Utility meter and total cost counter, how to deal with tariff's unit rate changes?

Configuration
1

I have a utility meter entity that tracks the kWh comsumed in the peak and off peaks periods.
I then have a template entity that calculate the yearly cost

sensor:
  - platform: template
    sensors: 
      yearly_cost:
       value_template: "{{ (((0.3935 * states('sensor.yearly_import_energy_day')|float))+(0.075 * states('sensor.yearly_import_energy_night')|float))|round(2)}}"
       unit_of_measurement: '£'

If I manually change the value (0.3935), the entire “yearly cost” metric will be screwed up, how can I deal with this?

2

Manually calculate the total until the date the tariff changed. Put that as a constant in your calculation.

3

Is there a way to avoid having to do that every time the tariff changes?

4

Store the tariff in a helper. Take your daily consumption (take the previous period attribute from a utility meter with a daily cycle). Do this by triggering an automation on a specific time and take this previous value together with the tariff helper to calculate the last day’s total. Make a second helper, an input number, to store the running total. Add your new (calculated) value to the helper’s value. Wrap this helper in a template sensor and give it a state class for long-term tracking.

Now, if you change the tariff today, with tomorrow’s run it will calculate the total for today using the new tariff.

5

How can I apply the result of an operation (unit rate * daily consumption) to an input number?
The only services I see are ‘increment’ and ‘set’ (which needs a defined value)

6

Set can take a template, so you can reference the current value, for the equivalent of i = i + 1.

7

In fact, you don’t even need that helper I just realised now. You only need the sensor I referred to as the wrapper sensor. In it’s value template you can reference the current value too.

8

Ok, but this way my sensor will only update every 24 hours, right? Right now it’s continuously updating (almost every second) but I can’t change the tariff without affecting its value

P.S. I don’t think input number set can take a template

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9

Read my comment again. Make a utility meter using your current sensor. That will give you a daily value for the previous day.

Do you really want to see the cost at a level more granular than daily? If so, make a utility meter with an hourly cycle.

Why do you say that? I don’t know about the UI, but in YAML you can.