All about the Insolation-adjusted Expected Energy Ratio (IEER), a configurable monthly weather-adjusted performance ratio for your sites

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Written by Locus Energy
Updated over a week ago

Insolation-adjusted Expected Energy Ratio (IEER) is a site-level type of monthly weather-adjusted performance ratio that is often used because of the simplicity and transparency of the calculation. The formula for IEER is:

IEER = (Em / Ee) / (Im / Ie)  


  • Em = Measured AC Energy

  • Ee = Expected AC Energy (i.e., long-term TMY-based expected energy)

  • Im = measured sunlight insolation

  • Ie = expected sunlight insolation (i.e., long-term TMY-based expected sunlight)

In order to give users full control over IEER, a new data set of user-entered insolation expectations has been created. Similar to user-entered energy expectations, LocusNOC provides an interface for users to submit monthly insolation expectations. This metric can only be configured through the LocusNOC config page.


To configure the inputs, select a site from the Search-Browse Pane, and navigate to the IEER Settings tab. Ensure that “Enable IEER” is checked. Select the drop down for each of the four inputs to configure IEER. Click “Save IEER” once you have finished.

Note: If a value is not available for one of the inputs see below on how to configure the inputs.

Measured Insolation (Im)

Measured sunlight insolation can be configured to draw from a variety of sources, both measured from a sensor and modeled based on location.

Adjustments based on measured insolation are a key addition to the platform. The user can specify any measured insolation field from weather components on the site. From the drop down for “Data Input for Measured Insolation” first select the component, followed by the measured irradiance field.

If a measured source is not available or deemed inaccurate, the Modeled Global Horizontal Insolation is available at the site level. This modeled value is derived from Locus Energy’s Virtual Irradiance and require a valid US address.

Note: Modeled fields that are available for sub-components will generate the same values as the site level modeled fields and are interchangeable.Note: In most instances Expected Insolation is output in Global Horizontal Insolation (GHI). In order to create an apt comparison, select a Global Horizontal source for measured irradiance sources in these instances.  In order to use a POA sensor for measured insolation, user must upload POA-based Expected Insolation values.

Expected Insolation (Ie)

Expected sunlight insolation is available from two sources:

  • Expected Insolation-TMY3: refers to the typical meteorological year drawn from the nearest TMY3 station. In order for this option to be visible the site must have a valid address in the US.

  • Expected Insolation: refers to the user input values on the Insolation Expectations tab. In order for this option to be visible the user must enter values for each month and click “Save Insolation Expectations.”

Measured AC Energy (Em)

Measured AC Energy will always come from the site level aggregated energy. The site level aggregation is dictated by the site’s hierarchy.

Expected AC Energy (Ee)

Expected AC Energy is uploaded under the Energy Expectations tab at the site level. See instructions specific to this tab to upload values. Ensure there are monthly values for all months in which the site is expected to operate.


Q: Where can I find IEER in the LocusNOC charting tool?
A: In the charting tool, IEER can be found on the left hand side of the screen under Modeled Data.

Q: Where can I find IEER in the LocusNOC reporting tool?
A: In the New Reporting Interface, select your site using the search box in the top left. IEER can be found under Modeled Data.

Q: How accurate is IEER?
A: The accuracy of this metric is highly dependent on the accuracy of the original design model’s expected energy output. For example, if the original design model was 5% high, this IEER metric will skew 5% low, and vice versa. Also, if the site is over-built with a high DC to AC ratio (e.g., 150 kW of DC capacity with a 100 kW inverter), the ratio adjustment will tend to be inaccurate.

Q: Is IEER replacing GEER?
A: GEER is a metric that has been available to users in LocusNOC in multiple views. It is a less flexible version of IEER, with Measured Insolation being supplied from Virtual Irradiance and Expected Insolation from TMY3 data. At this time, IEER is not replacing GEER. IEER will be available alongside GEER in the charting tool, reports and External API.

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