Hello William Ellwin,
Because you see a different outcome when unpaired vs. paired I am wondering if this could be driven by the shared features between your Apple Watch, iPhone, and iCloud.
This Apple Lithium-Ion Batteries information page has many recommendations for better battery performance. The most important ones in my experience, have been enabling Bluetooth on the iPhone and Watch as well as making sure your watch band is tight enough to maintain a constant "on your wrist" connection. If the band is loose it could be working extra hard to maintain a connected reading from the optional sensor.
https://www.apple.com/batteries/maximizing-performance/
"Adjust your settings.
There are a few ways you can preserve battery life on your Apple Watch:
- During running and walking workouts, turn on Power Saving Mode to disable the heart rate sensor. To do this, open the Apple Watch app on iPhone, go to My Watch > Workout, and turn on Power Saving Mode. Note that when the heart rate sensor is off, calorie burn calculations may not be as accurate.
- For longer workouts, you can choose to use a Bluetooth chest strap instead of the built-in heart rate sensor. To pair the Bluetooth chest strap with your watch, make sure the strap is in pairing mode, then open Settings on Apple Watch, select Bluetooth, and choose from the list of Health Devices.
- If you’re very active with your hands and your watch display turns on more than you think it should, you can prevent the display from turning on every time you raise your wrist. Open Settings on Apple Watch, select General, select Wake Screen, and turn Wake Screen on Wrist Raise off. When you want to turn on the display, just tap it or press the Digital Crown.
- Disabling Bluetooth on your iPhone increases the battery drain on your Apple Watch. For more power-efficient communication between the devices, keep Bluetooth enabled on iPhone."
I hope this helps.
