Accessories

Wireless Chargers: The Qi Standard, Wattage Classes, and Why Heat Slows Them Down

A 25W-rated wireless charger doesn't sustain 25W for long. The coil-to-coil handoff loses energy as heat, and heat is exactly what slows the next watt down.

F

Flossa

Published October 5, 2026 · 15 min read

Value Score

How Qi wireless charging actually transfers power

Qi (pronounced "chee") is the open wireless charging standard maintained by the Wireless Power Consortium (WPC), and it works through electromagnetic induction: a transmitter coil in the charging pad generates a fluctuating magnetic field, which induces a current in a receiver coil inside the phone, without any direct electrical contact between the two. It's a genuinely elegant mechanism — and, as with any energy transfer that isn't a direct wired connection, it's inherently less efficient than plugging in a cable.

Qi2's two speed tiers, and why the naming is confusing

"Qi2" on a product box currently refers to two different speed tiers, and packaging doesn't always distinguish them clearly:

Hitting the full 25W isn't just about buying a "25W" charger — every link in the chain (wall adapter, charging pad, and phone) has to support the higher tier, or the system falls back to the lower common denominator.

What the magnets actually fix

Qi2's headline feature is built-in magnetic alignment (a Magnetic Power Profile, conceptually similar to Apple's MagSafe), and it solves a real efficiency problem rather than just a convenience one: coil-to-coil misalignment — the phone sitting even slightly off- center on the pad — reduces charging efficiency and increases heat generation during transfer. By snapping the phone into a fixed, correct position magnetically, Qi2 charging is more consistently efficient charge to charge, not just easier to place down correctly.

The heat problem: why a 25W-rated charger rarely sustains 25W

Every coil-to-coil wireless power handoff loses a meaningful share of transferred energy as heat — this is true even under good alignment, and it compounds at higher wattages. That's why a charger rated for 25W frequently can't sustain that full output for an extended charging session: as the coils heat up, some chargers and phones throttle output specifically to manage temperature, meaning the rated wattage and the sustained real-world average wattage over a full charge cycle are two different numbers. Some higher-wattage wireless chargers now include active cooling fans specifically to manage this — a component that exists only because heat, not the coil design itself, is the practical ceiling on sustained wireless charging speed.

Wired vs. wireless: the efficiency gap that never fully closes

Independent of any specific product's design quality, wireless charging carries a structural efficiency disadvantage against a wired connection: commonly cited figures put wireless charging at roughly 10–20% more total energy loss than an equivalent wired charge, due to the induction handoff itself. Qi2's magnetic alignment narrows this gap compared to older, unaligned Qi charging (one estimate puts the heat reduction from proper alignment at roughly 30%), but it doesn't eliminate the fundamental physics gap between induction and a direct electrical connection.

What you actually need for the full rated speed

To actually reach a charger's advertised top wireless speed, every component in the chain needs to support it:

1. A phone with the matching wattage class (15W or 25W Qi2 tier)
2. A charging pad rated for that same or higher wattage
3. A USB-C PD wall adapter rated meaningfully above the wireless charger's output (commonly 30W+ for a 25W wireless charging pad, since the pad itself has conversion overhead)
4. A cable that supports the required USB PD protocol version, since an underspecified cable can prevent the higher wattage from negotiating at all

Missing any one link in that chain silently caps the whole system at whatever the weakest component supports — a genuinely 25W-capable phone on a genuinely 25W-capable pad will still charge at a lower speed if the wall adapter behind it isn't rated high enough.

How to read a wireless charger listing

1. Check whether "Qi2" means the 15W or 25W tier specifically — the name alone doesn't tell you, and most current Qi2 hardware is still the 15W tier.
2. Confirm your specific phone has built-in magnets or needs a case before assuming Qi2's alignment benefit applies out of the box.
3. Buy a wall adapter rated above the wireless charger's stated output, not equal to it, since the pad itself has conversion overhead.
4. Don't expect a charger's peak rated wattage to hold for an entire charging session — thermal throttling at higher wattages is normal behavior, not a defect, and is exactly why active-cooling models exist at the top of the market.

Bottom line

Wireless charging's speed and heat behavior come down to the same underlying physics: every watt transferred through induction rather than a direct wired connection loses more to heat than a cable would, and that heat is precisely what limits how long a charger can sustain its rated peak output. Qi2's magnetic alignment is a genuine, measurable efficiency improvement over unaligned Qi charging, not just a convenience feature — but it narrows the wired-vs-wireless efficiency gap rather than closing it, and a charger's advertised wattage remains a best-case peak, not a number you should expect to see sustained for an entire charge.