# Power Variability

Canonical HTML: https://effortsignal.com/learn/power-variability/

Last content update: 2026-09-20

Published: 2026-09-11

Author: [EffortSignal Editorial Team](https://effortsignal.com/editorial-standards/)

Power Variability compares Weighted Power with average timer-running power.

<!-- Connect the familiar VI terminology without implying it is one of the three cited trademarks. -->
Familiar with Variability Index (VI)? [Our power-metric naming guide](/learn/power-metric-names/) shows the equivalent ratio alongside PSS, PI and Weighted Power.

## Formula

```text
Power Variability (PV) = Weighted Power / average power
```

PV is dimensionless. Both values come from the timer-running projection; zeros remain valid and missing power remains missing. Weighted Power uses its complete 30-second windows while the average uses valid projected power samples. Those populations can differ at recording edges and gaps, so this is not a guarantee that every short record has a ratio above one.

## Verified examples

A constant one-hour recording at 200 W has Weighted Power 200 W, average power 200 W and PV 1.00. For the 60-second example with 30 seconds at 200 W and 30 seconds at zero, the values are approximately 135.3952 W and 100 W, giving PV 1.353952. That short example illustrates the arithmetic and is low confidence for activity interpretation.

## Limits

Unavailable Weighted Power or nonpositive average power prevents the ratio. FTP does not appear in this formula, so changing only FTP does not directly change PV. Smoothing, pauses, missing data and recording length can affect it; see [Weighted Power](/learn/weighted-power/). Use [interval analysis](/features/interval-analysis/) to locate changes in output instead of treating one whole-activity ratio as a description of every effort.

## Sources and implementation scope

[Coggan, power-based training metrics](https://www.trainingpeaks.com/learn/articles/normalized-power-intensity-factor-training-stress/) explains the research background. The formulas, windows, quality gates and examples above describe EffortSignal 0.1.13, checked September 10, 2026. Thresholds and rounding are EffortSignal rules.