How accurate is your smart trainer's power? (and why it disagrees with your power meter)

Your smart trainer says 240 W. Your power-meter pedals say 249 W. Which one is lying? Very possibly neither — and understanding why is the key to trusting your numbers at all. Power accuracy has two separate parts people constantly blur: how precise each device is, and where on the bike it's measuring. Two honest instruments reading different points of the same drivetrain should disagree. Here's what the accuracy claims really mean, why the disagreement is legitimate, and how to sanity-check your own setup in twenty minutes.

What the accuracy claims actually promise

A "±2%" claim means the device should report within 2% of true power under reasonable conditions — at 250 W, anywhere from 245 to 255 W. Here's how the classes stack up; all figures are manufacturers' claims and approximate, and a well-treated unit typically does better than its worst case:

Bar width shows the relative size of each class's worst-case error band — wider means the reading can wander further from the truth.

Why the trainer and your power meter legitimately disagree

Follow one pedal stroke down the bike. Your legs put power in at the pedals; the chain carries it to the cassette; the trainer absorbs it at the hub. At every link and tooth along the way, a little of it becomes heat:

So when pedals say 250 W and the trainer says 243 W, both are right about the point they're measuring. The ~2–3% gap is your drivetrain's efficiency bill (approximate; a filthy chain charges more). This also means the direction of the gap is predictable: the trainer should read lower than a pedal or crank meter. If yours reads consistently higher, or the gap is far beyond the two accuracy claims combined, then something genuinely is off — usually calibration.

Calibration: who needs it and when

Wheel-on trainers need it, regularly. Their power is inferred through a tyre pressed against a roller, and that interface changes with tyre pressure, clamping tension and temperature. The fix is the spindown: ride ~10 minutes to warm the tyre and roller first, then run the calibration — a cold spindown bakes the wrong friction number in for the whole session. Re-do it when you change tyre pressure or re-tension the roller.

Direct drives mostly handle themselves. Mid-range units are factory-calibrated and many auto-calibrate as you ride; an occasional manual spindown after firmware updates doesn't hurt. High-end units like the Tacx Neo measure directly and have no calibration routine at all. The one habit that helps every class: give the trainer ten minutes of riding before you trust the numbers, because electronics and strain gauges drift as they warm — the same reason your first interval of the day can look strangely cheap or expensive.

Consistency beats absolute truth

Here's the mindset shift that ends most power anxiety: for training, you don't need the cosmically true number — you need the same ruler every time. A trainer that reads 3% low but does so every single ride is a perfectly good training instrument: your FTP, your zones and your progress are all measured in its units, and they move together. Problems only start when you mix rulers — testing FTP on the pedals, then doing workouts off the trainer, then wondering why threshold feels wrong.

The practical rule: pick one power source per context and keep your zones per source. If you own both, either let the pedals be the source indoors and out (one ruler everywhere — the cleanest option), or keep an indoor FTP and an outdoor FTP and never compare a Tuesday trainer ride against a Sunday road ride watt-for-watt. This, incidentally, is one honest reason indoor FTP often tests lower — sometimes it's physiology, and sometimes it's simply a different ruler.

Sanity-check your own setup

If you have two power sources, twenty minutes settles whether yours agree within reason:

One convenience worth knowing: because Moveee Indoor runs in the browser and pairs devices over Web Bluetooth, you can pair your trainer for control and a separate power meter as the power source in the same session — the ERG resistance comes from the trainer while your watts come from the meter you've anchored your zones to. Whichever source you choose, choose it once, calibrate it warm, and then get on with the actual training. The watts don't need to be perfect; they need to be yours.