How Big Of A Power Bank Do You Need To Fully Charge A Laptop?

Most laptops' average battery life is about six to eight hours on a full charge, but some models are just built differently. For instance, the MacBook Pro M4 has been tested to provide up to 24 hours of runtime, while Samsung Galaxy Book4 Edge can last for 22 hours at most. Although more and more high-end laptops are coming out with longer-lasting batteries, this doesn't change the fact that the laptops will still require a recharge sooner or later.

This is an easy problem to fix if you're at home, in a library, or at school, where wall outlets are readily accessible. But if you're out camping or on a road trip across the country, your source of power might be limited to a portable power bank. Here's the thing, though: just because your power bank can fully charge your phone battery a couple of times doesn't mean it can also do the same for your laptop. A laptop requires higher specs.

Estimating how big of a power bank you need to fully charge your laptop begins with knowing what the battery capacity of the two devices are. That tells you how much charge it can hold. For laptops, this is measured in watt-hour (Wh) and usually ranges from 30 to 100Wh. The higher watt-hour a laptop battery has, the longer the runtime is. While you can readily find this figure on the laptop's spec sheet, that actually isn't the current capacity your battery can hold. Batteries degrade over time, and after multiple charging sessions, there will be a noticeable drop in capacity. So unless your laptop is completely new, don't rely solely on the spec sheet (we'll walk you through how to find your laptop's current battery capacity in the next section).

Just like laptops, power banks also have a battery capacity. They are, after all, essentially a portable battery pack. This capacity is the mAh typically printed on the power bank. It stands for milliampere-hour and tells you how much charge it can store, and therefore, provide to the connected device. However, it's important to note that the advertised mAh isn't the actual output you can get. Due to energy loss within the power bank, it's only about 60 to 70% efficient. That means a 25,000mAh power bank only has a usable capacity of 15,000 to 17,500mAh.

So, with all that being said, what does the power bank's mAh even have to do with your laptop's Wh? Well, the power bank capacity you need to pick will largely depend on how big the laptop battery capacity is and how many times you want to fully charge it. Let's go over how to calculate the required power bank mAh.

To know what power bank capacity you need to buy for your laptop, here's what you need to do:

• Find your Windows laptop's current battery capacity:
  1. Launch the Terminal app.
  2. Type powercfg /batteryreport into the command line.
  3. Hit Enter.
  4. Open the generated HTML file on your browser.
  5. Take note of the Full Charge Capacity under Installed batteries.
  6. Convert this to Wh by dividing by 1000.
• Look up what battery capacity your MacBook is at present:
  1. Visit the Apple Support Documentation page on your MacBook.
  2. Search for your MacBook model.
  3. Click the result with the label "Technical Specifications."
  4. In the Battery section, check for the watt-hour value.
  5. Check the MacBook's battery health by clicking the Hold + Apple menu.
  6. Select System Information.
  7. Go to Power on the left side panel.
  8. Record the Maximum Capacity under Health Information.
  9. To know your MacBook's current battery capacity, multiply the Maximum Capacity by the watt-hour value on the spec sheet. Then, divide it by 100. So, if your watt-hour value is 72.4 and the Maximum Capacity is 93%, the current battery capacity is 67.33Wh.
• Calculate how big of a power bank you need to fully charge your laptop at least once:
  1. Multiply your laptop battery's Wh by how many full charges you want to squeeze out of the power bank.
  2. To get the power bank capacity in Wh, divide the result to 0.6 (or 60%), which is the usual minimum efficiency most power banks deliver. This formula came from rearranging the original formula Output = Capacity x Efficiency.
  3. Convert Wh to mAh with the calculator from Inch Calculator. Simply input the Wh from your previous calculation and use 3.7 for the Volts (the internal battery of power banks are normally rated at 3.7V).

The mAh result you get tells you the smallest power bank capacity you should consider when shopping. So if you have a 40Wh laptop battery, you'll need at least an 18,000mAh power bank to fully charge it once and 36,000mAh to fully charge it twice.