Battery Life Calculator

Estimate how long a battery will last based on its capacity, the current draw of your device, voltage and efficiency.

Battery Life Calculator

Enter the battery's capacity and your device's current draw.
The estimated runtime updates live as you type.
For example, a 10,000 mAh power bank at 90% efficiency runs an 800 mA phone for about 11.25 hours.

Load an example

Battery settings

Battery capacity

Printed on the battery label.

Current draw

What the device pulls.

Voltage (V)

1.5 alkaline, 3.7 Li-ion, 5 USB, 12 lead-acid.

Efficiency (%)

Typically 80–95% for rechargeables.

Live mode

Recalculate automatically as you change any value. Turn this off to hold your results until you press the Calculate button.

Capacity and draw are converted to watt-hours and watts using the voltage, so entering watt-based units bypasses the voltage field for that value.

Your Battery Life Result

Estimated battery life

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Runtime (hours)

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Usable capacity

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Power draw

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Calculation breakdown Download

Fill in the battery details and press Calculate to see a live, visual breakdown of the estimated runtime.

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Overview

The Battery Life Calculator is a free online tool that estimates how long a battery will last powering a device. You enter the battery's capacity (in mAh, Ah or Wh), the current your device draws (in mA, A or W), the battery voltage and an efficiency figure, and the tool returns the estimated runtime together with the usable capacity and the actual power draw behind the result.

It is useful for electronics projects, choosing a power bank that survives a trip, comparing battery chemistries fairly, or working out whether a gadget will last a whole working day. Battery labels advertise capacity, not time, and real batteries deliver less than their rating – the efficiency field accounts for that gap. The tool is written in plain HTML, CSS and JavaScript using the site's shared Toolkit helper and Bootstrap for the layout, and every calculation happens locally in your browser – nothing you type is sent to any server.

What This Tool Does

You supply four things: the battery's capacity and its unit, the device's current draw and its unit, the battery voltage, and an efficiency percentage. The calculator then returns: • the estimated battery life, shown in minutes, hours or days depending on the size of the result; • the same runtime expressed in plain hours; • the usable (effective) capacity in watt-hours after the efficiency loss; • the device's power draw in watts.

The built-in "Load an example" link fills the form with a typical power bank and phone combination so you can see how the tool behaves in one click, and the Copy and Download buttons let you save the result as a formatted text file.

How the Calculation Works

Capacity and current are first normalised to energy and power using the voltage: milliamp-hours become amp-hours (divide by 1,000) and are multiplied by the voltage to give watt-hours, and the same conversion turns milliamps into watts on the draw side. Watt-hour and watt inputs skip the voltage step because they already describe energy and power directly. The efficiency percentage then trims the capacity down to what the battery can realistically deliver, and dividing that usable energy by the power draw gives the runtime in hours.

Battery life (hours) = (capacity in Wh × efficiency) ÷ power draw in W

The hourly result is presented in the most readable unit: under one hour it is shown in minutes, and anything over 48 hours is also expressed in days. The worked examples in the next section follow these rules exactly, and the full unit-conversion tables are listed in the Conversion Details section.

Examples

Each example below was calculated with the exact formula this tool uses, so you can type the same numbers in and reproduce the result.

1. Power bank and phone – a 10,000 mAh power bank at 3.7 V with 90% efficiency charging a phone that draws 800 mA: usable capacity is 10 × 3.7 × 0.9 = 33.3 Wh, the phone pulls 2.96 W, and the runtime is 11.25 hours.

2. LED camping lantern – four alkaline cells treated as a 2,400 mAh pack at 1.5 V with 85% efficiency powering a lantern that draws 100 mA: usable capacity is 3.06 Wh, the draw is 0.15 W, and the light lasts 20.4 hours.

3. 18650 flashlight – a 2,600 mAh Li-ion cell at 3.7 V with 90% efficiency driving a torch that draws 700 mA: usable capacity is 8.66 Wh, the draw is 2.59 W, and the high beam lasts 3.34 hours.

4. Camping fridge on a car battery – a 60 Ah lead-acid battery at 12 V with 80% efficiency running a fridge that draws 4 A: usable capacity is 576 Wh, the fridge pulls 48 W, and the battery delivers 12 hours of cooling.

5. Remote temperature sensor – a 1,500 mAh Li-ion cell at 3.7 V with 95% efficiency powering a sensor that draws just 5 mA: usable capacity is 5.27 Wh, the draw is 0.0185 W, and the runtime is 285 hours, or nearly 12 days of unattended readings.

Conversion Details

The calculator normalises every input to watt-hours and watts before dividing one by the other, so switching units never changes the result – 2,600 mAh and 2.6 Ah describe the same battery.

Capacity unit conversions (at voltage V):
• 1,000 mAh = 1 Ah
• Capacity in Wh from mAh = (mAh ÷ 1,000) × V
• Capacity in Wh from Ah = Ah × V
• Wh is used as-is (already an energy unit)

Current unit conversions (at voltage V):
• 1,000 mA = 1 A
• Power in W from mA = (mA ÷ 1,000) × V
• Power in W from A = A × V
• W is used as-is (already a power unit)

Common battery voltages:
• 1.5 V – alkaline (AA, AAA, C, D)
• 3.7 V – lithium-ion and lithium-polymer cells (the default)
• 5 V – USB power banks and USB-powered devices
• 12 V – lead-acid car batteries and solar systems

Efficiency: a percentage from 1 to 100 that scales the capacity down to what the battery really delivers. Typical rechargeable cells land between 80% and 95%; leave it at 100% only if you want the theoretical maximum runtime.

FAQs

FAQ 1: Where do I find the capacity and current draw?
Answer: Capacity (mAh or Ah) is printed on the battery itself or in its specifications; power banks often state it in both mAh and Wh. The device's draw is usually on its label, power adapter or datasheet, in amps or watts. If you only know watts for the device, pick the W unit and the voltage field is not needed for that input.

FAQ 2: Why is the voltage needed if the label already shows mAh?
Answer: Milliamp-hours only tell you charge, not energy. A 10,000 mAh pack at 3.7 V stores far less energy than 10,000 mAh at 12 V. Multiplying by the voltage converts charge into watt-hours, which is what actually determines runtime.

FAQ 3: What does the efficiency percentage represent?
Answer: Real batteries lose usable capacity to internal resistance, conversion electronics and discharge-rate limits, so they rarely deliver their full rated energy. The efficiency factor trims the capacity accordingly; 80–95% is realistic for healthy rechargeable cells, and lower values model ageing, cold weather or heavy loads.

FAQ 4: Why does my result differ from the manufacturer's advertised hours?
Answer: Marketing figures usually assume an ideal battery at a light, constant load. This calculator shows what happens with the efficiency you choose, so entering 100% reproduces the ideal case while a realistic value gives a more honest estimate.

FAQ 5: Does the result change if the device does not draw current constantly?
Answer: The calculation assumes a steady draw for every hour. For devices that sleep or cycle, use the average current: a sensor pulling 5 mA for one second every minute averages out far lower than 5 mA continuous, and entering that average is what makes the result meaningful.

FAQ 6: Is my data sent anywhere?
Answer: No. The calculator runs entirely in your browser using JavaScript, and no input or result is transmitted, logged or stored. You can even use it offline once the page has loaded.