Engadget Guide: Watt-Hours and Per-Port Watts, Not mAh, Determine Laptop Power Bank Performance

Power banks list capacity in milliamp-hours, but Engadget said that figure must be multiplied by voltage to get watt-hours. Most lithium-ion cells in power banks operate at about 3.7 volts. Because watts equal volts times amperes, and 1,000 milliamp-hours equal one amp-hour, a 10,000 mAh bank stores roughly 37Wh before conversion loss. A 20,000 mAh bank is about 74Wh if voltage stays consistent.

The next step is to find the laptop battery's watt-hour rating, which Engadget said should be available on the manufacturer's website. Without a wattmeter, average power use can be estimated by dividing the battery's Wh capacity by the hours it lasts. A 70Wh battery that runs for five hours averages about 14W, though instantaneous power varies with tasks such as scrolling a PDF or rendering video. Modern MacBook batteries range from about 37Wh to 100Wh, meaning a 10,000 mAh, 37Wh bank will not fully top up most MacBooks, and capacity alone does not show charging speed.

Engadget also cautioned that charging is not perfectly efficient. Under fast-charging standards such as USB Power Delivery, a bank must raise its voltage to push enough watts into a laptop. Some energy is lost in that conversion and as heat. The guide cited a general rule of 20 percent efficiency loss for normal charging, but said about 30 percent or more is realistic for fast charging.

Using those figures, Engadget estimated that a 20,000 mAh, 45W USB-C bank would have an effective capacity of about 52Wh after subtracting 30 percent overhead. For a laptop averaging 14W, that would extend runtime by roughly 3.7 hours. If the laptop draws more than the bank's 45W output, it will draw from its internal battery alongside the power bank, so the bank supplements rather than replaces the internal battery.

Capacity and output are not the same, according to the guide. A portable 50,000 mAh battery may store a lot of energy, but a 15W output rating will only slow a laptop's drain or may be rejected by the laptop. For laptop charging, Engadget said the label to look for is USB-C PD, which allows the bank and laptop to negotiate a higher charging level than an ordinary USB port. The guide also said buyers should check per-port output because a total rating can be deceptive. A bank described as 100W total may not deliver that much on any single port, and it may deliver less than 100W when charging a phone and a laptop at the same time.

For most people, Engadget described a 20,000 mAh, 74Wh bank as a reasonable emergency or partial-charge option, while a 27,000 mAh, 100Wh bank is closer to the sweet spot. In the United States, 100Wh is also the limit for power banks allowed on a plane without approval. The guide said its favorite laptop power banks cover a range of needs.

Engadget also listed reasons a power bank may fail to charge a laptop. A weak bank might provide power but not fast enough to keep up with the laptop's battery drain under use. Users should check whether their laptop supports charging through USB-C, because some USB-C ports handle data, video or accessories but cannot run a charger. They should also check whether 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 will not make a dent in a laptop drawing 65W or more. If the bank's output is below the laptop's needs, the computer will either drain its internal battery or fail to charge.