The calculations are as follows:
- Load = (sec x Normalised Watts x Intensity) / (FTP x 3600) x 100
- Intensity = Normalised Watts / FTP
The software does it for you, so no need to try calculate it. Use the workout builder for this.
Edit: The words Load (TSS), Normalised Watts (Normalized Power) and Intensity (Intensity Factor) are used on Intervals, because the other terms (in brackets) are registered trademarks of Peaksware LLC.