Heart rate and power both measure intensity, but they're measuring fundamentally different things. Confusing them leads to bad training decisions.
What each one actually tells you
Power is output. It measures the mechanical work your legs produce, in watts. 200 watts is 200 watts whether you're fresh, tired, dehydrated, or running on caffeine and spite. Objective. Instant. Indifferent to your feelings.
Heart rate is input. It measures how hard your cardiovascular system is working to deliver oxygen. The same heart rate can correspond to wildly different power outputs depending on fatigue, temperature, hydration, caffeine, stress, sleep, and altitude.
Why power wins for pacing
Power responds instantly. Start a 300W interval and your power meter reads 300W immediately. Your heart rate needs 30-90 seconds to catch up. For anything under 5 minutes, heart rate is basically useless for pacing because by the time it reflects reality, the interval is half over.
Power also doesn't care about conditions. On a 95-degree day, your heart rate might be 10 bpm higher at the same power. If you pace by heart rate, you'd back off the power, effectively undertraining. The watts tell the truth regardless.
Where heart rate is actually better
Heart rate gives you information power can't.
Recovery tracking. If your resting heart rate is 5+ bpm above baseline for two days straight, you're not recovered. No power meter can tell you that. Check recovery science for more markers.
Aerobic decoupling. On a long steady ride, compare heart rate to power in the first half vs. the second half. If heart rate drifts up more than 5% relative to power, your aerobic base needs work.
It's cheap. A heart rate monitor costs $30-80. A power meter costs $300-1500. For most recreational cyclists, heart rate plus a good training app gets you 80% of the way there. Check out apps that work without a power meter.
The best setup
If you have both, use power for pacing every workout and heart rate for recovery monitoring and long-ride analysis. They complement each other.
If you only have one, power is more valuable for structured training. But heart rate alone is still far better than guessing.
And if you have neither?
RPE (Rate of Perceived Exertion) on a 1-10 scale is surprisingly accurate for experienced riders. Research confirms that athletes who know their body can estimate training zones by feel well enough to train effectively. Nivvy supports all three: power, heart rate, and RPE.