How to check condition for certain amount of time before flow execution?

If I understand the use case correctly, it is a bit more complicated than @Adrian_Rockall described.
The power of the device in idle state is about 100 W. 100 W is also less than 900 W. So there would be no difference if the device has a problem (600 - 800 W) or just went into idle state (100 W), the notification will be send in both cases, because the power is in both cases less than 900 W
Therefore, the flow that is started after the timer expires needs to check the power again:

Flow #3 (like Adrian suggested)

– Timer is finished
– power is < 900 W (logic card)
– power is > 500 W (logic card, or an other value)
– Send notification