It says “Calculating charging speed…” or “Measuring...”. Is something wrong?
Usually not. Watt has to see the battery level change twice before it can work out a number, and the first number can take up to about 10% of charging. A slower charger or a larger battery makes that take longer, and most iPads take longer than iPhones. Keep the app open on screen until the first number appears. If the battery level does not move at all (a warm phone, or Optimized Battery Charging holding at 80%), there is nothing to work from until iOS starts charging again.
Why is the number lower than what my charger says?
A charger's label is the most it can supply at the plug. Your iPhone only takes what it wants: less when it is warm, and less above about 80%. Watt also shows only the power reaching the battery, which leaves out what the phone uses while it charges and the energy lost as heat.
How accurate is it, and how is it different from a USB power meter?
It is an estimate built on iOS's 5% battery steps, and it gets steadier the longer you charge. It is best for comparing chargers and cables on the same device, not for a lab-grade number. A USB power meter in the cable shows what the charger delivers, including what the phone uses and the heat lost on the way, so it reads higher than Watt. The two numbers answer different questions. Readings run high on a worn battery (roughly 15–20% high at 85% battery health).
Why does Watt ask whether my iPhone has a SIM card tray?
Phones sold in different countries have slightly different batteries. On the iPhone 17 Pro, iPhone 17 Pro Max, iPhone 18 Pro and the iPhone 18 Pro Max sold outside the US, the eSIM-only versions (sold in the US and a few other countries) have a somewhat larger battery than the versions with a SIM tray, and iOS cannot tell the two apart. The US iPhone 18 Pro Max reports its own model code, so it does not ask. Watt needs the battery size for its estimate, so in Watt 1.5 and later it asks once, with the likely answer already selected from your App Store country. Picking the wrong one moves the watts by about 3–7%. For the eSIM-only iPhone 18 Pro Max sold outside the US, the larger size is inferred from Apple's battery-life figures, not measured. If you are not sure, look at the side of the phone: a SIM tray slot means Yes. You can change the answer at any time under the gear icon, in Settings. Other iPhones and iPads never ask.
What does “Battery capacity estimated for this device” mean?
In Watt 1.5 and later this appears when Watt does not have your model's battery size in its table. Watt 1.5 has the battery size of every iPhone and iPad that can run iOS 18.6, so you should only see it on a model released after your version of Watt. It then uses a typical size (about 12 Wh for an iPhone, 30 Wh for an iPad), so the watts shown can be higher or lower than what actually reaches the battery. Tap the message to read more. The numbers still work for comparing chargers and cables on the same device.
Why does the number only change every few minutes?
iOS reports the battery level in 5% steps, and Watt can only work out a new number when the level changes. On a fast charge that is every few minutes; on a slow one the figure can stay put for ten minutes or more. Watt does not follow the battery second by second.
Why do two runs on the same charger give different numbers?
Where in a 5% step you start, how warm the phone is, whether the screen is in use and the battery level all change the result, and charging slows as the battery fills. For a fair comparison, use a benchmark test with the same battery range each time, and repeat it.
Why does the screen stay on?
iOS only lets Watt read the battery while the app is on screen, so it keeps the screen awake while your device is charging or a test is running. Unplug, or finish the test, and auto-lock works normally again.
What happens if I lock the phone or switch apps?
Watt can only follow the battery while it is on screen. Come back while you are still plugged in and it carries on. If you unplug while the app is closed or in the background, that session may not be saved.
What does “Charging Paused” mean?
The battery level has not moved for at least ten minutes, and for three times longer than the last step predicted, so iOS has probably paused charging: usually Optimized Battery Charging holding at 80%, or a phone that is too warm. Watt shows that instead of an outdated number. It can only do this once it has shown a first number.
How do benchmark tests work?
Create a test with a battery range, for example 30% → 80%, and run it once per charger or cable. Each result is saved with its average watts, time and curve, and the highest average gets a crown. Start at or below the start level. If the battery is already above it, Watt starts from where it is and warns you, because the result will not compare fairly. If the battery is already at or above the end level, the test cannot start. If the target is reached while Watt is in the background, it asks you to run the test again instead of saving a wrong result.
Can I name a charger, add notes or delete a session?
Yes. When you unplug after a session, Watt asks which charger you used. You can change the name and add notes later in the session details, and delete sessions from the history.
Does it work with MagSafe, wireless chargers and power banks?
Yes: anything that makes iOS report your device as charging. Wireless charging loses more energy as heat, so expect a lower number than the same adapter on a cable.
Which devices and languages are supported?
iPhone and iPad with iOS or iPadOS 18.6 or later. Watt 1.5 and later knows the battery size of every iPhone and iPad that can run iOS 18.6: iPhone XS, XS Max and XR up to iPhone 18 Pro Max (including iPhone SE 2nd and 3rd generation, iPhone Air, iPhone 16e and 17e), the iPhone Duo (both batteries counted together), iPad Pro (11-inch, 12.9-inch and 13-inch, up to M5), iPad Air (3rd generation up to M4), iPad (7th generation up to A16) and iPad mini (5th generation up to A17 Pro). Earlier versions know fewer models (iPhone 11 through iPhone 17 and only some recent iPads) and use a typical battery size for the rest without saying so. Watt 1.5 and later says so for any model it does not know, such as one released after it. The app is in English, Chinese (Simplified), Filipino, French, German, Indonesian, Italian, Japanese, Korean, Malay, Norwegian, Portuguese (Brazil), Spanish, Thai, Turkish and Vietnamese.
How do I send feedback or report a wrong number?
In Watt 1.5 and later, tap the gear icon and then Send Feedback (it is also at the bottom of the How It Works screen, under the info button). Mail opens with a message to wattchargingspeed@frekvens.ai and, below your text, the app version, your system version and your device model, which tell me which battery size a reading used. You can read and change all of it, and nothing is sent until you press Send in Mail. If Mail is not set up on the phone, Watt shows the address with a Copy Address button, so you can write from any email app.
Is my history synced or moved to a new phone?
Watt has no account and no sync of its own. Your history is stored on the device, so restoring a new phone from a normal backup usually brings it along; otherwise the new phone starts empty.
What does it cost? Is there a subscription?
Watt is a one-time purchase on the App Store: no subscription, no in-app purchases and no ads. The App Store shows the price for your country.
Does Watt collect any data?
No. Watt makes no network requests of its own, has no analytics, ads or third-party code, and your charging history never leaves your device. The one exception is a message you choose to send with Send Feedback: Mail sends it only when you press Send, and you can read and edit it first. The details are in the privacy policy.
