Most health apps hand you a wall of numbers — HRV, resting heart rate, sleep stages, active calories — and leave you to figure out what they mean for the rest of your day. The Energy Battery does the opposite. It's one number, 0 to 100, that answers a single question: how much do I have left right now?

62
Moderate — holding steady
Started the day at 86 after a solid night's sleep. A walk and a couple of active hours have brought it down to 62. Nothing urgent — just a normal day of spending.

Think of it less like a lab report and more like the battery indicator on your phone: it goes up overnight, it goes down while you use it, and it tells you at a glance whether you should push through or ease off.

How It Works, at a High Level

1
Morning fill
Every morning, the battery is "seeded" — filled to a starting level based on how well you slept and recovered overnight, with a small adjustment for how demanding yesterday was.
2
Daytime drain
Through the day, activity and physical stress spend down the reserve — a workout, a stressful hour, a long walk. Ordinary movement is expected and mostly forgiven; it's the above-normal effort that costs points.
3
Quiet recharge
Genuinely restful moments — sitting still, a calm heart rate, an afternoon nap — top the battery back up a little, though never past this morning's starting fill. The real recharge always happens overnight.
A typical day, shape only
Wake — filled Workout / activity Rest — small top-up Now

The line only ever moves in response to something real — a night's sleep, a workout, a quiet hour. There's no hidden math nudging it up or down for its own sake.

What Sets the Morning Starting Point

The starting fill is built from the same overnight signals a recovery score uses, each weighed by how strongly it reflects your actual recovery state and compared against your own rolling baseline rather than a population average:

Overnight signal weights
HRV level
30%
Resting HR
20%
Sleep duration
15%
HRV trend
15%
Sleep quality
10%
SpO₂
10%

If a signal is missing that morning — no SpO₂ reading yet, say — its weight is dropped and the rest are rescaled to fill the gap, rather than treating the missing value as zero. A fever-range body temperature also caps the whole composite from above (it can only pull the number down, never push it up), since elevated temperature is a much stronger "something's off" signal than any single recovery metric.

From there, the seed formula is simple in shape: starting fill = recovery score − a small previous-day-workout adjustment − a small leftover-fatigue adjustment, clamped between 5 and 100 so it's never reported as literally empty overnight. Both adjustments are intentionally capped low — around 10 points each at most.

Why the adjustments are capped so low. Yesterday's hard effort already shows up in tonight's HRV and resting heart rate — that's most of what a recovery score measures. Layering a large, separate workout penalty on top of that would count the same fatigue twice, which is exactly the kind of over-correction that makes a battery read unfairly harsh on training days. Capping both adjustments keeps them as a small residual nudge, not a second scoring system fighting the first.

Why it sometimes says "finalizing"

Your watch or ring doesn't always deliver last night's HRV and resting heart rate the instant you wake up — some devices back-fill that data minutes to hours later. So the starting fill is computed from whatever's available, tagged internally by how complete it is, and only treated as final once enough has arrived (or a few hours have passed, whichever comes first). If a device revises its own numbers shortly after — as some do — the starting fill can still follow that revision for a short morning window. By early afternoon it locks in place for the rest of the day, so the number you're tracking doesn't quietly shift under you later on.

What Moves It During the Day

From the morning fill onward, the battery responds hour by hour to what your body is actually doing, weighing three signals against each other — active-energy burn, heart rate relative to your resting baseline, and step count — and letting whichever one is dominant decide that hour's outcome:

−
Real exertion drains it.Logged workouts, a heart rate sustained well above resting (roughly a third higher), or active-calorie burn beyond an hourly allowance all cost points — the size of the drain scales with how far above baseline you are, up to a cap so no single hour can crater the number.
≈
Ordinary movement is close to free.Each hour gets a small allowance of active calories — roughly what a sedentary hour with normal fidgeting and errands burns — before anything counts as drain. Walking to the kitchen or standing up isn't "spending your reserve"; it's the baseline cost of being awake.
+
Genuine rest tops it up, gently.An hour with heart rate close to resting and low step count counts as rest, and recharges a small, steady amount — with a bonus once a rest stretch runs two or more hours back to back, since a real nap is worth more than a few quiet minutes.
↑
It never charges past this morning's fill.Daytime rest can slow the drain or nudge the number back up, but the real recharge — the kind that raises tomorrow's starting point — only happens overnight.

Reading the Number

85–100
Full
70–84
Charged
40–69
Moderate
20–39
Low
0–19
Depleted

A "Depleted" reading late in the day after a genuinely demanding day is expected and useful information — it's telling you tonight's sleep matters more than usual. What it shouldn't do is show up after an ordinary day of light activity. If the number and how you actually feel keep disagreeing, that's a signal the model needs tuning, not a signal to ignore your own judgment.

What "Moved It Today" Actually Shows

Every drop and rise on the detail screen is labeled by what genuinely caused it — a workout, a stretch of elevated heart rate, a period of calorie burn, a rest block — never a guess dressed up as a fact. Small movements that don't deserve their own headline are grouped into a single "background activity" line, so the list you see always adds up to the number on screen. Nothing is hidden, and nothing is invented.

Built on Assumptions, Not Guarantees

Like any model built from wearable data, the Energy Battery makes reasonable assumptions rather than measuring energy directly — there's no such thing as a literal "energy" reading. It assumes your own recent history is the right baseline for what's normal for you, that heart rate elevation and calorie burn are reasonable proxies for exertion, and that stillness plus a calm heart rate is a reasonable proxy for rest. Those assumptions hold well most of the time, and less well in edge cases — illness, unusual stress, or a wearable that misses a workout entirely can all throw the number off for a day. It's a strong daily signal, not a clinical measurement.

How to Actually Use It

✓ Do this
✓ Check it before deciding how hard to push a workout
✓ Read the "what moved it" list when a number surprises you
✓ Treat a low reading late in the day as a cue to wind down early
✓ Watch the pattern over days, not one snapshot
✗ Avoid this
✗ Treating it as a medical or diagnostic reading
✗ Panicking over a single low number on an otherwise fine day
✗ Comparing your number to someone else's
✗ Ignoring how you actually feel in favor of the number

No Data Ever Leaves Your Phone

The Energy Battery is computed entirely on your device from your own Apple Health data. Nothing about your sleep, heart rate, or activity is ever sent to a server to produce this number — no account, no cloud processing, no exceptions. It's yours, calculated locally, every time you open the app.

See your own reserve, right now.

Free, on-device, built from the Apple Health data you already have. No account, no AI key needed for the battery itself.

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