Every morning you wake up with some amount of capacity. Some days it’s obvious — you feel sharp, motivated, ready to go. Other days something is off, but you can’t quite name it. You push anyway, or you don’t, based on a guess.
The Battery Score is Capacity’s attempt to make that guess much smarter. Three signals, combined into one number. Here’s exactly how it works.
TL;DR
- ✓ Your Battery Score is the sum of three components: Sleep (32 pts), HRV (45 pts), and Strain (23 pts)
- ✓ HRV is the biggest component because it's the most direct physiological signal of recovery
- ✓ All comparisons are against your personal baseline — not population averages
- ✓ The score calibrates over 14 days and becomes more precise as it learns your patterns
- ✓ Strain is scored inversely: a heavy training day lowers today's score, prompting recovery
- ✓ Everything is processed on-device — your health data never leaves your phone
The formula
No black box. The math is simple:
You don’t need all three to get a useful number. If you don’t have an Apple Watch or wearable, HRV won’t be available — but Sleep and Strain still give you a meaningful signal. The score degrades gracefully rather than failing outright.
Sleep Score (up to 32 pts)
Most people think sleep is just about hours. It’s not — or at least, not only that.
Sleep contributes up to 32 points across three dimensions:
Duration — How many hours you actually slept versus your personal target. Short nights cut into this score aggressively. Oversleeping doesn’t penalize you.
Timing — How consistent your sleep schedule was with your usual pattern. Staying up three hours later than normal disrupts your circadian rhythm even if you still got eight hours. The body runs on a clock, not just a counter.
Quality — When sleep stages are available (Apple Watch, Oura, or a compatible wearable), we check for adequate Deep and REM sleep. A night of shallow, fragmented sleep scores lower than the same hours with proper stage distribution.
🔬 Why timing matters as much as duration
Circadian misalignment — sleeping at irregular hours — impairs cognitive performance and metabolic recovery even when total sleep time is adequate. Shift workers consistently show worse recovery markers than people sleeping the same hours on a fixed schedule. Consistency isn’t a bonus; it’s part of the sleep dose.
Sleep sits at 32% of the total score because it’s the foundation. HRV, mood, and performance all trace back to sleep quality. You can’t train your way out of chronic sleep debt.
HRV Score (up to 45 pts)
HRV is the heaviest component by design. It’s the most direct window into your autonomic nervous system — and therefore the most sensitive indicator of how recovered you actually are.
We use your RMSSD reading from overnight measurement and compare it against your rolling 14-day baseline. The score reflects how far above or below your personal average you sit today.
🔑 Why personal baseline beats population averages
An elite endurance athlete might have a resting RMSSD of 110ms. A healthy 50-year-old might have 38ms. Both can be fully recovered — but their numbers are completely incomparable. Generic thresholds would tell the 50-year-old they’re always in bad shape. Personal baseline tells both of them the truth: how they’re doing relative to themselves.
💡 The 14-day calibration period
Your HRV score is approximate during the first two weeks. This is normal — the algorithm is learning your baseline. Once you have 14+ readings, the score stabilizes and becomes significantly more precise. Resist the urge to interpret early numbers too literally.
If your HRV is above baseline, you’re trending recovered. At baseline, you’re neutral. Significantly below, your body is under stress — whether from training, sleep debt, illness brewing, or life pressure. The score reflects the gradient, not a binary pass/fail.
Strain Score (up to 23 pts)
This one surprises people at first: Strain is scored inversely. Less strain yesterday = more points today.
That’s intentional. If you ran 15km yesterday, your body is carrying physiological load right now. Your Strain score reflects that load and brings down your total, prompting recovery behavior. A rest day means your Strain score is near maximum — contributing to a higher Battery Score.
The metric uses whichever is higher:
- Steps × 0.04 — a simple proxy for general daily movement (10,000 steps ≈ 400 kcal equivalent, roughly moderate activity)
- Active calories from your wearable — captures the real metabolic cost of high-intensity work that steps miss entirely
💡 Why active calories matter
A 45-minute HIIT session might generate only 2,000 steps but burn 500 active calories. If we only counted steps, we’d massively underestimate the load on your body. By taking whichever signal is higher, we capture both the athlete who runs 10km and the one who does a heavy lifting session — neither gets a free pass.
The ceiling is calibrated around ~400 kcal of active burn as a reference for moderate-to-high strain. Above that, you’re in high-load territory and your score will reflect it.
The color system
Three zones. Each one comes with a clear behavioral implication — not just a label. Green means your body is ready for effort. Yellow means train, but don’t chase PRs. Red means protect your recovery above all else.
The zones are intentionally wide. A 68 and a 42 are both yellow, but they’re not identical situations. Use the color as a starting point, then let the trend over the past few days refine your read.
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What we deliberately don’t do
A few design choices worth naming explicitly:
- No comparisons to other users. Your score is yours. Someone else’s green means nothing about yours.
- No population averages as thresholds. Generic HRV or sleep benchmarks don’t account for age, fitness level, or individual variation.
- No server storage. All calculations happen on-device. Your health data never leaves your phone.
- No manual input required. Everything comes from Apple Health or Health Connect automatically.
The score is a tool to inform decisions — not to judge your day. A red score doesn’t mean you can’t exercise. It means your body is signaling it needs more recovery than usual. What you do with that information is still entirely your call.
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