When an inverter goes offline (returns an error, incomplete read, timeout, or other errors) entities will change state to unavailable so that stale or bad data is not reported: this a by design, not a bug. Templates that use these entities should be updated to handle this condition.

This is a template issue that needs to be modified to check the entity is numeric, add a default value, or possibly add availability_template to it. (Note: the integration author does not use templates so these suggestions should be tested and/or updated then remove this note.)

Use states() rounded flag

When you see an error like this:

2023-08-11 22:57:25.547 ERROR (MainThread) [homeassistant.helpers.template_entity] TemplateError('ValueError: Template error: round got invalid input 'unavailable' when rendering template '{{ states('sensor.solaredge_b1_state_of_energy') | round(0) }}' but no default was specified') while processing template 'Template<template=({{ states('sensor.solaredge_b1_state_of_energy') | round(0) }}) renders=786>' for attribute '_attr_native_value' in entity 'sensor.battery_in_percent'

Instead of using round() use the rounded flag like this:

{{ states('sensor.solaredge_b1_state_of_energy', rounded=True) }}

Using the rounded flag not only avoids the use of round(0), but also applies the entity the default precision setting.

Use availability_template

Use availability templates to flag templates as unavailable. Without an availability template, template sensors assume always available.

https://www.home-assistant.io/integrations/template/#availability_template

https://www.home-assistant.io/docs/configuration/templating/#states

https://community.home-assistant.io/t/template-sensor-with-unavailable-sensor/416676

https://community.home-assistant.io/t/how-to-prevent-unavailable-in-template-sensors/398409/3