How I can automate the low-power mode of the Apple Watch during the day?

Greetings Positive_Nerd, 

There doesn’t seem to be a way in Settings > Battery or the Control Panel that automates enabling Low Power Mode after disabling it for overnight use. 

Here is some information on using Low Power Mode though and the features/functions it impacts - Use Low Power Mode on your Apple Watch. Perhaps there are some other things that can be adjusted to help with battery life, if that is your concern.

Low Power Mode turns off these features

• Always On display

• Heart rate notifications for irregular rhythm, high heart rate, and low heart rate

• Background heart rate measurements

• Background blood oxygen measurements

• Start workout reminder

Low Power Mode turns off these features when your iPhone isn’t nearby

• Wi-Fi and cellular connections

• Incoming phone calls and notifications

When you're in Low Power Mode without your iPhone nearby and you open an app that requires a data connection, Wi-Fi or cellular turns on so that the app can work. This can cause significant battery consumption.

Missed phone calls and notifications are retrieved once an hour when you're in Low Power Mode.

Low Power Mode affects these features

• Making a phone call can take longer

• Background app refresh happens less frequently

• Complications update less frequently

• Siri can take longer to process a request

• Some animations and scrolling might appear less smooth

Thanks. 

Since what you've asked about, is not an available option on Apple Watch, feel free to make a feature suggestion to Apple here --> Feedback - Watch - Apple