Use and interpret PV forecasts

Know which solar estimate you are using

Vega keeps three kinds of estimated solar production separate because they answer different questions:

• Production Forecast is a short-term operational forecast for the current local day through two days ahead (D through D+2). Use it when planning around likely near-term PV production.
• Expected Production is a historical benchmark calculated from observed weather. Use it to assess completed periods, not as a substitute for a missing operational forecast.
• PVGIS Calculation is a long-term monthly and annual reference. It describes typical production and variability rather than the expected output of a particular upcoming day.

This guide concerns Production Forecast. See Evaluate solar production and self-consumption for historical performance indicators and the PVGIS reference.

Check that the component can produce a forecast

Operational forecasting is available only for a supported PV component with the required configuration and weather-data context. Before relying on it:

1. Open Inventory and select the site.
2. Confirm the site has valid coordinates on its General tab.
3. Open Components and select the directly connected PV component.
4. On Settings, confirm that it is marked as a PV installation and has valid tilt and azimuth values.
5. Check that the component synchronizes production measurements and that the site can synchronize weather data.
6. Review Measurements and confirm that recent production data exists.

Virtual components and calculated PV-array components do not provide the operational forecast view. If PV Forecast reports that forecasting is unavailable, review the site coordinates and component geometry, then use Connect a solar power plant or contact the person responsible for the connection.

Open the PV Forecast timeline

1. Open Inventory from the left navigation.
2. Select the site and open Components.
3. Select the supported PV component.
4. Select PV Forecast.
5. Wait for the current local month to load.
6. Select Refresh when you need to reload the latest stored values.

Use the left chevron to inspect an earlier month. The right chevron moves forward again but stops at the current month. Select Current month to return directly to the operational horizon.

Read the forecast table

Each row represents one day in the component’s time zone. The table places Actual and Forecast values side by side for:

• Production (kWh), the daily PV energy total, and
• HPOA (kWh/m²), the daily plane-of-array irradiation total.

The Today, D+1, and D+2 badges identify the operational forecast horizon. Forecast updated shows the issuance time associated with that day’s complete forecast inputs, formatted in the component’s time zone.

Vega shows actual daily totals only for completed local days. It does not present a partial current-day actual total in this table. An em dash means that the value is unavailable; it must not be read as zero production or zero irradiation.

An amber warning beside forecast production means its inputs are incomplete or come from different refreshes. Treat that row as unsuitable for a precise forecast-versus-actual conclusion until a coherent forecast is available.

For earlier dates, Vega retains the final same-day forecast available for each date. This allows historical review, but it is not a record of every forecast revision issued before that day.

Ask Vega for a forecast

Use the exact solar component and a date within the operational horizon. For example:

For @Main Site / Roof PV, show the Production Forecast for today, D+1, and D+2 as daily kWh. Include the forecast issuance time and identify any date with missing or inconsistent forecast data.

Vega can return a total, table, or chart using the Production Forecast energy metric. To compare near-term forecast production with recorded production, ask for both metrics over the same component and interval:

For @Main Site / Roof PV, compare hourly Production and Production Forecast for today and show them as a table and line chart. State the forecast issuance time and leave unavailable values empty.

When Vega produces a visualization, select Open using the external-link action on the response. In the detailed chart window, confirm Energy, Production Forecast, the component, period, and interval before interpreting the result. See Select metrics, meters, and sites for the controls.

Forecast series use a dashed line in charts, and unavailable forecast points remain gaps rather than being filled with zero. A total can hide when values are missing, so use Hourly or Daily plus Table when checking coverage.

Compare forecasts with actual production correctly

A fair comparison uses the same component, time zone, period, and aggregation interval. Follow these steps after the day is complete:

1. Open the component’s PV Forecast tab and locate the completed day.
2. Confirm that both actual and forecast production are present.
3. Check for the amber inconsistent-input warning.
4. Record the displayed forecast issuance time.
5. Compare daily production in kWh, then inspect hourly values in Chat if you need to understand the timing of the difference.
6. Check measurement coverage before describing forecast error.

The current-day forecast contains a complete local-day profile, including hours earlier than its displayed issuance time. Those earlier hours were not predictions available before they occurred. Do not describe the entire current-day profile as an advance forecast or use it for a strict forecast-accuracy calculation without accounting for the issuance time.

Forecast values can change between synchronization cycles. If one result combines more than one issuance, component, weather model, or production model, ask Vega to narrow the period and component and report the provenance separately. Do not attribute a mixed result to a single forecast run.

Resolve missing forecast data

If the tab says Operational PV forecasting is unavailable for this component:

1. Confirm that you selected a directly connected PV component rather than a virtual or calculated component.
2. Verify the site coordinates.
3. Verify the component’s PV designation, tilt, and azimuth.
4. Check that weather and production synchronization have started for the site.

If forecasting is available but the month contains no values, or Chat reports that no forecast was generated:

1. Return to the current month and check whether the requested date is within D through D+2.
2. Select Refresh.
3. Open Measurements and inspect the Production Forecast calendar when available.
4. Select a day to review hourly forecast production and its issuance, provider, weather model, production model ID, and horizon.
5. Check Health and the Event Log for missing inputs or synchronization findings.
6. Contact support if a correctly configured component repeatedly has no current forecast.

A message that no data was found for the requested period can also include the dates covered by stored forecasts and their latest issuance. Change the requested period only when those dates answer the original question; never silently substitute Expected Production or PVGIS values.

Use forecasts as planning evidence

Forecasts describe a modeled outcome, not a guarantee. Before using one for an operational decision, record the component, dates, time zone, units, issuance time, horizon, and any completeness warning. Consider uncertainty and the cost of forecast error, and verify critical decisions against current operating conditions.