How We Calculate

1. Energy in a battery

Watt-hours are the product of charge and voltage:

Wh = mAh × V ÷ 1000

and the inverse, used when only watt-hours are printed:

mAh = Wh × 1000 ÷ V

V is the nominal voltage of the cell, not a USB output rail. A power bank with a 3.7 V cell and 10,000 mAh holds 37 Wh. Substituting 5 V for that 3.7 V would produce 50 Wh — a 35 % error introduced by reading the wrong line of the label. The converter therefore requires a voltage value and warns when a value looks like a USB rail rather than a cell voltage.

2. Estimated full charges

charges = power bank Wh × overall efficiency ÷ device battery Wh

The result is shown as a range, because the efficiency figure is a range. Two things follow:

• The number is an estimate of full charges under favourable conditions, not a promise.
• It is not a runtime. "How many charges" and "how many hours" are different questions, and runtime needs an average load in watts: hours ≤ usable Wh ÷ average W.

3. What the efficiency figure covers

Efficiency here means the whole path from energy stored in the power bank to energy stored in the phone's battery: boost conversion, cable loss, heat, and the phone's own charging circuit. Typical well-made combinations land somewhere in the 60–65 % band, and the exact value depends on the devices, the cable and the conditions.

The calculators do not ship a single "official" efficiency percentage. The range is an input you can change, and the default is labelled as an assumption to be replaced with your own if you have better information. The test range used in the automated tests (70–75 %) exists to check the arithmetic, and is not a recommendation for real devices. Wireless charging has its own, larger losses and does not inherit the wired assumption.

4. Charger output

W = V × A, applied to one output profile at a time. A charger marked 5 V/3 A, 9 V/3 A, 15 V/3 A, 20 V/5 A offers four alternatives; its highest single-profile output is 100 W. Adding them (5×3 + 9×3 + …) would produce a meaningless total, so the code never does it — there is a test that fails if it ever starts to.

A profile figure is a rated output, not a measurement. It says what the port can deliver, not what a device will draw or how long a charge will take.

5. Simultaneous use of several ports

When two or more ports are in use, the only reliable source is the manufacturer's port-allocation table for that specific combination. The calculators look for an exact match:

• Combination found → the per-port figures from the table are reported.
• No table, or no matching row → the status is needs verification.

Two shortcuts are refused. First, total wattage is never treated as proof that every port still receives its target. Second, a missing allocation table is never converted into a verdict of "incompatible" — absent information is not negative information.

6. Travel plugs and voltage

The check runs in order: socket shape, then supply voltage and frequency, then grounding and rated current, then the difference between a plug adapter and a converter. A destination with more than one socket type returns more than one answer, because a single "the adapter you need is X" statement would be false in half the country.

Countries without a verified record return "data not yet verified". They never return a default "compatible".

7. Data rules

• Unknown values are null with an explicit status, never 0.
• Battery energy, charger output and AC input rating are separate fields. They are not interchangeable.
• Region and model variant are part of the record: a specification for one SKU is not reused for another.
• Development fixtures carry a visible test-only marker and are excluded from any published recommendation.

8. Why some inputs are rejected

Zero, negative numbers, blank fields, text, Infinity and NaN are refused rather than coerced to a number. A coerced value would produce a confident-looking output from meaningless input. When a field is rejected the message appears next to it and the other values stay where they were, so nothing has to be retyped.

9. Worked example

The same figures are in the automated test suite, and the reverse conversion is checked to return 10,000 mAh from 37 Wh at 3.7 V.

10. What the numbers do not include

• Charging while the phone is in use: the pack drains faster and the phone fills slower.
• Temperature: cold reduces usable capacity noticeably; heat accelerates wear.
• Battery health: an aged phone battery accepts less energy than its original rating.
• Device power management, which may slow charging near the top of the range.
• Cable quality, which is invisible to any formula and can cost double-digit percentages.

These are the reasons the site says "estimated". A precise-looking figure would misrepresent what the arithmetic can support.

11. Sources

Formulas are standard electrical relationships. Country electricity values come from standards and regulator publications, recorded per country with the date they were checked. Manufacturer documentation is the source for any device or charger specification. Where a claim could not be traced to such a source, it was not published.

• IEC World Plugs — plug type, nominal voltage and frequency reference data, accessed 6 November 2026. It is the base reference for every country record. Where it is the only source for a value, the records say so.
• Terna S.p.A. — Grid Code, Chapter 1 Section 1C — states "the nominal frequency (50 Hz) on the grid", read 9 November 2026. Used as the second independent source for the Italian frequency.
• Legislation and national documents used for the verified country records: the Electricity Safety, Quality and Continuity Regulations 2002 (regulation 27) and the Plugs and Sockets etc. (Safety) Regulations 1994 for the United Kingdom; the Electricity Business Act Enforcement Rule as published by the National Law Information Centre for South Korea; the TANESCO distribution page and a Tanzania Bureau of Standards draft standard for Tanzania. Each carries its own access date in data/countries.json.
• Device and charger nameplate labels — the only accepted source for a battery's nominal voltage, a charger's output profiles and an AC input range. The site asks you to read them rather than guessing a model's specification.

The full source list for each data record lives with the data (data/countries.json, data/devices.json, data/chargers.json, data/products.json) and on the relevant page, together with its review date. A record with no source is shown as unverified, never as a fact.

12. Tests

A fixed-case test suite covers energy conversion, charge estimation, invalid input rejection, profile handling, multi-port allocation, and the travel rules including the high-power appliance exit. It runs with node tests/run-tests.cjs and needs no packages. A test failure blocks a release. Recorded results, including the reference cases on this page, are published in tests/RESULTS.md.

Last reviewed 6 November 2026.