Some sites have a "watt calculator". Is this type of resource a good tool for evaluating a ride/workout?

Forget if the formula is correct or not. If you're always using the same formula, won't it balance out?

If use watts, then what is the normal variance between workout? The problem with using average speed is it doesn't take into account the differences in wind speed and I thought a watt tool might help keep up the enthusiasm when the speed drops but the intensity is higher.

Here is the site I've been using: http://www.kreuzotter.de/english/espeed.htm

I'm seeing swings of 120 watts, and that seems a bit too much variation to me.