How Many Watts Is A 10000mAh Power Bank? | Real-World Math

If you’re sizing a charger for your phone, earbuds, or a small tablet, the number on the label can be confusing. The milliamp-hour figure describes capacity, while watts describe charging power. Below you’ll find the quick math, a broad table of scenarios, and plain guidance so you can pick the right brick without guesswork.

10000mAh To Watts: Fast Math And Assumptions

First convert milliamp-hours to watt-hours. The standard formula is:
Wh = (mAh ÷ 1000) × voltage. Commercial packs use lithium-ion cells with a nominal cell voltage near 3.6–3.7V, so the stored energy for a 10000mAh unit is roughly 36–37Wh. The pack then boosts that cell voltage to the USB output level you use, which introduces conversion loss. The energy figure describes how much “fuel” the pack holds; the watt number printed beside the USB ports describes how fast that fuel can be delivered.

Want the primary sources? The FAA watt-hour formula matches the calculation above, and the USB Power Delivery overview shows the power profiles that ports can advertise and negotiate.

What “Watts” Means On A Power Bank

Watts are rate. The number printed next to a USB-A or USB-C port tells you the maximum charging power the port can supply at that moment, based on the negotiated voltage and current. Many compact 10000mAh units top out around 10–20W on a single port, while larger or newer packs can go higher when they support advanced USB-C PD modes. The energy in the pack (≈37Wh) doesn’t change, but a higher watt rating can fill a phone faster when the device accepts it.

Why 37Wh Is Not What Your Phone Actually Receives

Two factors shrink delivered energy: conversion loss and heat. A boost converter steps the cell voltage up to the requested output voltage; even efficient designs burn a little energy. At typical loads for phones, many modern converters land near the 85–93% zone. Cold weather can also reduce effective capacity during discharge. Expect delivered energy from a 10000mAh pack to fall near the low-30-Wh range on a 5V output in normal room conditions.

How To Read Your Label And Box Specs

Energy Line

Look for a line that reads something like “37Wh (typ)” or “3.7V 10000mAh.” That pair implies nominal cell voltage and rated capacity. The energy number is your fuel tank size. Many brands print both values.

Port Lines

Under each port, you’ll see voltage and current figures such as “5V⎓2A” for USB-A or “5V⎓3A, 9V⎓2A, 12V⎓1.67A” for USB-C. Multiply V×A to get watts. A label such as “PD 20W” means the USB-C port can negotiate one of the PD profiles that totals near 20W. The device being charged must accept that profile to see the higher rate.

Charging Speed: What You’ll See In Daily Use

Phones

Most modern phones draw 15–20W from mainstream PD-enabled banks, then taper as they near full. Older phones stick closer to 10W. The screen you’re using and background tasks can nudge real charging time.

Tablets And Handhelds

Small tablets may accept 18–20W. Larger tablets want 27W or more, which many 10000mAh packs can’t supply. You still get energy, just at a slower clip than the tablet’s wall charger.

Wearables And Earbuds

Tiny batteries sip power. Even at low rates, a 10000mAh unit can top up a watch or earbud case many times before it needs a recharge.

Method: From Label To Practical Numbers

Step 1: Convert mAh To Wh

Use the standard formula. For a 10000mAh pack at 3.7V, that’s ≈37Wh. If you only see mAh on the case, assume 3.6–3.7V unless the spec states a different chemistry. The calculation comes from the same rule aviation agencies use to classify battery size.

Step 2: Account For Conversion Loss

Multiply by an efficiency factor. With mainstream boost converters, 0.85–0.93 is a reasonable bracket near phone-charging loads. That turns ≈37Wh into ≈31–34Wh delivered at 5V.

Step 3: Read The Port’s Watt Rating

Check the port’s printed V and A or any PD watt badge. That watt number caps charging speed, not total energy. A 20W port simply lets a compatible device take energy faster than a 10W port, until the device begins to taper.

Real-World Outcomes With A 10000mAh Pack

To help you plan a flight day or a weekend away, here’s what that energy looks like across common device classes. Numbers are ballpark, since devices vary in battery size and charging behavior.

Common Questions People Have While Shopping

Why Do Some Boxes Say 27W Or Even 30W?

Those figures refer to port power under USB-C PD profiles. The pack still holds ≈37Wh; higher port power just shortens the time to reach a given state of charge if your device accepts that profile. Some laptops need 45–65W or more, which is beyond most 10000mAh units.

Does Temperature Matter?

Yes. Low temperatures reduce available capacity during discharge. If you’re out in winter, stash the bank in an inside pocket so it stays warm enough to deliver energy efficiently.

What If My Pack Lists 3.85V Cells?

Some brands use slightly higher nominal-voltage cells. Repeat the same formula with the listed voltage. A 10000mAh rating at 3.85V yields ≈38.5Wh, before any conversion loss.

USB-C PD Profiles And What They Mean

With USB-C PD, the bank and your device negotiate a voltage step such as 5V, 9V, 12V, or higher on bigger chargers. The goal is to supply the same or higher power at a voltage the device prefers, which can reduce cable current and losses. Compact packs in this capacity often top out near 18–20W, while larger models and wall chargers can go far higher under PD 3.1. For a phone, 15–20W support is a sweet spot for speed without bulk.

Efficiency: What Designers Target Inside The Pack

Inside, a boost converter elevates the cell voltage to the requested output. Good designs post peak efficiencies in the 90% range near moderate loads common to phones. Light loads and heavy loads sit lower. This is why the same pack feels “longer-lasting” when you trickle charge earbuds than when you fast charge a tablet.

Travel Tip: Watt-Hour Labels Win At Airports

Airlines and security staff classify lithium batteries by watt-hours. If your bank prints the Wh number, you can show it instantly. The general policy allows packs up to 100Wh in carry-on, with larger packs sometimes permitted with airline approval. If your case only lists mAh, you can compute Wh on the spot with the same rule described above. You can verify this on the FAA’s site and its calculator.

Worked Examples You Can Copy

Example 1: Single-Port 10W USB-A Bank

Energy: ≈37Wh. Assuming ~90% conversion and minor cable loss, expect ≈31–33Wh delivered. Port power is 10W, so a phone that accepts 10W spends longer on the wire yet still receives the same energy in the end.

Example 2: 20W USB-C PD Bank

Energy: ≈37Wh. Delivered energy similar to above. The difference is time: a compatible phone fills faster early in the charge curve, then tapers to protect its battery. The pack empties in fewer minutes of plug-in time, not fewer watt-hours.

Buying Checklist For A 10000mAh Bank

• Port power: Seek 15–20W USB-C if you want faster phone top-ups.
• Clear labels: Look for printed Wh and port profiles.
• Two ports: Handy for phone + earbuds; total power may split when both are active.
• Cables: Short, good-quality cables shave loss during fast charging.
• Temperature: Keep it warm in winter for better performance.

Bottom Line For Everyday Use

A 10000mAh unit stores about 37Wh at the cell level. After conversion, plan on around 31–34Wh reaching your devices and a charge rate near 10–20W on mainstream packs. That’s enough for two large-phone fills or a day of mixed top-ups without hunting a wall outlet.