the \(mAh \div mA\) shortcut is useful. If you know load watts, compare energy to power using watt-hours. For related electrical load planning, the Electrical Load Calculator can help with larger connected-load estimates.
Battery Life Calculator Formula
The simplest battery life formula divides capacity by average current. A more complete runtime estimate converts capacity to usable energy and divides by the effective load power.
Simple current-based runtime
Use this when capacity and current use matching charge units, such as \(mAh\) and \(mA\), or \(Ah\) and \(A\).
Energy-based runtime with real-world factors
In this formula, \(P_{eff}\) means the average load-side power demand before converter losses. If your measured power already includes converter or inverter losses, set converter efficiency to \(100\%\) to avoid double-counting losses.
Capacity and energy conversions
Active and sleep average current
This is the key formula for microcontrollers, sensors, remote devices, and IoT equipment that only draw high current for a short part of each cycle.
Required battery capacity
What the Battery Life Variables Mean
Each variable represents either stored battery capacity, load demand, or a real-world adjustment. The more accurately you estimate the load and usable capacity, the more useful the runtime result becomes.
\(t\)
Runtime or battery life. This is usually reported in hours, then converted to days, weeks, or months when the result is long.
\(C\)
Battery charge capacity. Use \(mAh\) with \(mA\), or \(Ah\) with \(A\), to avoid scale errors.
\(I\) or \(I_{avg}\)
Average current draw. This is usually the most important input because runtime is inversely proportional to current.
\(E_{Wh}\)
Stored energy in watt-hours. Use watt-hours when comparing batteries at different voltages or loads rated in watts.
\(P_{eff}\)
Effective load power. This can come from a device label, measurement, or conversion from current and voltage using \(P=VI\).
\(\eta_{conv}\), \(r\), \(s\), and \(m\)
Converter efficiency, reserve margin, monthly self-discharge rate, and storage duration in months. These factors reduce ideal runtime.
How to Use the Calculator
Start with the answer you need: battery life or required battery capacity. Then choose the load input mode that matches the data you actually know.
Select the solve mode
Choose Battery Life when you already know the battery size. Choose Required Battery Capacity when you know the target runtime and need to size the battery.
Choose current, power, or active/sleep mode
Use current draw for \(mAh\) and \(mA\) problems. Use power draw for \(Wh\) and watts. Use active/sleep profile for devices that cycle between operating modes.
Enter battery voltage and capacity
Voltage is required when converting between \(mAh\), \(Ah\), and \(Wh\). Do not compare two \(mAh\) ratings unless the battery voltages are the same.
Set advanced assumptions
Use battery chemistry, usable capacity, converter efficiency, reserve margin, and self-discharge to make the result more realistic.
Review the result and quick checks
Compare battery energy, usable energy, effective current, effective power, and the calculation steps. If the result looks too optimistic, reduce usable capacity or increase the reserve margin.
How to Interpret Battery Life Results
The output is an estimate of runtime under the load and assumptions entered. It is not a guarantee, because real batteries have discharge curves, cutoff voltages, temperature sensitivity, aging, and rate-dependent behavior.
What to do with the result
Use the result to compare battery sizes, check if a target runtime is realistic, or estimate how much reserve capacity is needed before buying or designing a pack.
What changes the result most?
Average load current or power usually dominates the result. Doubling the average load approximately cuts runtime in half.
Quick sanity check
A 2000 mAh battery powering a 100 mA load gives about 20 hours before derating. If your result is far from that ratio, check units first.
Low-voltage systems need extra caution
In 12 V, 24 V, solar, RV, LED, and battery backup systems, conductor voltage drop can reduce usable load voltage. If your battery is far from the load, check wiring losses with the Voltage Drop Calculator.
Input Checklist Before You Trust the Answer
Most battery life errors come from entering peak current instead of average current, confusing mAh with Wh, or assuming 100% of rated capacity is usable.
Use average load
Runtime depends on average current or average power over time, not only the maximum current printed on a label.
Confirm voltage
Use nominal battery voltage for energy conversion. A 5000 mAh 3.7 V battery stores much less energy than a 5000 mAh 12 V battery.
Derate usable capacity
Many batteries should not be treated as if 100% of the nameplate capacity is available for your load.
Check converter efficiency
Boost converters, buck converters, inverters, and USB power banks all lose energy during voltage conversion.
Watch decimal scale
One amp equals 1000 milliamps. Entering \(0.25\) with the unit set to \(mA\) means \(0.25\,mA\), not \(0.25\,A\). For \(0.25\,A\), enter \(250\,mA\) or select \(A\) and enter \(0.25\).
Account for standby loads
Small sleep currents can dominate long-duration devices if the device spends days, weeks, or months in standby.
Battery Life Worked Example
This example uses the most common runtime question: how long will a battery last when capacity and current draw are known?
Formula
Substitution
Calculation
Final answer
The estimated battery life is about 16 hours after applying an 80% usable-capacity factor. The ideal result is 20 hours, so 16 hours is reasonable for a conservative real-world estimate.
Power-based check
The same battery stores \(E=\frac{5000\times3.7}{1000}=18.5\,Wh\). Applying 80% usable capacity gives \(14.8\,Wh\). The load power is \(P=VI=3.7\times0.25=0.925\,W\), so \(14.8\div0.925=16\,hr\), matching the current-based result.
More quick examples
Small device
A \(2000\,mAh\) battery at \(100\,mA\) gives \(2000\div100=20\,hr\) ideal runtime before derating.
12 V battery
A \(12\,V\), \(100\,Ah\) battery stores \(1200\,Wh\). A \(50\,W\) load gives \(1200\div50=24\,hr\) ideal runtime.
IoT duty cycle
If a device uses \(100\,mA\) for 10 seconds and \(1\,mA\) for 90 seconds, \(I_{avg}=10.9\,mA\).
How to Visualize the Battery Life Calculation
Battery life is easiest to understand as a simple flow: start with the battery’s stored energy, reduce it for real-world losses, then divide the usable energy by the load. This is more useful than a decorative diagram because it shows exactly where runtime is gained or lost.
Start with battery capacity
Use the battery rating from the label or datasheet. If the battery is listed in \(mAh\) or \(Ah\), convert it to watt-hours when the load is given in watts.
Reduce for usable capacity and losses
The full nameplate capacity is rarely available in real use. Battery chemistry, reserve margin, converter efficiency, temperature, battery age, and cutoff voltage can all reduce usable energy.
Divide by the average load
Runtime is controlled by the average load, not just the peak load. For cycling devices, calculate active and sleep current first, then use the average value.
What this means in practice
A larger battery increases runtime, but higher load power, lower converter efficiency, colder temperatures, aging, and reserve margin all reduce runtime. If the result seems too optimistic, the first inputs to check are average load current, usable capacity, and converter efficiency.
Battery Life Reference Checks
Battery runtime ranges vary widely by chemistry, load, temperature, and cutoff voltage, so fixed reference values should be treated as reasonableness checks instead of design guarantees.
| Scenario | Quick Check | Why It Helps |
|---|---|---|
| Small electronics | \(2000\,mAh \div 100\,mA \approx 20\,hr\) ideal | Good first check for USB devices, sensors, and small DC loads. |
| 12 V battery load | \(100\,Ah \times 12\,V = 1200\,Wh\) | Converts amp-hours into energy before dividing by watts. |
| Power-bank comparison | Compare \(Wh\), not just \(mAh\) | mAh ratings only compare directly at the same voltage. |
| Long standby device | Check sleep current and self-discharge | Microamp or milliamp standby loads can dominate month-long estimates. |
Manufacturer data matters
For final sizing, use the battery datasheet discharge curve at the expected current, temperature, cutoff voltage, and end-of-life condition. Rated capacity is often measured under specific test conditions that may not match your application.
Design Notes and Practical Battery Sizing Ranges
For a practical battery design, the calculated runtime should normally include margin. A system sized to exactly match the ideal runtime may fail early when the battery ages, gets cold, powers a higher load, or reaches cutoff voltage sooner than expected.
Usable capacity
Lead-acid systems are often derated more heavily than lithium systems because deep discharge can shorten life. Lithium batteries may allow more usable capacity but still need cutoff and protection limits.
Reserve margin
A reserve margin helps cover battery aging, manufacturing tolerance, cold weather, underestimated load, and future load additions.
C-rate and high current
If the load current is large compared with battery capacity, the battery may deliver less usable capacity than expected. A 1C discharge means the battery would discharge in about one hour under ideal conditions.
Low-current devices
For very long runtimes, leakage current and self-discharge can matter as much as the main load current.
Battery pack configuration note
Cells in series increase voltage. Cells in parallel increase amp-hour capacity. Total pack energy should be checked in watt-hours using the full pack voltage and full pack amp-hour rating.
Battery Life Units and Conversions
Battery calculations are unit-sensitive. The most common mistake is treating \(mAh\) as energy when it is actually charge. Voltage is needed to convert charge into watt-hours.
Current and charge units
Power and energy units
Hidden unit trap
A 10,000 mAh battery is not automatically “twice as much energy” as a 5000 mAh battery unless both ratings use the same voltage. Convert both to watt-hours before comparing different battery types or voltage levels.
mAh vs Ah vs Wh for Battery Runtime
Use \(mAh\) or \(Ah\) when capacity and load current are measured at the same voltage. Use \(Wh\) when comparing batteries at different voltages or when the load is rated in watts.
mAh
Best for small electronics and same-voltage comparisons. It measures charge, not total energy by itself.
Ah
Common for larger batteries such as 12 V, 24 V, and deep-cycle systems. Convert to Wh with \(Ah \times V\).
Wh
Best for runtime from watts, power banks, laptops, solar storage, and cross-voltage comparisons.
When current, resistance, and voltage relationships are part of the same circuit, the Voltage Divider Calculator can help evaluate simple resistor-divider behavior, while RMS and AC applications may require the RMS Voltage Calculator.
Common Battery Life Calculation Mistakes
Battery runtime errors are usually caused by using the wrong load value, wrong unit scale, or ideal assumptions that do not match the actual battery system.
Do
- Use measured average current when possible.
- Convert \(mAh\) to \(Wh\) when the load is given in watts.
- Apply usable-capacity and efficiency factors for real systems.
- Check active and sleep current separately for cycling devices.
- Verify the battery voltage used for the conversion.
Don’t
- Do not confuse \(mA\) with \(A\).
- Do not assume nameplate capacity is fully usable.
- Do not ignore converter or inverter losses.
- Do not compare \(mAh\) ratings across different voltages.
- Do not use peak current as the average load unless it is always on.
Troubleshooting Unrealistic Battery Life Results
If the answer looks impossible, start with unit scale and load assumptions. A mathematically valid runtime can still be misleading if the entered current, voltage, or usable capacity does not match the real device.
Runtime is too high
Check whether current was entered in \(mA\) instead of \(A\), whether sleep current was understated, or whether usable capacity was left at 100%.
Runtime is too low
Check whether a peak startup load was used as average current, or whether a watt value was entered in milliwatts or vice versa.
Runtime is over 30 days
Check self-discharge, standby current, leakage paths, and battery shelf-life assumptions. Long runtimes are often limited by small losses.
Required capacity is huge
The target runtime may be too long for the selected battery type, or the load may need reduction, duty cycling, or a larger battery bank.
Assumptions and Limitations
This calculator is best used for estimating and comparing battery runtime, not for certifying final product performance. Real battery behavior depends on chemistry, discharge rate, temperature, age, cutoff voltage, protection circuitry, load profile, and manufacturer test conditions.
Rated capacity is conditional
Battery capacity is usually specified under defined test conditions. High current, low temperature, and aging can reduce available capacity.
Voltage is not constant
Battery voltage changes during discharge. Some devices stop working before the battery is completely empty because they reach a cutoff voltage.
Converters are not perfect
Inverters, boost converters, buck converters, and USB power electronics reduce runtime through conversion losses.
Safety-critical systems need review
Backup power, emergency equipment, field instrumentation, and product designs should be checked against manufacturer data, applicable standards, and qualified professional judgment.
Battery Life Glossary
These terms appear often when estimating battery runtime and required battery capacity.
mAh
Milliamp-hour, a charge-capacity unit often used for small batteries. It must be paired with voltage to estimate watt-hours.
Ah
Amp-hour, equal to 1000 mAh. It is common for larger batteries such as 12 V deep-cycle batteries.
Wh
Watt-hour, a unit of stored energy. It is often the best unit for comparing batteries and calculating runtime from watts.
Usable capacity
The portion of rated capacity that can realistically be used after chemistry limits, cutoff voltage, reserve, and aging are considered.
Duty cycle
The fraction of time a device spends in each operating state, such as active mode and sleep mode.
Self-discharge
Capacity lost over time even when the battery is not powering the main load.
Battery Life Calculator FAQ
How do you calculate battery life from mAh and mA?
For an ideal estimate, divide battery capacity in mAh by average current draw in mA. For example, a 2000 mAh battery supplying 100 mA gives about 20 hours before real-world losses.
Is Wh better than mAh for battery runtime?
Watt-hours are usually better when comparing batteries at different voltages because Wh measures stored energy. Milliamp-hours measure charge and only compare directly when voltage is the same.
Why is my real battery life lower than the calculator result?
Real battery life can be lower because of usable capacity limits, converter losses, temperature, battery age, voltage cutoff, high discharge rate, standby loads, and self-discharge.
How do you calculate required battery capacity?
Multiply the target runtime by the average current draw to estimate required Ah, or multiply target runtime by load watts to estimate required Wh. Then divide by usable capacity and efficiency factors if real-world derating is needed.
How do active and sleep current affect battery life?
Active and sleep current affect battery life through the average current over one full cycle. A short high-current active period may have little impact if the device spends most of its time in low-current sleep mode.