Confirm a real shutdown
Choose Shut Down from the Apple menu, wait for the display and system activity to stop, and do not confuse a locked screen, closed lid, or restart with shutdown. At the next startup, note the battery percentage and elapsed time. Repeat for comparable periods without peripherals attached if possible.
A small difference between readings can be affected by how battery charge is estimated and displayed. What merits attention is a repeatable, material change across controlled shutdown periods, especially if the Mac is also difficult to start or behaves unexpectedly when powered on.
Do not apply sleep troubleshooting to shutdown
Sleep logs, wake reasons, and apps preventing sleep apply when macOS is running or sleeping. They do not supply a history for the time after shutdown. WattGuard therefore has no magic repair for shutdown drain and should not be used to claim that a particular process consumed power while the system was off.
Similarly, do not change hibernation settings in an attempt to solve a powered-off symptom. Hibernation and sleep configuration do not establish the cause of a battery loss during a confirmed shutdown.
Check condition and isolate the environment
Check the battery's condition in System Settings. A battery that needs service can provide less usable runtime, but condition alone does not identify every shutdown symptom. For a comparison, fully shut down, remove nonessential accessories, use the same temperature range, and record charge before and after one or more similar periods.
If the pattern is repeatable, run Apple Diagnostics and contact Apple Support or a service provider with the record. Mention that the test was performed after a confirmed shutdown and include elapsed time, battery readings, charging state, and whether accessories were disconnected.
Use WattGuard where it fits
Use WattGuard before shutdown to distinguish a prior sleep-drain or wake issue from the later powered-off period. Its local sleep timeline can help you avoid treating an earlier wake as shutdown loss. Once the Mac is shut down, diagnostics must rely on the next boot, battery condition, and service investigation rather than background monitoring.