What Is Rated Capacity In A Power Bank? | Plain Facts

Shoppers see two numbers on portable chargers all the time: a glossy “capacity” on the box and a smaller “rated capacity” on the label or spec sheet. Those two aren’t the same thing. One tells you how much energy sits inside the battery cells. The other tells you how much a user can draw under defined test conditions. This guide breaks the terms down, shows the math behind them, and helps you predict real-world run time without guesswork.

Capacity Terms That Actually Matter

Before diving into formulas, let’s pin the words you’ll meet on packaging or spec pages.

Term	What It Means	Where You’ll See It
Capacity (mAh)	Charge stored in the internal cells measured at the cells’ nominal voltage (often 3.6–3.7V).	Big number on the box: “10,000 mAh,” “20,000 mAh.”
Energy (Wh)	Total energy content. Calculated from capacity × voltage. Best for apples-to-apples comparisons.	Spec labels, airline rules, safety documents.
Rated Capacity	Usable output measured by the manufacturer under specified load, temperature, and cutoff voltage.	Data sheets, fine print on the shell; often lower than the headline mAh.
Output Voltage	USB output levels while charging your device (5V, 9V, 12V, 15V, 20V with PD/PPS).	Port labels, user manuals.
Efficiency (%)	Energy lost in voltage conversion, heat, and electronics. Typical range 70–85% for many packs.	Rare on packaging; seen in reviews and lab tests.

Rated Capacity Of A Portable Charger — Plain-Language Guide

Manufacturers test batteries under controlled conditions so users have a consistent yardstick. Rated capacity reflects that yardstick. It’s the measured output when the pack discharges through a defined load, at a set temperature, down to a specific cutoff voltage chosen by the maker. That’s why the label can show “Rated: 6,500 mAh” on a unit sold as “10,000 mAh.” The label isn’t a downgrade; it’s the realistic figure for what you can draw at the output side.

Why The Headline mAh And The Label Don’t Match

A pack’s cells store energy at their own nominal voltage, often near 3.7V. Your phone, tablet, or handheld needs 5V or a higher PD level at the USB port. The power bank must boost voltage, and that conversion costs energy. Some energy also turns to heat. The result: usable output at the port is lower than the raw cell number suggests. That usable figure is the rated capacity.

The Simple Math: mAh, Wh, And A Quick Reality Check

Two tiny formulas give you clarity:

• Wh = (mAh × cell voltage) / 1000  → converts the headline mAh into energy.
• Estimated output mAh at 5V = (Wh × efficiency) / 5 × 1000  → predicts what the USB port can deliver at 5V.

Say a pack is labeled 10,000 mAh at 3.7V. Energy is 10,000 × 3.7 / 1000 = 37 Wh. With 80% efficiency, a 5V output looks like 37 × 0.80 / 5 × 1000 ≈ 5,920 mAh. If a phone has a 3,000 mAh battery, you’d expect close to 1.5–2 fills depending on device draw and charge curve. Rated capacity figures land in this ballpark because they’re measured under repeatable limits rather than best-case assumptions.

What Authorities Mean By “Rated Capacity”

In aviation and battery safety documents, rated capacity is a defined term. It’s the measured capacity of a cell or battery under a specified test procedure and cutoff set by the maker. That’s why energy in watt-hours is used for safety rules and transport limits, and why many spec labels show Wh next to mAh. You’ll see this language in the IATA lithium battery guidance and on FAA PackSafe pages for travelers.

How Makers Decide On The Test Setup

Makers choose a discharge current, temperature window, and cutoff point that reflect common use. Higher discharge rates or lower temperatures reduce measured capacity. A conservative cutoff protects cell health. All three choices nudge the result down from the headline energy number. That’s why one model can show a slightly different rated figure than another model with the same nominal mAh.

Estimating Real-World Run Time Without Lab Gear

You don’t need instruments to make a good estimate. Use the steps below anytime you’re comparing two power banks or checking if a pack fits your trip.

1. Find the Wh on the label. If only mAh is listed, convert it with the first formula using 3.6–3.7V as the cell voltage for most packs.
2. Pick an efficiency range. Many mainstream units land near 70–85% total, including electronics and heat.
3. Apply the second formula for 5V output. If your device negotiates 9V/12V/20V with PD, use that output voltage instead of 5V.
4. Divide by your device battery size to get a rough number of charges. Reduce the result by another slice if you plan to use the device while charging.

Worked Scenarios You Can Copy

Everyday Phone Pack

Label: 10,000 mAh at 3.7V → 37 Wh. With 75% efficiency at 5V, output mAh is ≈ 5,550. A 3,200 mAh phone sees around 1.5 fills depending on screen-on time and background tasks.

Tablet-Friendly Pack

Label: 20,000 mAh at 3.7V → 74 Wh. With 80% efficiency at 5V, output mAh is ≈ 11,840. A tablet with an 8,000 mAh pack gets one full charge and change, or two light top-offs.

Why Wh Beats mAh When Comparing Models

Energy in watt-hours accounts for voltage, so it lets you compare different designs without guesswork. Some high-capacity banks use slightly higher-voltage cell chemistry, which skews mAh alone. Wh levels the field, and it’s the unit airlines care about. Many travel pages set a 100 Wh limit per spare battery, with some carriers allowing up to two at 101–160 Wh with approval. Full details live on the FAA’s airline passengers and batteries page.

What Rated Capacity Doesn’t Tell You

The label is a controlled snapshot, not a promise for every scenario. Fast-charge protocols raise voltage and current; cables add resistance; heat changes efficiency. Long sessions that keep the pack near peak output can show a bit less delivered energy than a slow trickle. None of that makes the label wrong; it just means conditions moved away from the test window.

Reading The Label Without Getting Lost

Look for these three lines on the shell or spec card:

• Energy (Wh) — the most reliable single number for travel and real comparison.
• Rated Capacity (mAh at X V) — often shows the output mAh referenced to 5V, which reflects USB use.
• PD Output Levels — the port’s max, such as 20W, 45W, 65W, 100W. This affects how fast a device charges, not total energy.

How Rated Capacity Shows Up On Different Boxes

Single-Cell 10K Units

These use two to three 18650/21700-class cells in parallel to hit the energy target. Expect a rated number near 5,500–6,500 mAh at 5V depending on electronics quality and heat handling.

High-Output Laptop Banks

These packs add more cells and beefier converters for PD at 60–140W. The rated figure can look lower than you’d expect because high-watt sessions push losses higher. A 24,000 mAh model (~88–90 Wh) feeding a 65W laptop may deliver fewer watt-hours than the same pack feeding a phone at 10W, even though the energy stored is the same.

A Quick Reality Check On Marketing Claims

Some boxes only print a big mAh without the Wh. That makes comparison harder. When two packs both say 20,000 mAh, the one with the higher Wh actually holds more energy if the cell voltage differs. If both list Wh and the numbers match, choose based on weight, output power, port mix, and verified efficiency.

How To Choose The Right Size For Your Gear

Match the energy to your longest stretch away from wall power. A phone-only day hike may be fine with ~30–40 Wh. A camera, action cam, and phone need more headroom. A laptop day out needs 70–100 Wh depending on power draw and work habits. Keep airline rules in mind if you fly with spare batteries.

Practical Tips To Get Closer To The Rated Number

• Use short, well-made cables. High resistance wastes energy.
• Charge devices when they’re cool. Heat increases losses.
• Avoid stacking adapters. Go direct from the bank’s port to the device when you can.
• Let the bank rest between heavy sessions. Electronics stay more efficient when not heat-soaked.

Safety Notes That Keep Trips Smooth

Power banks are treated as spare batteries when you fly. They ride in carry-on, not checked bags. Most carriers allow units up to 100 Wh without approval, with limited exceptions above that range. Check the current guidance on the IATA guidance document and the FAA PackSafe lithium batteries page before you pack.

From Box To Reality: Estimator Table You Can Use

The numbers below give a quick feel for what common labels deliver at the USB side under typical ranges for total efficiency. Real results depend on device draw, cable, and charge profile.

Label (mAh @ 3.7V)	Energy (Wh)	Estimated Output mAh @ 5V (70–85%)
10,000	≈ 37	≈ 5,180–6,290
20,000	≈ 74	≈ 10,360–12,580
24,000	≈ 88.8	≈ 12,430–15,130
26,800	≈ 99.2	≈ 13,880–16,880

What To Check When Specs Show Two mAh Numbers

Many labels print both a big capacity and a smaller rated figure. The big number almost always refers to the internal cells at nominal voltage. The smaller rated number refers to output at a fixed voltage and load. When you see that pair, trust the figure that includes voltage or Wh and aligns with the formulas above. That’s the one that predicts charging sessions more closely.

How Fast-Charge Modes Affect The Picture

PD and PPS raise output voltage during parts of a session. Power equals voltage × current, so higher voltage at the same output power can reduce current and cable losses a bit, yet high-watt bursts increase converter stress and heat inside the bank. Net delivered energy across a full session can land near the same totals as 5V charging, just with a different time profile. Rated capacity is still a solid guidepost for the total energy you’ll get.

FAQ-Free Takeaways You Can Act On

• Rated capacity is the usable output under a defined test. It’s lower than the headline mAh because of voltage boost and losses.
• Convert to Wh first, then estimate output mAh at the voltage your device will use.
• Efficiency swings with load, temperature, and port power. Expect a range, not a single magic number.
• For travel, watch the Wh number. It’s the unit airlines use.

A Short Checklist Before You Buy

• Find the Wh on the spec label. If missing, compute it from the printed mAh × 3.7V.
• Scan for a printed rated capacity at 5V. That tells you what the USB port can really hand off.
• Match PD watts to your device needs. A laptop-class bank without the right wattage won’t keep up during use.
• Pick a realistic efficiency range. Use 70–85% unless you have verified test data for that model.

Bottom Line For Real-World Buyers

Rated capacity exists to set clear expectations. It’s the practical figure you can plan around, and it lines up with how airlines, labs, and spec sheets treat energy. Use Wh for comparison, keep an eye on the rated output at common voltages, and you’ll choose a size that matches your gear without guesswork.