Power banks provide their capacities in milliamp-hours, but for a proper measure of capacity across devices, you need to multiply this value by the voltage to get the true amount of energy stored in watt-hours.
Doubling the mAh doubles the Wh as long as voltage stays consistent, so a 20,000 mAh bank is about 74Wh.
For fast charging standards like USB Power Delivery, the bank has to increase its voltage to push enough power to charge your laptop quickly enough that it doesn't drain.
With a laptop averaging 14W, the power bank would extend its runtime by around 3.7 hours.
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Don’t let the big mAh number fool you. While most good power banks can theoretically extend your laptop's runtime by providing a small amount of charge.
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While most good power banks can theoretically extend your laptop's runtime by providing a small amount of charge, a proper laptop power bank will store enough energy and charge faster than the battery drains. Power banks provide their capacities in milliamp-hours, but for a proper measure of capacity across devices, you need to multiply this value by the voltage to get the true amount of energy stored in watt-hours.
Watts are equal to volts times amperes, and you must divide the mAh count by 1,000 since 1,000 mAh is one amp. Doubling the mAh doubles the Wh as long as voltage stays 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 be able to find on the manufacturer's website.
Using modern MacBooks as an illustration, their batteries are rated from ~37Wh up to 100Wh.
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 efficient. For fast charging standards like USB Power Delivery, the bank has to increase its voltage to push enough power to charge your laptop quickly enough that it doesn't drain.
With all these values, you can estimate how much more time a power bank will run your laptop for. With a laptop averaging 14W, the power bank would extend its runtime by around 3.7 hours.
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