This is an unrealistic problem. The player is very unlikely to leave the game running unpaused and walk away from the computer for an extended period while the computer is running on battery. Gamers understand that games use battery power. Also, think about when a player is likely to leave their device on battery power. You're most likely to use the battery if you're outdoors, traveling, or in a public place without easy access to outlets (e.g. at school). In any of these circumstances, the user is unlikely to leave his or her device unattended because it could get stolen. In places where it's safe to leave a device unattended, such as at home, the user likely has access to outlets to plug in his or her device.
You should of course create a pause menu for when the player needs to leave the game unattended for a short period. While the game is paused, you can do various things to reduce power usage, such as:
Time.timeScale = 0
- Temporarily disable components that don't need to run while the game is paused
- Disable your main camera, and enable a camera that uses the Culling Mask to only render the pause menu.
Players who need to step away from the game for an extended period are more likely to minimize (if not quit) the game. In the Player Settings, you can disable the "Run in Background" option to greatly reduce battery usage while the game is minimized.
If it's a multiplayer game that can't be paused - well, players shouldn't be leaving the game running unattended while they're in multiplayer, so why optimize for this edge case?
When developing a game, it's best not to let yourself get distracted trying to solve hypothetical problems that are unlikely to come up in the real world. Focus your time on actually developing and publishing your game, not on unnecessary features.