Don’t let the large mAh number fool you. The better comparison is watt-hours.
While most good power banks can theoretically extend your laptop’s runtime by providing a small amount of charging, a real laptop power bank will store enough energy and charge faster than the battery drains. Two values explain the experience you have: Watt-hours (Wh) for capacity and Watts (W) for output. Power banks provide their capacities in milliamp-hours (mAh), but for a true measure of capacity across devices (especially for a higher drain device like a laptop), you need to multiply this value by the voltage to get the correct amount of energy stored in watt-hours.
Most power banks store energy in lithium-ion cells at about 3.7 volts (V). Watts equals volts times amps, and you need to divide the mAh count by 1,000 because 1,000 mAh is one amp. So for example, a 10,000 mAh bank stores about 37Wh (3.7V x 10Ah) before conversion loss. Doubling the mAh doubles the Wh as long as the voltage remains consistent, so a 20,000 mAh bank is about 74Wh.
Next, you need to know the Wh rating of your laptop’s battery, which you should find on the manufacturer’s website. Without a wattmeter, it’s hard to know how much power your laptop uses on average, so we can estimate it by dividing the Wh capacity by how many hours the battery lasts. For example, a 70Wh battery that runs for 5 hours averages 14W power consumption. Of course, actual power consumption differs based on what you’re doing (scrolling a PDF uses much less energy than rendering video).
Using modern MacBooks as an illustration, their batteries are rated from ~37Wh (MacBook Neo) to 100Wh (16-inch MacBook Pro). That means charging your MacBook with a 10,000 mAh (37Wh) bank won’t even provide a full top-up in most cases. And the capacity doesn’t tell you how fast the charging happens.
Understand what your power bank is capable of
Before calculating how long a power bank should run your laptop, you also need to consider that the charging process is not perfectly efficient. For fast charging standards like USB Power Delivery, the bank needs to increase its voltage to push enough power (watts) to charge your laptop fast enough that it doesn’t drain. Some energy is lost in this conversion, as well as heat in general. 20 percent efficiency loss is a general rule of thumb for normal charging, but around 30 percent (or even higher) is more realistic when charging fast.
With all these values you can finally estimate how much additional time a power bank will run your laptop. Say you connect a 20,000 mAh 45W USB-C bank to the theoretical laptop mentioned above. Subtracting 30 percent for the overhead, the 74Wh power bank has an effective capacity of ~52Wh. With a laptop with an average of 14W, the power bank would extend its runtime by approximately 3.7 hours. If the laptop ever needs to draw more than the 45W the bank can provide, it draws power from its internal battery next to it – the bank therefore supplements the internal battery to extend its life.
The important point is that you cannot assume that the capacity of a power bank is the same as its output. A portable 50,000 mAh battery is great, but that capacity doesn’t tell you how much power it’s pushing. A large capacity bank rated at 15W will only slow down the laptop’s power drain, or it could be completely rejected. For laptop banks, the label to look for is “USB-C PD.” In simple language, this means that the bank and the laptop negotiate a higher charging level than a normal USB port.
Always check the bank’s per-port output as well, as the total output figure can be misleading. For example, if the bank description says “100W total”, it may not be capable of this full output on any given port, or it may deliver less than 100W when charging a phone and a laptop at the same time.
For most people, a 20,000 mAh / 74Wh bank is a reasonable emergency or partial charge option, while a 27,000 mAh / 100Wh is closer to the sweet spot. In the US, 100Wh is also the limit for power banks that are allowed on an aircraft without approval. if you need suggestions, our favorite laptop power banks span all needs.
Why can’t my power bank charge my laptop?
Similar to plugging your laptop into your car’s USB port, a weak power bank can provide power to your laptop, but not as quickly as the laptop’s battery drains under use. There are several other possibilities. Check if your laptop supports charging via USB-C. Some USB-C ports handle data, video or other accessories, but cannot run a charger. If there is no indication on the ports themselves, check the manual if you still have it or check the product specifications online.
Next, check if the bank supports USB-C Power Delivery at a high enough per-port wattage. A basic 15W USB-C output can charge a phone, but won’t make a dent in a laptop drawing 65W+. If the output of the bank is below the needs of the laptop, the computer will either refuse it or only charge under light use after warning you of a “slow charge”.
Finally, check the cable, as not every USB-C cable is rated to carry high-wattage power. A standard USB-C cable generally supports up to 3 amps, while higher power 100W and Extended Power Range connections require a 5 amp rated cable. USB PD 3.1 can provide up to 240W, but only if the laptop, bank and cable all support the required power level.
As long as the hardware is compliant, your USB-C PD devices negotiate a compatible voltage and current. Avoid anything that’s damaged or ancient cables you’ve pulled out of a cardboard box—those won’t handle today’s power needs.