Energy dashboard: support cost tracking (entity/static price) for external statistics

[loganrosen]

Problem

When an energy source uses an external statistic (e.g., from Opower — opower:cned_elec_xxx_energy_consumption), the energy dashboard's "Use an entity with current price" and "Use a static price" options are disabled in the frontend. Users cannot set up cost tracking for any integration that provides external statistics rather than regular entities.

This affects every Opower user whose utility doesn't provide cost data through the API — ConEd, National Grid, PECO, and others. See core#108551 (48 comments, 10 👍, open since Jan 2024) and this community forum thread.

Why it happens

EnergyCostSensor in homeassistant/components/energy/sensor.py is built around real-time entity state tracking:

1. _process_sensor_data() calls valid_entity_id(stat_energy) — this rejects external statistic IDs (which use : instead of .), so cost sensors are never created.
2. Even if that check were removed, _update_cost() calls hass.states.get() and async_track_state_change_event() on the energy source — neither works for external statistics since they don't live in the state machine.

The frontend disables the price options to match this backend limitation (dialog-energy-grid-settings.ts).

Secondary issue: WebSocket API allows invalid configuration silently

The energy/save_prefs WebSocket API accepts entity_energy_price alongside an external statistic without any validation:

• Config saves successfully, no error
• energy/info returns cost_sensors: {} with no explanation
• No log warnings, no UI indication anything is wrong

This creates a silent failure pit for anyone using the API directly. This issue is tracked in home-assistant/core#165581.

Proposed solution

Support cost calculation for external statistics. The cost sensor would need a statistics-based update path rather than entity state tracking:

• Poll get_last_statistics() periodically (e.g., hourly, matching typical external stat update frequency) instead of relying on state change events
• Remove the valid_entity_id() gate in _process_sensor_data
• Remove the disabled flag on price radio buttons for external statistics in the frontend

As a smaller complementary fix, energy/save_prefs should validate that entity_energy_price/number_energy_price aren't set when stat_energy_from is an external statistic, or at least log a warning — matching the frontend's existing behavior.

Related

• core#108551 — Opower ConEd $0 cost (2+ years, 48 comments)
• Community forum: Electricity price cannot be set when using an external statistic
• architecture#1335 — Energy dashboard evolved (broader energy dashboard evolution discussion)