# Add Lights

Lighting loads represent the electrical power consumed by lighting systems within a building simulation. This includes both artificial lighting fixtures and any associated control systems, where applicable.

Lighting loads are a key part of energy simulations, as they contribute to total energy use, affect peak electrical demand and influence the thermal performance of the space.

## Add Lights

To add lights, navigate to 'Spaces' and select the 'Default Space' or any other space available. Then navigate to 'Lights' and either 'Select from Template' or 'Create Custom' from the 'Set Lights' dropdown menu. 

'Select from Template' gives access to templates linked to your workflow. To view all templates, you must be in Designer Mode. In this example, select 'Create Custom'.

Next, select the 'Design Level' and click the panel showing the light density (W/m²).  When annual variability is required, these can be defined in detail. [See Set Multiple Occupant Density & Schedules](https://docs.betterbuilding.io/user-guide/2d-building/building/spaces/add-occupants#set-multiple-occupant-density-and-schedules).  Adjust the light density as needed, then click 'Update' to save. 

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## **Adjust Light Units**

To adjust light units to W, as opposed to the default W/m², navigate to 'Spaces' and select the 'Default Space' or any other space available. Then navigate to 'Lights' and 'Switch Schedule Type' to Watts.

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## **Set Light Heat Gains**

The heat generated by electrical lighting systems contributes to zone loads, with nearly all electrical input eventually converting into heat. This heat is divided into four components: Return Air Fraction, Radiant Fraction, Visible Fraction, and the remaining Convective Fraction, which directly warms the zone air. The Convective Fraction is calculated as:

*Convective Fraction = 1.0 – (Return Air Fraction + Radiant Fraction + Visible Fraction)*

Make sure the total of all fractions does not exceed **1.0** to avoid simulation errors.

### Return Air Fraction

This is the portion of lighting heat extracted by the return air system before reaching the occupied space. A LED downlight installed in a return plenum ceiling may have a Return Air Fraction of 0.80, meaning 80% of its heat is removed by the return air system, significantly reducing its impact on the zone.

### Radiant Fraction

This represents the long-wave thermal radiation from the fixture that heats surfaces like walls and furniture.  A LED downlight typically emits little radiant heat. For instance, with a Radiant Fraction of 0.05, only 5% of its heat contributes to surface heating in the zone.

### Visible Fraction

This is the short-wave (visible light) portion of heat, which eventually becomes thermal load after being absorbed by interior surfaces.  A LED downlight may have a Visible Fraction of 0.10, where 10% of its energy output initially enters the zone as light, later converting to heat.

### Convective Fraction

This is the residual fraction of heat that directly warms the air in the zone. For a LED downlight with Return Air = 0.80, Radiant = 0.05, and Visible = 0.10, the Convective Fraction is:

*1.0 – (0.80 + 0.05 + 0.10) = 0.05*

Only 5% of its heat directly convects into the zone air.

To set light heat gains, navigate to 'Spaces' and select the 'Default Space' or any other space available. Next, select 'Settings' and nominate the 'Visible Fraction', 'Radiant Fraction' and 'Return Air Fraction' then click 'Update' to save.

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