A battery percentage is an estimate of the charge remaining. Depending on the equipment, it may use measured current over time, battery voltage, information from the battery itself, or a combination of methods.
Compare like-for-like readings
If two screens disagree, check what each one is showing and which battery it refers to. A percentage from one device is not necessarily calculated in the same way as a percentage from another. Voltage-based estimates can also change when a load or charging source is switched on or off.
Some monitoring systems need correct capacity settings or a documented synchronisation process. Follow the manual for your exact equipment; do not force a reset to 100% simply to make two screens agree.
Gather a useful pattern
Note the reading before and after charging, which appliances are running and any warnings. A photo of the display and the relevant model names can help your installer interpret the behaviour.
If the reading jumps repeatedly or the battery shuts down much earlier than expected, arrange a system check. Until the cause is understood, avoid relying on the percentage alone when planning how long you can remain away from a charging source. See estimating battery runtime for the wider picture.