I'm often asked by athletes if they should buy a Power Meter or not for training? My answer...you guessed it:: it depends! It depends on what you want to get out of your training. Do you want to maximize the amount of training time you have? If so, then yes..I think buying a Power Meter will definitely optimize your training time. Do you want a tool that can measure cycling dosage? Then absolutely, the PM is the tool for you. Can you get by with a Heart Rate monitor alone for training? Sure, cyclists got by (albeit not as well as today) before Power Meters came on the scene.
Like a Heart Rate Monitor, the Power Meter is just another tool in the cyclists toolbox. (Notice I said cyclists toolbox rather than cycling mechanics toolbox. I don't think most cycling mechanics know what a PM is..haha). And, just like the tools pictured above..each one has a specific purpose. Just like you don't buy a screw driver to loosen a bolt..you wouldn't buy a Heart Rate Monitor to try and measure Power Output. (Funny, nobody asks if they should buy a PM for racing. I wonder why. After all, some of your BEST power data comes from race day.)
Power is a measure of cycling dosage (i.e. your output) and Heart Rate is a measure of response (to that particular output). It's the most basic training paradigm I often talk about in my blogs- the dosage-repsonse paradigm. Does Power correlate well with Heart Rate? Sure, my Lactate Threshold Heart Rate is consistent with my Threshold Power on long continuous rides. i.e. If I'm riding for 20 minutes at my present Threshold Power (FTP) of 260w I can pretty much bet that my LTHR will be 175 bpm +/- 5 bpm. i.e. provided I'm properly hydrated, I'm not overheated, etc. Does this correlation fall apart for shorter more intense efforts? Absolutely. That is why I don't recommend training with a Heart Rate monitor alone when doing hi-intensity interval work. Your Heart Rate will lag. The Heart Rate Monitor is NOT a substitute for a Power Meter..it has a different purpose. One measures dosage and the other measures response. Therefore, one tool actually compliments the other..rather than a replacement for.
When I ride (and race), you will almost always find me with both a Power Meter and Heart Rate monitor on my bike. That is, my power output and heart rate are displayed on my cycling computer when I train and when I race. Why? For the same reason I always look at my speedometer, tachometer and temp gauge when I drive my car. I want to periodically monitor how things are running...especially if I'm driving my car hard/fast. Likewise on the bike, if I'm pushing hard (high watts) I want to monitor what my heart rate is doing. Is it staying level, drifting, escalating? Can I power up or should I back it down? In either case, driving a car or riding a bike HARD, I don't want to blow-up. That's why I periodically monitor heart rate and power output when I train/race my bike. Blow-up and it's game over..at least for that particular ride (car or bike).