nordpool_diff custom integration for Home Assistant

Requires https://github.com/custom-components/nordpool

Applies non-causal FIR differentiator¹ to Nord Pool SPOT prices, resulting in predictive sensor that:

Gives positive output when electricity prices are going to increase in the next few hours
Gives negative output when electricity prices are going to decrease in the next few hours
Gives ~zero output when electricity prices are going to stay ~constant for the next few hours

The output can be used for e.g. adjusting target temperature of a heater so that it will heat more before prices will go up (to allow heating less when prices are high), and let the temperature go down a bit before prices will go down.

Apart from potentially saving some money, this kind of “temporal shifting of heating” can also save the environment, because expensive peaks are produced by dirtier energy sources.

Installation

Install and configure first.
Copy the nordpool_diff folder to HA
/custom_components/nordpool_diff/
Restart HA. (Failing to restart before modifying configuration would give “Integration ‘nordpool_diff’ not found” error message from the configuration.)
Add the following to your configuration.yaml file:
```
sensor:
  - platform: nordpool_diff
    nordpool_entity: sensor.nordpool_kwh_fi_eur_3_095_024
    filter_length: 5
```
Modify the nordpool_entity value according to your exact entity value.

The filter_length value must be an integer, at least 2. Smallest possible value 2 produces FIR [-1, 1]. Value 5 produces FIR [-1, 0.25, 0.25, 0.25, 0.25]. First entry is always -1 and the filter is normalized so that its sum is zero. This way the characteristic output magnitude is independent of the filter length. Values larger than 8 have the problem that prices typically update 8 hours before midnight (in Finland), so at 15:59 you only know prices for the next 8 hours. But the filter algorithm pads missing data by using the last entry, so the result should still be quite reasonable.
Restart HA again to load the configuration. Now you should see nordpool_diff_N sensor, where N corresponds to filter_length. You can set up several nordpool_diff entities, each with different filter_length.