How Big of a Power Bank Do I Need?

A practical sizing guide that matches power bank capacity to your devices, your trip length, and your reason for carrying one.

Quick answer

For most people, a 10,000 mAh power bank is the sweet spot. It carries roughly one to two phone charges, fits in a jacket pocket, and stays well under airline limits. Step up to 20,000 mAh if you also charge a tablet, share the bank with a partner, or routinely go a full day or more without an outlet. Step up to 25,000 mAh and a USB-PD output port if you need to charge a laptop.

Capacity is rated two ways: mAh and watt-hours

Most power banks are sold by milliamp-hours (mAh) at the cell voltage of about 3.7 volts. That number is fine for shopping comparisons within the same product category, but it does not tell you how much usable energy reaches your device.

mAh is convenient and prominent on the box.
Watt-hours (Wh) is more honest because it includes voltage. Wh = volts × (mAh ÷ 1000). A 10,000 mAh bank at 3.7 V is about 37 Wh.
For laptops and travel planning, Wh is the better number to compare across products.

What size to buy by use case

These ranges assume modern phones with batteries of about 4,000 to 5,000 mAh and account for typical conversion losses. If you want to dig into where those losses come from, the companion guide on charge counts walks through it.

Your situation	Capacity to look for	Approximate Wh
Phone topup, daily commute	5,000 mAh	~18 Wh
One full phone day, one device	10,000 mAh	~37 Wh
Phone plus tablet, full day	20,000 mAh	~74 Wh
Multi-day trip, two devices	20,000 to 27,000 mAh	~74 to 99 Wh
Laptop charging on the go	20,000 to 27,000 mAh with USB-PD output	~74 to 99 Wh
Emergency or extended outage	27,000 mAh, or a portable power station	99 Wh and up

How to size for a phone

Start with your phone's battery, listed in the spec sheet or in the device settings. Recent flagships sit between 4,000 and 5,000 mAh.

5,000 mAh bank: useful for a topup or an emergency boost, not a full charge cycle.
10,000 mAh bank: roughly one to two real phone charges. Best all-rounder for one person, one device.
20,000 mAh bank: roughly three to four real phone charges. Worth the bulk if you go a long day without an outlet or share with a partner.

How to size for a tablet

Tablets have batteries two to three times the size of a phone, so charge counts drop fast.

A 10,000 mAh bank usually covers a partial tablet recharge, not a full one.
A 20,000 mAh bank is the realistic minimum if a full tablet recharge matters to you.
Pay attention to output wattage. Many tablets charge slowly off a 10W port and properly off an 18W or 30W USB-PD port.

How to size for a laptop

Laptops are a different category. Capacity in mAh is misleading here, so think in watt-hours and check that the bank actually supports laptop charging.

You need a USB-C port that supports USB Power Delivery (USB-PD), ideally 45W to 65W or higher. A standard 5V USB-A port will not charge most laptops.
Laptops typically have batteries between 40 Wh and 80 Wh. A 74 Wh power bank (about 20,000 mAh) gives most lightweight laptops a partial-to-full top-up after losses.
For a near-full laptop recharge, look for a 27,000 mAh bank rated 99 to 100 Wh with a 65W or higher USB-PD output.
Check your laptop charger's wattage. If it ships with a 100W brick, a 30W power bank will trickle-charge but may not keep up under heavy use.

How to size for travel

Travel sizing is about clearing two day-long stretches: a long flight or train ride, and the gap between hotel outlets. Charging speed becomes a comfort factor.

For one phone: a 10,000 mAh bank covers most travel days comfortably.
For phone plus laptop or tablet: 20,000 to 27,000 mAh with USB-PD is the practical range.
Most airlines cap power banks at 100 Wh in carry-on baggage. A 27,000 mAh bank at 3.7 V is roughly 99 Wh, just under the line. Always check your specific airline policy and verify the Wh rating printed on the bank.
Power banks must travel in carry-on baggage, never checked.

How to size for emergencies

Outage planning is different from daily backup because you usually want to power more than one device, possibly for more than one day, and you may need to charge things you do not normally carry, like a flashlight or a portable radio.

Two to three people for one outage day: at least one 20,000 to 27,000 mAh bank, kept charged.
Multi-day outage planning: a portable power station in the 200 to 500 Wh range gives you several phone-and-tablet days plus headroom for a CPAP, laptop, or small fridge cycle.
Whatever you choose, recharge it on a schedule. A power bank that has been sitting in a drawer for a year often holds far less than its rated capacity.

Output wattage matters as much as capacity

Two banks with the same mAh can feel completely different in daily use because of their output ports.

5V/2A USB-A (10W): works for any phone, but charges modern phones and tablets slowly.
18W to 30W USB-C PD: fast-charges most phones and small tablets. Common on midrange banks.
45W to 65W USB-C PD: needed to charge most laptops at a useful rate.
100W and above USB-C PD: useful for larger laptops or for fast pass-through charging the bank itself.

Common sizing mistakes

Buying for the maximum trip you might ever take instead of the trip you actually take. A 26,800 mAh brick lives in the drawer if you never carry it.
Treating mAh as the only number that matters. A 20,000 mAh bank with only a 5V/2A USB-A port is the wrong answer for a laptop, regardless of capacity.
Forgetting the Wh rating before flying. Banks above 100 Wh need airline approval, and banks above 160 Wh are usually banned outright.
Not checking how long the bank takes to recharge itself. A big bank with a slow input can take a full overnight to refill.

For real-world charge counts after losses, see How Many Charges Will a Power Bank Give? When the use case is closer to outage backup than daily carry, the Portable Power Station Runtime Calculator is a better fit than a power bank.

Bottom line

Pick the smallest bank that covers your real use case, not the largest you can afford. A 10,000 mAh bank is the right answer for most one-phone, one-day situations. A 20,000 mAh bank handles two devices or a long day. Step up to 27,000 mAh with USB-PD only when you charge a laptop or plan for two-plus days off-grid. Whatever you choose, check the watt-hour rating before flying and recharge it on a schedule so it actually delivers when you need it.