Home Assistant templates: states are strings, and 'unavailable' breaks your math

Three facts that cause most template bugs

1. Every state is text. "21.5" is a string until you convert it.
2. Missing entity → "unknown". A typo in an entity ID doesn't raise an error; states() returns unknown.
3. Offline device → "unavailable". unknown means "exists, value not known right now". unavailable means "can't be reached" (device offline, integration failed to load).

So {{ states('sensor.outdoor_temp') | float + 5 }} fails exactly when the sensor goes offline, often at night, in an automation nobody is watching.

Patterns

{# Convert with a fallback: 0 is used when the state is unavailable/unknown #}
{{ states('sensor.outdoor_temp') | float(0) + 5 }}

{# Better when 0 would be a misleading value: skip the calculation #}
{% if has_value('sensor.outdoor_temp') %}
  {{ (states('sensor.outdoor_temp') | float * 1.8 + 32) | round(1) }}
{% else %}
  unavailable
{% endif %}

{# Attributes: use state_attr(), compare with is_state() / is_state_attr() #}
{{ state_attr('climate.living_room', 'current_temperature') }}
{{ is_state('binary_sensor.front_door', 'on') }}

For template sensors

• Pick the fallback deliberately. float(0) makes a dead temperature sensor report 0 °C, which can trigger heating automations. Prefer an availability condition (has_value(...)) so the template sensor itself becomes unavailable.
• Test in Developer tools → Template with the source entity switched off or renamed.
• Use states('...') rather than attribute access on states.domain.entity, so missing entities give you unknown instead of an error.