Apple 7 watch charged 100% @ 9:00 AM. but down to 45% by 9:00 PM

To understand Apple Watch performance and its relation to your battery, see:

Apple Watch - Battery - Apple

Apple Watch Battery and Performance - Apple Support

“Battery life” is the amount of time your device runs before it needs to be recharged. “Battery lifespan” is the amount of time your battery lasts until it needs to be replaced. Maximize both and you’ll get the most out of your Apple devices, no matter which ones you own. See the "Tips for Apple Watch" section in:

Batteries - Maximizing Performance - Apple

Comparatively, the MacBook and iPad can also receive up to 1000 complete charge cycles while retaining up to 80% of original battery capacity. iPhones receive up to 500 complete charge cycles, and iPods receive up to 400 complete charge cycles before the battery may deplete further.

“Battery lifespan” is the amount of time your battery lasts until it needs to be replaced. Maximize both and you’ll get the most out of your Apple devices, no matter which ones you own.