Why does my laptop last only 3 hours when it is rated for 10?

A maker's headline hours come from one named test, not from your day. Apple browses 25 popular websites over Wi-Fi with the display set 8 clicks from the bottom; Lenovo runs the MobileMark office suite at 250 nits. Your own draw is whatever your screen, your apps and your background tasks add up to, and the only honest way to compare is to divide your battery's watt-hours by the watts you actually measure. Age is the second half of the answer: Apple prints that a MacBook battery is designed to keep up to 80% of its original capacity at 1000 complete charge cycles, and a pack at 80% health returns 20% fewer hours at the same draw.

Why do two laptops with the same battery size last different lengths of time?

Because the draw differs, not the battery. Lenovo prints 8.7 hours of local 1080p video on the 80Wh Legion Pro 5 and 4.7 hours on the MobileMark 30 office benchmark for the same machine, which works out at 9.2W and 17W from the identical pack. Apple's 99.6Wh MacBook Pro 16-inch prints 21 hours streaming video and 14 hours on wireless web, or 4.7W against 7.1W. Capacity sets the ceiling; the workload decides how fast you spend it.

Should I keep my laptop plugged in all the time or cycle the battery?

Modern laptops manage charging themselves, so the choice matters less than it used to. Lenovo publishes a conservation mode that keeps the charge within 75% to 80% for machines that live on a desk, and similar caps exist on other brands under different names. Apple states the design target plainly: up to 80% of original capacity at 1000 complete charge cycles. If your laptop offers a charge cap and you are mostly plugged in, turn it on; if it does not, the loss over a few years is small enough that it is not worth rearranging your habits for.

Laptop Battery Runtime Calc — Wh & Watts

7 min read

A laptop battery's runtime is its watt-hour capacity divided by the average watts it draws. Lenovo prints a 57Wh battery in the ThinkPad X1 Carbon Gen 11 and 13.95 hours on the MobileMark 25 benchmark at 250 nits. Divide one by the other and Lenovo's own figures imply an average draw of 4.1W, which is what this page loads by default: enter it and the answer comes back at 13.9 hours. Makers do not test at idle. Apple browses 25 popular websites over Wi-Fi at a set screen brightness; Lenovo runs an office application suite. What separates their hours from yours is the draw, so the useful move is to find your own watts rather than to discount their claim by a rule of thumb. The same Wh-divided-by-watts math applies to every battery chemistry, as our battery runtime formula guide explains.

Laptop battery runtime banner: a 57 Wh ThinkPad rated 13.95 hours by Lenovo, implying a 4.1 W average draw.

How Long Does a Laptop Battery Last?

Battery capacity sets the ceiling on how long a laptop runs unplugged. Every capacity below is a figure a maker prints for a shipping machine, and every column is a draw backed out of that maker's own published battery life (the arithmetic is in the next section):

Battery Size	4W (light, maker-test class)	9W (1080p video on a gaming laptop)	17W (office benchmark on a gaming laptop)
57 Wh (ThinkPad X1 Carbon Gen 11)	14.3 hours	6.3 hours	3.4 hours
70 Wh (Zenbook A14)	17.5 hours	7.8 hours	4.1 hours
72.4 Wh (MacBook Pro 14-inch)	18.1 hours	8 hours	4.3 hours
80 Wh (Legion Pro 5 16IAX10)	20 hours	8.9 hours	4.7 hours
99.6 Wh (MacBook Pro 16-inch)	24.9 hours	11.1 hours	5.9 hours

Runtimes assume a battery still holding 100% of its original capacity, so each cell is a plain division with nothing hidden in it. Apple prints that a MacBook battery is designed to keep up to 80% of its original capacity at 1000 complete charge cycles; at 80% health every figure above drops by 20%. Batteries above 100 Wh need airline approval to fly and nothing over 160 Wh is allowed in the cabin, which is why no laptop pack in the table goes higher. For external packs that extend these times, see how long a power bank lasts.

Chart of laptop power draw implied by printed battery life, from 2.1 W on a thin-and-light to 17 W on a gaming laptop. — A gaming laptop's printed productivity benchmark implies 17 W; a thin-and-light's printed video test implies 2.1 W.

How Many Watts Does a Laptop Use?

No maker publishes a wattage. They publish a battery size, a number of hours and the test they ran, and the draw falls out of the division. Each row below is one printed triple with its arithmetic:

Laptop	Battery	What the maker prints	Implied average draw
ASUS Zenbook A14 (UX3407NA)	70 Wh	over 33 hours, 1080p video playback	70 ÷ 33 = 2.1 W
Apple MacBook Pro 14-inch	72.4 Wh	24 hours video streaming	72.4 ÷ 24 = 3 W
Lenovo ThinkPad X1 Carbon Gen 11	57 Wh	13.95 hours, MobileMark 25 at 250 nits	57 ÷ 13.95 = 4.1 W
Apple MacBook Pro 14-inch	72.4 Wh	16 hours wireless web	72.4 ÷ 16 = 4.5 W
Apple MacBook Pro 16-inch	99.6 Wh	21 hours video streaming	99.6 ÷ 21 = 4.7 W
Apple MacBook Pro 16-inch	99.6 Wh	14 hours wireless web	99.6 ÷ 14 = 7.1 W
Lenovo Legion Pro 5 16IAX10	80 Wh	8.7 hours, local 1080p video at 150 nits	80 ÷ 8.7 = 9.2 W
Lenovo Legion Pro 5 16IAX10	80 Wh	4.7 hours, MobileMark 30 at 250 nits	80 ÷ 4.7 = 17 W

Two things fall out of that spread. The same 80 Wh gaming laptop implies 9.2 W playing video and 17 W running an office suite, so the machine matters less than the task. And nobody prints a figure for gaming under load, a video call or a compile, which is why this page gives you none: those draws are real but unpublished, so measure your own instead. The next section shows how, and it takes about a minute on either platform.

How to Find Your Laptop's Battery Specs

Windows: Open a Command Prompt and type powercfg /batteryreport. The generated HTML report shows your battery's design capacity in mWh. Divide by 1,000 to get Wh. It also shows current full charge capacity vs original — a measure of battery health.
Mac: Hold Option and click the Apple menu, then choose System Information. Under Power, find "Full Charge Capacity (mAh)" and voltage. Multiply mAh by voltage and divide by 1,000 for Wh. Or check Apple's spec page for your model's Wh rating.
Check real-time power draw: On Windows, install HWMonitor or check Task Manager's "Power Usage" column. On Mac, Activity Monitor shows energy impact per app. These tools reveal which apps drain your battery fastest, and a few minutes of watching the figure while you work gives you the number to type above.

Worked Examples

Reproducing Lenovo's Printed 13.95-Hour Claim

Context

Lenovo's spec sheet for the ThinkPad X1 Carbon Gen 11 prints two numbers: a 57Wh battery and up to 13.95 hours on the MobileMark 25 benchmark at 250 nits. Neither number is a wattage, so the draw has to be worked out from the pair.

Calculation

Implied draw: 57 Wh ÷ 13.95 hr = 4.09 W, so enter 4.1 W.

Runtime: 57 Wh ÷ 4.1 W = 13.90 hours.

Interpretation

13.9 hours against Lenovo's printed 13.95, a gap of about a third of a percent that comes entirely from rounding 4.09 to 4.1. The division is doing exactly what the maker's own figures say it should. Feed it a draw that matches your work rather than an office benchmark and the same division gives your answer.

Takeaway

Any published battery-life claim can be turned into a wattage this way, which makes it comparable with your own measurement. For packs quoted in amp-hours instead of watt-hours, our battery capacity calculator converts between the two.

What a Gaming Laptop Gives You Unplugged

Context

The Legion Pro 5 16IAX10 carries an 80Wh battery. Lenovo prints 4.7 hours for it on the MobileMark 30 office benchmark at 250 nits, and 8.7 hours playing local 1080p video at 150 nits. Same machine, same pack, two very different answers.

Calculation

Implied draw: 80 Wh ÷ 4.7 hr = 17.0 W.

Runtime: 80 Wh ÷ 17 W = 4.71 hours.

Same battery, Lenovo's video test: 80 ÷ 8.7 = 9.2 W, and 80 ÷ 9.2 = 8.7 hours.

Interpretation

The office workload costs 17W and the video workload 9.2W, so the same battery runs 4.7 hours or 8.7 hours depending on what you ask of it. Actual gaming draws more than either, but no maker prints a gaming figure, so this page will not invent one. What the pair does show is how far a workload alone can move the answer.

Takeaway

If you need a real number for a heavy task, measure it rather than borrow it. To model a separate power station or battery bank instead, enter its specs into our battery runtime calculator.

Frequently Asked Questions

Glossary

Watt-Hour Capacity

The total energy stored in a laptop battery. A 57Wh battery can deliver 57 watts for one hour, or 14.25 watts for four hours. Larger capacities mean longer runtime at the same power draw.

Battery Health

A percentage showing how much of the original battery capacity remains after charge cycles. At 80% health, a 57Wh battery effectively holds 45.6Wh. Most laptops show battery health in system settings or diagnostics.

Implied Power Draw

The average watts behind a maker's printed battery life, found by dividing the battery's watt-hours by the hours the maker publishes. Lenovo's 57Wh ThinkPad X1 Carbon Gen 11 at 13.95 hours implies 4.1W; the 80Wh Legion Pro 5 at 4.7 hours on an office benchmark implies 17W. It is the one wattage figure on this page that comes from a published number rather than a guess.

Calculate what your laptop costs to run per month with the kWh calculator.

The number this page gives you is only as good as the wattage you feed it, and that is the figure worth spending five minutes on. Take a battery report, watch the draw while you do your normal work, then come back and enter it. Need to convert your laptop's mWh rating to standard Wh or Ah? Our Wh to Ah converter handles the math. If you are building a portable power station from individual cells, the battery pack calculator sizes the configuration.

More Battery calculators

Browse all battery calculators — LiFePO4, lead-acid, AGM, and lithium-ion runtime, charge time, and capacity sizing.

Last updated: September 22, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.

Methodology reviewed by Doc. dr. sc. Damir Topić, Assistant Professor, FERIT Osijek.