How Much Can A Power Bank Charge? | Real-World Math

Shoppers ask one thing: how many times will this pack refill my phone, earbuds, tablet, or laptop? The label shows milliamp-hours (mAh) or watt-hours (Wh), but ports, voltages, and conversion losses muddy the picture. This guide clears that up with plain math, sample builds, and quick tables you can trust. You’ll know what to expect before you leave the house.

How The Math Works For Real Devices

Every power bank stores energy in cells near 3.6–3.7 volts. Your devices charge at 5V USB or higher USB-C PD voltages. The pack must convert its stored energy to the output voltage, and that step wastes some juice. That’s why a 10,000mAh label never means ten full 1,000mAh refills at the port.

Step-By-Step Formula

1. Convert the pack to watt-hours: Wh = (mAh ÷ 1000) × 3.7.
2. Estimate efficiency: use 0.7–0.85 for small to mid packs; use 0.8–0.9 for USB-C PD packs with short cables.
3. Usable energy: Usable Wh = Pack Wh × Efficiency.
4. Convert your device battery to Wh: phones often sit near 3.8V nominal; a quick rule is Device Wh = (mAh ÷ 1000) × 3.85 or use the maker’s Wh rating if listed.
5. Charges: Full Charges = Usable Wh ÷ Device Wh.

That’s it. The only variables you tweak are the efficiency and the device’s true energy. If the device shows only mAh, the 3.85V rule of thumb keeps estimates tight.

Quick Example With A Mid-Size Phone

Say the pack is 10,000mAh. Pack Wh = 10 × 3.7 = 37Wh. With 80% efficiency, Usable Wh = 29.6Wh. A phone with a 4,300mAh battery at 3.85V is 16.56Wh. Estimated full recharges: 29.6 ÷ 16.56 ≈ 1.8. In real life, screen-on charging and background apps trim that, so plan on about 1.6–1.7.

Wide Comparison: From Earbuds To Big Phones

This first table gives broad, early guidance. It uses a mid-range efficiency of 80% and a 3.85V device baseline. Pick the device size closest to yours, then scan across pack sizes to see practical refills.

Pack Label	Usable Energy (Wh)	Approx. Full Recharges*
5,000mAh (18.5Wh)	14.8Wh	Earbuds 200–500mAh: 6–18 • Small phone 3,000mAh: ~1.0 • Big phone 5,000mAh: ~0.6
10,000mAh (37Wh)	29.6Wh	Earbuds: 12–36 • Small phone: ~2.0 • Big phone: ~1.2
20,000mAh (74Wh)	59.2Wh	Earbuds: 24–72 • Small phone: ~4.0 • Big phone: ~2.4
27,000mAh (100Wh)	80Wh	Earbuds: 32–96 • Small phone: ~5.4 • Big phone: ~3.6

*Phones estimated with 3,000mAh (~11.6Wh) and 5,000mAh (~19.3Wh). Real results vary with signal strength, screen use, cable losses, and charging profile.

Why The Label Never Matches The Port

Labels are printed at cell voltage. Output happens at 5V or higher. Energy stays the same, but current in mAh shifts with voltage. Makers also subtract losses to show a rated output figure. Anker explains the math with a 10,000mAh case and shows why you see ~7,400mAh at 5V after conversion and circuit overhead; see the-maker guide in rated capacity details. That reference mirrors what your recharge count math will show.

Close Variant: How Many Phone Refills From My Pack

Use this mini checklist for fast estimates on a phone.

• Find pack Wh: the label might list Wh directly; if not, convert from mAh with 3.7V.
• Pick efficiency: 0.75 if cables are long or warm, 0.85 with short cables and neat PD output.
• Find phone Wh: convert from mAh at 3.85V if Wh isn’t listed.
• Divide: usable pack Wh by phone Wh. Round down a bit for screen-on charging.

Edge Cases That Change The Count

• Screen usage during charging: GPS, gaming, or hot weather cut gains fast.
• Long or thin cables: more resistance equals more heat, so fewer watt-hours land in the battery.
• Older batteries: aged phones don’t store their original energy; cycle wear drops capacity.
• High-draw boosts: fast charging raises conversion losses a bit. Counts dip against the plain math.

Tablets, Handhelds, And Laptops: What Changes

When devices need more watts, USB-C PD steps up the voltage. USB-IF lists PD modes up to 240W with fixed 28V, 36V, and 48V levels, plus adjustable ranges for fine control; see the standard overview at USB Charger (USB Power Delivery). With higher PD voltages, cables must be certified for the current you plan to pull.

The energy math stays the same. You still divide usable pack Wh by device Wh. What changes is whether the pack’s max wattage can keep up with the device’s draw. A thin ultrabook might sip 15–30W while idle and 45–65W while under load. If the device demands more watts than the port can supply, charging slows or pauses while you use it.

Worked Example: Small Laptop On A 65Wh Pack

Assume a 65Wh power bank with 85% efficiency. Usable energy is ~55Wh. A notebook with a 50Wh battery can see about 55 ÷ 50 ≈ 1.1 full cycles from empty, but only if the pack can meet the notebook’s watt draw. If the port tops out at 30W and the notebook pulls 45W while you edit video, the gauge may hold steady or crawl until the load drops.

Port Specs, Cables, and Heat

Ports: USB-A sits near 5V and a few amps. USB-C without PD gives up to 15W. USB-C PD negotiates voltage and current to match the device, which cuts conversion losses and speeds things up. The pack’s display or spec sheet lists each port’s cap; pick the one that meets your device draw.

Cables: short, thick cables waste less power. E-marked USB-C cables are needed for the highest PD levels. If your device keeps dropping the charge rate, swap the cable before blaming the battery.

Heat: warm packs lose efficiency. Keep them shaded, avoid tight pockets during fast charging, and don’t stack the phone on the pack during high-wattage bursts.

Real-World Scenarios You Can Copy

Weekend City Trip With A 10,000mAh Pack

You start at 100% on Friday. Across a camera-heavy Saturday plus maps and rides, you’ll refill a large phone once and still have a slice left for the evening. Sunday light use turns that slice into top-ups at lunch. Budget two full phone charges if you keep screen time in check.

Camp Night With Headlamp, Watch, And Earbuds

A 5,000mAh pack is plenty. Small cells on accessories sip energy. Expect a string of tiny refills across two or three nights. Keep cables bagged to dodge loss and dirt.

Conference Day With A USB-C Laptop

Bring a 20,000mAh PD unit that can output 45–65W. You’ll stretch a thin-and-light through a keynote and one workshop, then refill again at lunch from an outlet. If the pack sits near 74Wh, you can cover a phone and a notebook with careful scheduling.

Second Table: Device Energy And Expected Cycles

Pick a device type, then scan to see how many cycles a 59Wh usable pack may deliver. This assumes a PD pack with decent cables.

Device	Battery Energy (Wh)	Cycles From ~59Wh Usable
Wireless Earbuds Case	0.8–2.0	~30–70
Smartwatch	0.3–0.7	~80–190
Compact Phone	10–12	~4.9–5.9
Large Phone	17–20	~3.0–3.5
8–10″ Tablet	25–30	~2.0–2.4
13″ Ultrabook	45–55	~1.1–1.3

Numbers assume you aren’t using the device hard while charging. Heavy draw during the session cuts cycle counts.

How To Read Pack Labels Without Guesswork

Look For Watt-Hours

Wh tells the truth across voltages. If the box only lists mAh, multiply by 3.7V and divide by 1000 to estimate Wh. Many boxes now print Wh to meet air-travel rules, which helps you compare models.

Check The Port Matrix

Match the device draw with the right socket. A USB-A port that tops at 12W won’t feed a tablet at its peak. A USB-C PD port marked 9V/12V/15V delivers cleaner power to phones that support it and helps reach the estimate you calculated.

Mind The Cable

A flimsy cable wastes power and warms up. Pick short runs. If you need long, choose better wire gauge and rated plugs.

Air Travel Notes In One Glance

Most airlines ask for spare lithium packs in carry-on with a cap near 100Wh, with higher ranges needing approval. IATA’s passenger guide lays out limits by Wh and device type; see passenger lithium battery rules. Labels with clear Wh markings speed checks and reduce back-and-forth at the gate.

Fast Reference: Build Your Own Estimate

Phone

• Pack: 20,000mAh — 74Wh.
• Efficiency: 0.8 — 59Wh usable.
• Phone: 5,000mAh — ~19.3Wh.
• Refills: 59 ÷ 19.3 ≈ 3.0 full cycles with light screen use.

Tablet

• Pack: 27,000mAh — 100Wh.
• Efficiency: 0.85 — 85Wh usable.
• Tablet: 28Wh pack.
• Refills: 85 ÷ 28 ≈ 3.0 light-use cycles; heavy streaming trims that.

Laptop

• Pack: 65Wh bank with PD at 65W.
• Efficiency: 0.85 — ~55Wh usable.
• Laptop: 50Wh battery, idle draw near 10–15W, peak near 45–65W.
• Refills: near one full cycle if you plug in while the machine is idle or sleeping between sessions.

Tips To Squeeze More Recharges

• Start higher: plug in at 20–40% phone level, not near zero; conversion losses rise when the system is hot and loaded.
• Use PD when available: matched voltage cuts waste and shortens the session.
• Kill background drains: flip low-power modes, dim the screen, and pause sync during the session.
• Go wired: skip wireless pads with packs; induction adds heat and cuts delivered energy.
• Store cool: room temps help both the pack and the device hold capacity longer across months of use.

FAQ-Style Clarity Without The FAQ Block

Why Do Two Packs With The Same mAh Perform Differently?

Cell quality, converter design, cable choice, and port limits all matter. Two “10,000mAh” units may land different usable Wh at the port. A pack with PD that can step to 9V or 12V for a modern phone often wins.

Does Fast Charging Reduce My Total Cycles From A Pack?

Fast profiles waste a bit more energy, so cycle counts dip a little. The time saved may be worth it on busy days. If you need every last Wh, use a slower port and short cable near the end of the session.

What About The mAh Numbers On Big Laptop Packs?

Ignore the mAh and look for Wh. Large packs often list 20,000–27,000mAh because that’s common in marketing, but Wh tells you the real energy across any voltage.

One-Screen Recap You Can Use

• Core math: convert both pack and device to Wh, apply 0.75–0.85 efficiency, and divide.
• Small gear sips: earbuds and watches barely dent even tiny packs.
• Phones: a 10,000mAh unit lands about 1.6–2.0 full cycles on mid phones, more on compact models.
• Tablets & laptops: check watts as well as Wh; the port must meet the draw to charge while in use.
• Cables & heat: short, thick leads and cooler temps add back a surprising chunk of energy.

Method Notes And Sources

Energy math follows simple conservation rules across voltage conversion with realistic loss ranges and real-world device baselines. For PD power levels and voltage steps, see the USB-IF PD overview. For why output capacity is lower than the headline mAh, see Anker’s rated capacity explainer. Both help you translate a box label into the recharges you’ll see in daily use.