Emerald Chronometer Faces for Wear OS by Google
Copyright © 2018, Emerald Sequoia LLC
To save energy in this "ambient mode", Emerald Chronometer uses a low power (mostly black) version of the face that you've selected; this face is shown to the right of the "interactive mode" face on each of the watch face description pages in this help site. (Most watch devices today are OLED devices, where each pixel uses power proportional to the intensity of the pixel's color, so black is the lowest-power color, and white is the highest).
Considerable attention has been paid to the design of these "ambient mode" faces. In particular:
The idea is that you should be easily able to tell the time on the watch in any ambient mode, even outside (though to be clear, in sunlight and other bright daylight *nothing* will be visible on the face on today's devices). And inside in a relatively dark room, you should be able to see almost all of the information available on the face, even in ambient mode.
Whether the extra power required for "Always on" is worth it depends on your particular situation; different devices have differing battery capacities and different power curves, and if you need the GPS or WiFi or the cell modem (on watches that have these functions), it may be that you'll need to conserve energy as much as possible. But note that since Emerald Chronometer doesn't need the network except for time and location syncing, you can always put your watch in Airplane mode and pretend you have a "dumb" mechanical watch. :-)