Since it's winter it's cold and dark so I've been riding my trainer. However, how can I tell what my wattage is? I've got an old Trek 2100 mounted on an old Performance Peak Load 3 Stationary Training Unit. I can calculate cadence, speed, and elapsed time, but I would like to also calculate calories burned.

There are some trainers that measure power output. If yours is one of them, this info should be available in the manual. My guess, though, is that it is not, especially if it's "old" and cost less than about $500

In that case, without a power meter of some sort (SRM, PowerTap, Ergomo), the best you'll be able to do is a very rough and close to worthless estimate.

Knowing your wattage is going to set you back $400+

A heart rate monitor will give you a fairly accurate measure of calories burned and is easily obtained for well under $100.

If you know how many calories you've burned (which a HRM can tell you, as slowandsteady said), you can use that info to find the power (wattage).

Take your energy burn for one hour and divide by 4.3, and you have a pretty good estimate of your power output for that hour.

Just because some heart rate monitors give you a number, that doesn't make them accurate. There is no good correlation between heart rate and calories burned since so many things beside workload can affect heart rate. Anyone who's ridden at constant power indoors with a heart rate monitor will have observed heart rate drift as their heart rate continues to increase over time while power remains constant. Obviously in this case, calories consumed does not increase as heart rate climbs.

HR and power agree very well for a single individual. Sure, it can drift a bit up or down, but it still serves as a good reference for this purpose. If you come up with 180 or 200 W, does it really matter?

This is not really true. HR can vary significantly, independent of power output. For example, if you are feeling ill, your HR may increase while your power output takes a dive. And more than "a bit" -- sometimes quite a lot.

As to whether you come up with 180 vs 200: it may not matter; it depends on what the information is being used for. If you're trying to correlate power to calories burned, say, for purposes of monitoring weight (calories in/calories out), then an estimate may be sufficient. But if you're "training with power", that 20W difference could be a big deal.

Well, if you're ill, you shouldn't be on your bike anyway, so that's not really an argument, is it?

The point is that HR and Power do not necessarily track. Illness is only one of many factors that may have an effect. Other factors include recovery (lack of) and hydration (or lack of), weight fluctuations, and so on. Very few of us can control all of these things to be the same every time we ride the bike, yet they all will have different effects on power, effects that a HR monitor alone would not be able to give sufficient information about.

And even if we could elliminate these factors, heart rate drift would cause heart rate to rise with time for a constant effort. Riding at threshhold, I see my heart rate rise 5-8 bpm over a 20 minute interval even with a fan and good cooling. So what would the calorie models predict for the change in calories consumed going from 172 bpm to 180 bpm (202 max, 180 lbs.)

No, the amount of calories burned may be increasing. The amount of work being done to the bicycle is the same, but your body may be using more calories to do the same amount of work.

Think about it, the heart muscle doesn’t pump for free; so increased HR at same power level will be burning more calories. Many other processes require energy that may not be going to the bike as work.

Other examples:
-Active cooling, as your body temp rises more energy is spent to maintain a safe temperature, yet power still remains constant.

-The illness example above is perfect, physiological process related to fighting the infection are gobbling up calories, even with a reduced power output. Ever notice how fast a person with a chronic illness loses weight?

-More obvious things, like moving your arm to drink out of your bottle uses calories too, but yet still does not effect your cycling power.

-Digestion of those gels etc. I could go on.

So, in reality, HR may be a more reliable way to measure calories burned. However, the power meter will be a far better tool for measuring cycling intensity (and thus better for setting training zones).

You may live to ride, but your body just lives for that purpose alone.

"May be" or "Is"? Do you know or are you guessing. It has been shown through direct measurement that efficiency for all individuals varies only slightly from about 22%-26%. For a single rider, at a fixed workload, efficiency doesn't change to any meaningful degree. So while what you propose may in theory be possible. Data shows it is not.

The efficiency at converting calories to work on the bike is almost fixed. I know that sitting on the bike flapping my arms while not pedaling burns calories, and elevates HR, are you really that naïve as to think that because your power meter says 0 you are using no energy?

The reason I used “may” is because scientists do not ever convey fact, as no such thing exists. Theories can only be confirmed, never proven. “May” was also appropriate as other conflicting processes may/are occurring; such as warming of the muscles decreases their viscosity, thus lowering the caloric cost of the work done.

No meaningful energy. So put a number to it. How many extra calories are burned cycling at 250W and 180 bpm compared to 250W and 172 bpm for the same individual?

Man, I am agreeing with you that power and HR are poorly correlated. I am only saying that HR is better correlated to calories than is cycling power. Studies that suggest otherwise are limiting their data collection in very narrow ways, like you are.

What is with power meter users that they think it has to be ultimate measure of every bloody physiological process? It is probably the best tool for measuring cycling, but that is it.

You want a quantitative measure of how many calories it takes for your body to support eight heartbeats? Ok take your BMR (say 1700kcal/day) and dived it by your resting HR (say 50bpm or 72000/day) and you get about 0.024 kcal/beat. Now if you want to use a difference of only 8bpm that works out to ~11.5 extra calories/hour. Not much, I agree, but you are using ridiculously similar situations. Furthermore, that is only the energy to drive the heart, not all the other possibilities I mentioned.

You have to accept the obvious reality that your body uses calories for other things besides cycling and that the heart only beats faster when demand has increased.

Just a reminder of the thread's title (even though the post actually says something about calories...).

BTW I'm not a power meter owner.

Hmm... The subject I put may be misleading. I thought that in order to find out the # of calories burned while riding my trainer, I would first need to know the wattage. What I'm trying to do is put in an accurate # of calories burned into FitDay to keep track of my overall caloric intake/burned.

The OP would be best served (cost:benefit) with buying a HRM, not a PT.

Not if he really wants to measure wattage. If he just wants to estimate calories burned, I agree with you. It's impossible to tell without clarification from him.

Edit: sorry, missed his clarification above. Go for a HRM, you don't need to measure Power.

Ok, mac, then ignore the pedal power.
Just use the HRM directly. It will be reasonably accurate for that purpose.

^^^ 3 posts above. What I really want to find out is # of calories burned while riding my trainer. Would I burn more in higher gears and mashing the pedals, or lower gears and spinning?

Not to be a smartass, but, this is something you'll probably be able to tell us once you get your HRM and some kind of formula to estimate your calories based on HR. Not everyone is going to react the same, though generally the higher cadence/smaller gears will engage your aerobic system more and produce a higher HR.

Work is work. Figuring out the cadence that allows maximal sustainable power output is individual, and difficult to determine without getting very technical.

You really just need to figure out "what is the highest HR (more correctly power) I can hold for a moderate amount of time, say 20min?" Just below that intensity is where you do most of your workouts, in order to burn the most calories within a reasonable amount of time.