Here's a stress graph (from Golden Cheetah) showing W' for my latest ride today:
Not sure why Golden Cheetah gave me a W'bal of 20 kJ to start with. I suppose it's based on my FTP of 220 that I entered into Golden Cheetah. The segment of my ride highlighted in red is a 3 mile Time Trial on my Mountain Bike on the Tow Path. You can see I started out a little hot, close to 300w, and backed it down to about 250w. I should have done this TT fresh, if I wanted a personal best time, but I had just ridden 8 miles prior. Nonetheless, you can see my W'bal plummet because 250w is definitely above my FTP or CP of 220w. In fact, W'bal went negative at the end of my TT. Was I exhausted at W'bal=0, hell yeah, but you could see I still had about 1.5 miles to go. So, at 250w (30w above my FTP) it only took about 5-6 minutes before I was near 0 W'bal. It doesn't take long. Lessons learned, if you want to do a TT do it fresh and don't start out too hot (rookie mistake). You can also see that my W'bal didn't start to recover until I walked over the bridge from Bulls Island, NJ to the PA side. The PowerCal HR monitor/meter showed 100w when in reality I wasn't even on the bike. I was walking my bike across the bridge because of the heavy pedestrian traffic. Anyway, that's W'bal and I think it's a neat/cool metric for post-ride analysis. Probably even better for post-race analysis.
Power ON! Coach Rob