Unfortunately it doesn t solve the issue

[topaz vale]

the issue is that it's a sensor itself, so it's part of states.sensor. It's filtered out, but that doesn't prevent the warning.
maybe this does:

                        'sensor.custom_unmonitored_power',
                        'sensor.quadro_phase_a_active_power',
                        'sensor.quadro_quadro_total_active_power',
                        'sensor.ups_potenza_reale_nominale'
                      ]
%}
{% set sum_entities = states.sensor
                          | rejectattr('entity_id', 'in', ignore_list)
                          | rejectattr('entity_id', 'search', '^sensor.consumi_')
                          | selectattr('attributes.unit_of_measurement', 'defined')
                          | selectattr('attributes.unit_of_measurement', 'eq', 'W')
                          | map(attribute='entity_id')
                          | list
%}
{% set monitoredpower = sum_entities
                          | map('states')
                          | map('float', default=0)
                          | sum
                          | round(2)
%}
{%- set totalpower = states('sensor.quadro_phase_a_active_power') | float(0) -%}
{%- set delta = totalpower - monitoredpower -%}
{{ delta if delta > 0 else 0 }}

[sonic nexus]

thx for the quick follow-up

[topaz vale]

Does it work now?

[sonic nexus]

Seems good so far, 30 min without any single warning

It just seems a bit slower