WattTrue
Charger test

Does your charger deliver what it claims?

WattTrue measures the power that actually reaches your phone's battery — with the screen off, so the display's own draw is not in the reading — and shows the verdict on your computer.

How it works Open a saved run

Android and Windows first. Not in any app store yet.

A phone lying on a charger with its screen dark, sending readings over Wi-Fi to a computer that draws the power as a line. screen off
How it works

Three steps, and the phone does the measuring.

  1. Phone on the charger

    Cable or wireless pad. Start below 80 % charge — above it every phone slows its own charging, and the test would measure the phone instead of the charger.

  2. Lock the screen

    WattTrue keeps measuring with the screen off and streams each reading to your computer over your own Wi-Fi, once a second. Nothing goes anywhere else.

  3. Read the verdict on the desktop

    What reached the battery, against what the label claims — with the reasons, and with every refusal said out loud when a run cannot answer the question.

Why the screen must be off

The screen is the biggest thing in the measurement.

A lit phone display draws 1–2 W from the same battery the charger is filling. That is more than the difference between most chargers — so a test you can watch on the phone mostly measures the phone's screen.

WattTrue measures with the display off and puts the readout somewhere else: your computer. That is why there is a desktop app, and why it is the honest answer to "can't I just use a battery app?".

Screen on

The reading includes the display's own draw. Useful as a live glance; not a charger measurement. WattTrue labels every screen-on sample as such.

Screen off

The battery sees what the charger delivers, minus what the phone needs to stay awake. This is the run WattTrue judges.

What the verdict looks like

A sentence you can argue with, not a badge.

The panel below is not a screenshot. It was computed when this page was built, by the same code the apps run, over a run of readings written by hand for this page.

Example run150 hand-built frames — not a measurement of any real charger

What the label claims: 20 W, cable

Labelled 20 W, delivered 6.01 W

Conversion and cable losses explain a gap of maybe 30 %. They do not explain this one. What this charger delivers and what its label says are not describing the same device.

6.01 Wat its best
5.83 Wmost of the time
10.0 Wit offered
141 s142 samples judged
  • The charger never went above 5.0 V

    A charger that can really do 20 W raises its voltage to 9 V or more when a phone asks. The phone reported ordinary USB voltage for the whole run, which is what any old phone charger does. Some phones report a flat 5 V here even on a genuine fast charger, so this supports the verdict rather than proving it.

Every wattage here is what reached the battery, not what the charger pulled from the wall. The two are never equal.

power · 150 frames · screen off throughoutbattery powerscreen on (none in this run)

Computed when this page was built, by checkCharger() from @voltlab/protocol — the same function the phone and the desktop run. Open this run in the viewer to change the label and watch the verdict change.

Honesty

What WattTrue measures, what it estimates, and what it refuses to say.

Measured

Android

Android phones report real battery current and voltage, so WattTrue measures the power reaching the battery, plus temperature and whether the phone is charging, paused or refusing the supply.

Manufacturers disagree about units and signs. WattTrue keeps the raw number exactly as the phone reported it and tells you which assumption every watt figure rests on.

Estimate only

iPhone

Apple gives apps the battery level in 5 % steps and whether it is charging — no current, no voltage, no temperature. WattTrue times the steps, labels the result an estimate, and never turns it into watts.

iOS also pauses apps when the screen locks, so the screen-off measurement is not possible on an iPhone. WattTrue says so rather than pretending.

Refused, not footnoted

When a run can't answer

  • Above 80 % charge: the phone is slowing itself down, so WattTrue refuses to judge the charger.
  • Readings from a preview or test source: generated, not measured — no verdict.
  • Too short a run: fewer than 20 readings say nothing about a charger.
  • An iPhone run: never a wattage.

Every wattage is what reached the battery, not what the charger drew from the wall, and readings are comparative, not lab-grade: good for telling a charger that delivers from one that doesn't, on the same phone. A cable charger is judged against 70 % of its label and a wireless pad against 45 %, to allow for conversion and coupling losses — and the verdict says so.

Platforms

Built Android and Windows first.

Coming soon

Android

The phone app that measures. Keeps sampling with the screen locked and streams to your computer.

Coming soon

Windows

The desktop companion: live readout, saved runs, the verdict, and comparisons between chargers.

Estimate · later

iPhone

Charge-rate estimates from 5 % steps, clearly labelled. No wattage, because iOS does not provide one.

Available now

Browser

Open a saved run in any browser and see the same verdict, computed on your computer.

Open the run viewer

Questions

Frequently asked.

Why does the screen have to be off?

A lit display draws 1–2 W from the battery while it is charging. Most chargers differ from each other by less than that, so a test watched on the phone mostly measures its own screen. WattTrue keeps measuring after you lock the phone — Android shows a notification while it does — and the verdict is read on your computer.

If some samples were taken with the screen on, WattTrue counts them and says so beside the result, before any number.

Why won't it test above 80 %?

Every phone slows its own charging as the battery fills. Above about 80 %, even the best charger in the world measures like a weak one, because the phone is the one holding back. A verdict there would be about the phone, and people would repeat it about the charger.

So WattTrue refuses, and tells you to test again from a lower level. A run that crosses 80 % is judged on its samples below it, and says how many it left out.

Why doesn't WattTrue say a charger is "fake"?

Because it can't know that. "Fake" is a claim about where a charger came from; WattTrue sees one phone's battery. What it says instead is exactly what it observed — labelled 20 W, delivered 6 W, and it never went above 5 V — and lets that stand. A sentence like that is stronger than a badge, because you can check it.

It also works the other way: when a charger offered its full rating and the phone is what limited the charge, WattTrue says the phone is the limit and does not blame the charger.

Does it work with wireless pads and MagSafe?

On Android, yes: the phone reports when it is on a wireless charger, and you tell WattTrue it is a pad. Pads lose more energy between the coils than a cable does, so a pad is judged against 45 % of its label instead of 70 %. If a pad under-delivers, alignment is the first thing WattTrue asks you to rule out.

On an iPhone, including MagSafe, WattTrue can only estimate how fast the level rises. It can compare two pads on the same iPhone; it cannot give either a wattage.

Why can't the iPhone version show watts?

Apple does not let App Store apps read battery current, voltage or temperature. The only public information is the battery level, in 5 % steps, and whether the phone is charging. Reading more would take private interfaces, which Apple rejects.

WattTrue times the 5 % steps and calls the result what it is: an estimate, good for comparing chargers on the same iPhone.

Is it lab-grade?

No. It measures what reaches the battery, as the phone's own fuel gauge reports it. That is excellent for comparing chargers on the same phone and for catching a charger that delivers a fraction of its label. It is not a USB power meter, and WattTrue never presents it as one.

Does anything leave my network?

No. The phone sends readings to the computer you pair it with, over your own network, and runs are saved on that computer. There are no accounts, no analytics and no ads, and this website makes no requests to anyone else. Details are in the privacy policy.