Provide JavaScript example for persisting total price (openhab#16889)

Signed-off-by: Jacob Laursen <jacob-github@vindvejr.dk>

bundles/org.openhab.binding.energidataservice/README.md

@@ -58,6 +58,8 @@ It will not impact channels, see [Electricity Tax](#electricity-tax) for further
 | co2-emission-prognosis | Number:EmissionIntensity | Estimated prognosis for CO₂ emission following the day-ahead market in g/kWh |
 | co2-emission-realtime | Number:EmissionIntensity | Near up-to-date history for CO₂ emission from electricity consumed in Denmark in g/kWh |
 
+#### Total Price
+
 _Please note:_ There is no channel providing the total price.
 Instead, create a group item with `SUM` as aggregate function and add the individual price items as children.
 This has the following advantages:
@@ -69,6 +71,41 @@ This has the following advantages:
 If you want electricity tax included in your total price, please add either `electricity-tax` or `reduced-electricity-tax` to the group - depending on which one applies.
 See [Electricity Tax](#electricity-tax) for further information.
 
+##### Time Series
+
+Group items with aggregate functions are not automatically recalculated into the future when the time series for child items are updated.
+Therefore, the `SUM` function mentioned above will only work for the current price.
+Calculation of future total prices can be achieved with a rule (in this example file-based using Rule Builder):
+
+:::: tabs
+
+::: tab JavaScript
+
+```javascript
+rules.when()
+ .channel('energidataservice:service:energidataservice:electricity#event').triggered('DAY_AHEAD_AVAILABLE')
+ .then(event => {
+ var timeSeries = new items.TimeSeries('REPLACE');
+ var start = time.LocalDate.now().atStartOfDay().atZone(time.ZoneId.systemDefault());
+ var spotPrices = items.SpotPrice.persistence.getAllStatesBetween(start, start.plusDays(2));
+ for (var spotPrice of spotPrices) {
+ var totalPrice = spotPrice.quantityState
+ .add(items.GridTariff.persistence.persistedState(spotPrice.timestamp).quantityState)
+ .add(items.SystemTariff.persistence.persistedState(spotPrice.timestamp).quantityState)
+ .add(items.TransmissionGridTariff.persistence.persistedState(spotPrice.timestamp).quantityState)
+ .add(items.ElectricityTax.persistence.persistedState(spotPrice.timestamp).quantityState);
+
+ timeSeries.add(spotPrice.timestamp, totalPrice);
+ }
+ items.TotalPrice.persistence.persist(timeSeries);
+ })
+ .build("Calculate total price");
+```
+
+:::
+
+::::
+
 #### Currencies
 
 There are some existing limitations related to currency support.