I have trained without power for the last 6 months. I still get power envy from those who are training with it. I keep all my intervals on a 1.2 mi wind-protected loop. I do all my intervals on RPE and track my progress through speed. Of course this is about the most unscientific way to go about it, but it has forced me to work harder to get aero and stay aero - in addition to becoming stronger. I like to compare my speeds on this loop to the speeds of races. (This loop was used for our weekly crit series for 20 years).
Just as an aside - wired powertaps are dirt cheap. Not glamorous, but they do the job.