Before you calculate how long a power bank should run your laptop for, you also must take into account that the charging process isn’t perfectly efficient. For fast charging standards like USB Power Delivery, the bank has to increase its voltage to push enough power (watts) to charge your laptop quickly enough that it doesn’t drain. Some energy is lost in this changeover, as well as generally from heat. 20 percent efficiency loss is a general rule for normal charging, but around 30 percent (or even higher) is more realistic when fast charging.
With all these values, you can finally estimate how much additional time a power bank will run your laptop for. Say you connect a 20,000 mAh 45W USB-C bank to the theoretical laptop mentioned above. Subtracting 30 percent for the overhead, that 74Wh power bank has an effective capacity of ~52Wh. With a laptop averaging 14W, the power bank would extend its runtime by around 3.7 hours. If the laptop ever needs to draw more than the 45W the bank can provide, it will draw power from its internal battery alongside it — the bank is thus supplementing the internal battery to extend its life.
The important point is that you can’t assume a power bank’s capacity 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 pushes. A large-capacity bank rated at 15W will only slow the laptop’s power drain, or it could be rejected entirely. For laptop banks, the label to look for is “USB-C PD.” In simple language, this means the bank and laptop negotiate a higher charging level than an ordinary USB port.
Always check the bank’s per-port output too, because the total output number can be deceptive. For instance, 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 you’re 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 allowed on a plane without approval. if you need suggestions, our favorite laptop power banks span all needs.
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