How Many Charges Does a 20,000mAh Power Bank Really Give You?

A 20,000mAh power bank gives a typical 5,000mAh phone about 2.5 to 3 full charges, not four. The maths only looks broken until you realise the two mAh numbers are measured at different voltages, and then you lose more energy again converting between them.

Why mAh is a misleading number

Milliamp-hours only mean something alongside a voltage. Power bank cells are rated at 3.7V nominal. Phone batteries are usually rated at 3.85V nominal. The USB output between them runs at 5V or higher.

Convert both sides to watt-hours and the comparison becomes honest:

• 20,000mAh × 3.7V ÷ 1,000 = 74Wh in the bank
• 5,000mAh × 3.85V ÷ 1,000 = 19.3Wh in the phone

74 divided by 19.3 is 3.8. So even before losses you were never getting four charges. And you don't get 3.8 either, because converting 3.7V up to 5V and back down into the phone's battery wastes energy as heat.

The real efficiency number

End-to-end efficiency for a decent power bank and cable is usually somewhere between 60% and 80%. Around 70% is a fair planning figure. That gives you roughly 52Wh usable out of a 74Wh bank.

Efficiency gets worse when:

• You use fast charging. More watts means more heat, and heat is wasted energy. A 65W PD charge is measurably less efficient than a slow 10W one.
• The cable is cheap or long. Thin conductors drop voltage and the bank compensates by pushing harder.
• It's hot. A power bank in a car on a summer day in Perth is fighting itself.
• The bank is old. Capacity fades with cycles, same as any lithium cell.

Real charges by device

All calculated from 52Wh usable, which is a 20,000mAh bank at 70% end-to-end efficiency.

Charges assume the device starts flat and reaches 100%. Topping up from 40% goes further.

How to read a power bank label properly

Ignore the giant number on the front. Look for the small print on the back or the side, which is legally required for shipping and usually lists:

• Cell capacity in mAh at 3.7V. This is the headline number.
• Rated capacity or output capacity. Often quoted at 5V, and it'll be much lower, typically 12,000mAh to 13,500mAh for a 20,000mAh bank. That's the honest figure and it's roughly what your device will see.
• Watt-hours (Wh). The number that matters for both maths and airline rules.

If a bank claims 20,000mAh and lists 60Wh or less, the cell rating and the maths don't agree, which is a red flag.

Flying with it: the Australian rules

Lithium batteries must travel in the cabin. Never in checked baggage.

Based on the standard international dangerous goods thresholds Australian carriers apply. Confirm with your airline before you fly, since individual carrier policies vary and have been changing.

A 20,000mAh bank is 74Wh, so it's comfortably under the 100Wh line and needs no approval. A 27,000mAh bank is about 100Wh and sits right on the boundary, which is exactly why so many "airline-friendly" banks stop at 26,800mAh.

Separately, several airlines tightened their rules during 2025 on using or charging power banks in flight, including keeping them visible rather than in the overhead locker. Check your specific airline's dangerous goods page before you fly, because these policies have been moving.

What size should you actually buy?

• 5,000mAh (18Wh): one phone top-up. Pocketable. Fine for a night out.
• 10,000mAh (37Wh): about 1.3 phone charges. The best size-to-weight compromise for daily carry, roughly 200g.
• 20,000mAh (74Wh): about 2.7 phone charges, or one laptop charge plus a phone. Around 350g to 450g. The travel default.
• 26,800mAh (99Wh): the biggest you can take on a plane without asking. Heavy, around 550g, and it counts against your carry-on limit.

If you want to charge a laptop, the watt-hours matter less than the output. You need USB-C Power Delivery at 60W or more, and the bank has to support it. Plenty of large-capacity banks only do 18W, which will charge a laptop slower than the laptop drains.

Getting more out of the one you have

• Charge slowly when you're not in a hurry. Low and slow is meaningfully more efficient.
• Use a good short cable. A 2m no-name cable costs you real energy.
• Put the phone in low power mode or aeroplane mode while it charges. You're not fighting the screen and the modem.
• Keep the bank out of direct sun and out of a hot car.
• Store it around 50% to 70% if it's going in a drawer for months, not at 100% and not flat.

Related calculators

• Power Bank Capacity Calculator to run your own numbers
• Carry-On Weight Calculator to check whether the bank pushes your bag over the airline limit
• Battery Runtime Calculator for amp-hour batteries and inverters
• Power Station Runtime Calculator for portable stations rated in watt-hours

Frequently asked questions

How many times can a 20,000mAh power bank charge a phone?

About 2.5 to 3 full charges for a typical 5,000mAh phone. The bank holds 74Wh at 3.7V, the phone holds about 19.3Wh at 3.85V, and end-to-end efficiency is usually around 70%, leaving roughly 52Wh usable.

Why doesn't a 20,000mAh power bank fully charge a 5,000mAh phone four times?

Because the two mAh figures are measured at different voltages, and converting from the bank's 3.7V cells up to 5V and back down into the phone wastes 20% to 40% of the energy as heat.

Can I take a 20,000mAh power bank on a plane in Australia?

Yes. A 20,000mAh bank is about 74Wh, under the 100Wh threshold that needs no airline approval. It must travel in the cabin, never in checked baggage. Banks between 100Wh and 160Wh need airline approval, and anything over 160Wh is banned from passenger aircraft.

Does fast charging waste power bank capacity?

Yes. Higher wattage produces more heat, and heat is wasted energy. Charging at 10W instead of 65W noticeably increases how many full charges you get from the same bank.

What does rated capacity mean on a power bank?

Rated or output capacity is usually measured at 5V and reflects what your device actually receives. For a 20,000mAh bank it typically lands between 12,000mAh and 13,500mAh. It is the more honest number of the two.