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Laptop Battery Life Calculator Runtime from Wh and watts

Enter the battery’s watt-hours, its health and how many watts the laptop draws. You get the runtime now, and what a fresh battery would give back.

Health worked out from design and full charge capacity
Second mode for comparing an old and a new battery
Inputs match the Windows battery report

Built by Marcus Reed, laptop and desktop repair technician

Battery life calculatorhours = Wh × health ÷ watts

What do you know?

Battery capacityWh on the battery label or design capacity in the battery report

Average power drawWatts the laptop uses while you work

Or work out watts: energy drainedA row from Battery usage in the battery report

mWh

Over how longThat row’s duration

min

Stop atCharge left when you plug in

Battery health nowOr fill in the two report numbers below

Design capacityFrom the battery report, any unit

Full charge capacitySame unit as design capacity

Example

Estimated runtime

–

Runtime moves with screen brightness, workload and power mode, so the result is only as good as the watts you enter. Health is full charge capacity divided by design capacity.

6 hfrom 54 Wh at 9 W, full health
4 h 30 mfrom the same battery at 75% health
under 50%of the original runtime: one of Dell’s replace signs
2capacities in powercfg /batteryreport

Quick answer

Laptop battery life in hours is the battery’s capacity in watt-hours, times its health, divided by the watts the laptop draws. A 54 Wh battery at 100% health running at 9 W lasts about 6 hours; the same battery at 75% health lasts about 4 and a half. Enter your numbers in the calculator above, or switch to the second mode to see what a new battery would give back.

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Where each number comes from

Capacity (Wh). Printed on most laptop batteries, and listed as design capacity in the Windows battery report. To create the report, open Command Prompt, run powercfg /batteryreport and open the HTML file it saves; Microsoft documents the command on its powercfg command-line options page. If your battery is rated in mAh instead, the mAh to Wh calculator converts it.

Health. Dell defines design capacity as the most charge a new battery can store and full charge capacity as what it can hold now. Health is the second divided by the first. Type both numbers from the report into the calculator and it fills in the percentage.

Power draw (W). This is the number people guess wrong most often, because it changes with brightness and workload. If you don’t know it, use the second mode: it works from the hours you actually get, which already include your own habits.

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Measure your laptop’s real power draw

Guides that list typical figures, such as 10 to 15 W for browsing, give you someone else’s laptop. The Windows battery report already holds yours. Its Battery usage section lists recent stretches on battery, each with how long it ran and how much energy it drained in mWh.

Create the report with powercfg /batteryreport and open the HTML file.
Scroll to Battery usage and pick a row that looks like a normal session for you, ideally 30 minutes or longer.
Type that row’s mWh and its duration in minutes into the “work out watts” fields of the calculator. It fills in the power draw.

The math is energy divided by time. TechRepublic’s walk-through of the report shows a session of 1 hour 16 minutes that drained 13,142 mWh: 13.142 Wh over 1.27 hours is about 10.4 W. With that number, the runtime estimate describes how you actually use the machine, not a lab test.

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Battery health and capacity on a Mac

Macs don’t have the Windows report, but the same three numbers are easy to find:

Capacity in Wh. The Tech Specs page for your model on Apple’s site lists the battery in watt-hours.
Health. In System Settings, open Battery and click the info button next to Battery Health to see maximum capacity as a percentage. Enter it as battery health.
Cycle count. Hold Option, click the Apple menu, choose System Information, then Power. Apple’s battery cycle count page lists 1,000 cycles for recent Mac laptops and says the battery is designed to keep up to 80% of its original capacity at that count.

Real wear varies a lot with heat and charging habits. In a 2026 r/GamingLaptops thread asking owners for their battery health, one wrote “18 months in, plugged in every gaming session, and it’s at 66%.” In another thread, an owner who had kept the charge limited to 80% for a little over two years reported 97.8% health after 28 cycles. If your laptop stays plugged in most of the day, look for a charge limit setting in the maker’s app or the BIOS.

Buying a used or refurbished laptop? Run the battery report in the first days, while the warranty or return window is open. Refurbished buyers on r/laptops post their reports asking whether the numbers are normal, and the health figure is the answer: divide full charge capacity by design capacity, as the calculator does.

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Will a new battery be worth it?

The second mode answers the question behind most battery searches. If a full charge lasts 3 hours at 70% health, a new pack at 100% should last about 4 hours 17 minutes for the same use, since runtime scales with usable capacity. If the gain is small, the battery isn’t the main problem: something is keeping the processor busy, and that is a Task Manager problem before it’s a parts problem.

If the numbers do point to a new pack, laptop battery replacement covers testing the old one first and matching the part by voltage, capacity and connector.

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Small changes that move the result

Because runtime is capacity divided by watts, anything that lowers the draw stretches the battery: a dimmer screen, closing apps that run in the background, and a power mode set for battery life. Windows 11 lists power mode under Settings, System, Power & battery. Each watt saved on a 9 W workload adds more time than the same watt on a 20 W gaming load, because it’s a bigger share of the total.

FAQ

Questions people ask

How do I find my laptop battery’s Wh rating?

Check the label on the battery or the underside of the laptop. On Windows, run powercfg /batteryreport in Command Prompt and open the HTML file it writes: the installed battery section lists the design capacity. If it shows the value in mWh, divide by 1,000 for Wh.

How do I know how many watts my laptop uses?

The easiest honest number comes from your own use. Note how many hours a full charge lasts on a normal day, then use the Hours I get now mode, which needs no watts at all. A USB-C power meter between the charger and a fully charged laptop also shows the draw directly.

What battery health means it is time to replace it?

Dell lists run time under 50% of the original as one of its replace signs, along with a Fair or Poor health status, sudden shutdowns while charge is still showing, and any swelling. A swollen battery is a stop-using-it problem regardless of the numbers.

Why did my runtime drop after a year or two?

Lithium-ion batteries hold less charge as they age, and Dell notes that run time often drops after 18 to 24 months of use. Put the design and full charge capacity from the battery report into the calculator to see how much capacity is gone.

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