How to Pick the Right Power Bank Capacity

mAh numbers on power bank packaging are almost always bigger than what you actually get in real device charges — and NZ airlines now enforce strict Wh limits on what you can even carry. Here's how to size a power bank properly.

Why mAh on the box overstates real-world charges

Power banks lose energy converting their internal battery voltage (usually 3.7V) up to the 5V or higher output your phone needs, plus further losses through the cable and charging circuitry. In practice, expect to get roughly 60-70% of the rated mAh capacity as usable charge in your device — so a "10,000mAh" power bank realistically delivers more like 6,000-7,000mAh to your phone, not the full number printed on the box.

Matching capacity to your devices

• iPhone (battery roughly 3,200-4,400mAh depending on model): A 10,000mAh power bank gives roughly 1.5-2 full charges after conversion losses — a solid everyday-carry size.
• Android flagship phones (battery roughly 4,500-5,500mAh): Similar 10,000mAh bank gives around 1-1.5 full charges.
• Tablets and larger devices: A tablet battery can be 7,000-10,000mAh on its own, so a small power bank barely gets you one partial charge — you'll want 20,000mAh or more if you're regularly charging a tablet or tablet-sized device on the go.
• Laptops via USB-C PD: Laptop batteries are commonly 50-90Wh, so you'll want a high-capacity 20,000-27,000mAh power bank with USB-C Power Delivery output rated at 60W+ to meaningfully top up a laptop, and even then expect only a partial charge.

The airline rules that now limit your choice

As of December 2025, Air New Zealand allows a maximum of two power banks per person in carry-on baggage, each capped at 100Wh, and power banks cannot be used or charged during taxi, take-off or landing, and must stay in the cabin (not in checked baggage or an overhead locker) — kept with you or in your seat area. Jetstar has gone further, banning the use and in-flight charging of power banks entirely on all its flights from December 2025, while still allowing up to two in carry-on baggage that simply can't be used onboard.

Practically, this means: check the Wh rating printed on any power bank you plan to fly with (not just the mAh figure), and if it's not clearly labelled in Wh, do the maths yourself — a 27,000mAh bank at 3.7V comes out to roughly 99.9Wh, technically under the 100Wh cap but close enough that airline staff may scrutinise it if the label isn't clear.

Our practical recommendation

• Everyday phone-only use: A 10,000mAh bank is enough for most people and easily clears airline limits with room to spare.
• Frequent flyer who also wants tablet/laptop backup: Stay under the 100Wh line — a 20,000mAh (roughly 74Wh) bank is a good upper-limit choice that still flies without issue.
• Avoid anything above 27,000mAh/100Wh if you fly regularly — some banks in this range require airline approval or are outright refused at the gate, and it's not worth the hassle for occasional extra capacity.