Title 24 Daylighting Control Requirements, Explained

If your building has windows or skylights bringing in daylight, California's Title 24 energy code likely requires automatic controls that dim your electric lighting when there's enough natural light. These are called daylight-responsive controls, and the rules tightened in the 2025 update of the code, which takes effect for permits and projects under the new version starting January 1, 2026.

This guide explains the Title 24 daylighting requirements in plain terms: what daylight-responsive controls are, which zones in your building are covered, the wattage threshold that triggers the requirement, and how to comply. Everything below is drawn from Section 130.1(d) of Title 24, Part 6.

For the full picture of Title 24 lighting controls, see our main Title 24 lighting controls compliance guide.

What Daylighting Controls Are

Daylight-responsive controls (also called automatic daylighting controls or photosensor controls) reduce general lighting in response to increasing daylight. The idea is simple: when sunlight is already lighting a space, your electric lights don't need to run at full power. A photosensor measures available light and signals the lighting controls to dim down, saving energy without leaving the space too dark.

Title 24 requires these controls in areas that receive ample and consistent daylight. The control system continuously matches electric light output to how much daylight is present.

The Daylit Zones: Primary Sidelit, Secondary Sidelit, and Skylit

Title 24 defines three types of "daylit zones" — the areas near windows or skylights where daylighting controls may be required:

• Primary sidelit zone — the area directly adjacent to vertical fenestration, such as windows. This is the zone closest to the glass, where daylight is strongest.
• Secondary sidelit zone — the area adjacent to the primary sidelit zone, set back further from the windows, where daylight is weaker but still meaningful.
• Skylit zone — the area under toplighting, such as skylights, where daylight comes from above.

The first compliance step is identifying these zones in the building and defining their dimensions. Under Title 24, this zone information must be shown on the building plans. Zone dimensions depend on factors like the window head height (the distance from the finished floor to the top of the window); confirm the exact zone-depth definitions for your project against the current Section 130.1(d) text.

Note: if a single luminaire falls into both a skylit zone and a primary sidelit zone, it must be controlled as part of the skylit daylit zone.

The 75W Threshold and What's Required

Here's the trigger. The 2025 code requires that in any enclosed space where there is 75W or more of general lighting in any primary sidelit, secondary sidelit, or skylit daylit zone, automatic daylighting controls must be provided for the qualifying zones.

The 2025 update lowered the threshold for daylight response compared to the prior code, meaning more spaces now fall under the requirement than before.

A few specifics that come with the requirement:

• Photosensors must be installed in each daylit zone, positioned so they are not readily accessible to unauthorized personnel.
• Continuous dimming is required in zones that have multilevel lighting controls. The daylight-responsive controls must respond to sensor signals by dimming the general lighting continuously rather than in coarse steps.
• The 90% reduction rule: where daylight-enhanced light levels are greater than 150% of the general lighting design level, the general lighting power in that daylit zone must be reduced by at least 90%. In plain terms, when there's far more than enough daylight, the electric lights in that zone have to drop nearly all the way down.

Note: This article covers the general daylighting requirements. Title 24 includes specific exceptions and additional rules for certain space types (for example, parking garages handle sidelit zones differently). Always check the current code text for exceptions that may apply to your project.

How to Comply: Continuous-Dimming Photosensor Controls

Compliance comes down to installing daylight-responsive photosensor controls that dim general lighting continuously in your daylit zones, wired and commissioned to meet Section 130.1(d).

As an authorized Wattstopper and Lutron dealer, Rock Lighting & Electric specifies and supplies the daylighting control hardware that meets these requirements — photosensors, dimming controls, and the related lighting control devices. Both manufacturers offer Title 24-oriented daylighting and dimming product lines designed for continuous-dimming response in sidelit and skylit zones. Browse our Wattstopper collection for daylighting and lighting control options.

Because Title 24 also requires lighting controls acceptance testing (under Section 130.4) for installed controls, plan for that step as part of your project.

Talk to Rock Lighting & Electric

Not sure whether your daylit zones cross the 75W threshold, or which photosensor and dimming products fit your layout? Rock Lighting & Electric can help you spec compliant Wattstopper and Lutron daylighting controls for your California project. Contact us or browse the Wattstopper collection to get started.

FAQ

This article is general information, not compliance advice. Title 24, Part 6 is detailed and contains exceptions, space-specific rules, and acceptance testing requirements not fully covered here. Code language and effective dates can change. Always confirm requirements for your specific project with a qualified Title 24 professional and against the current adopted code text before designing, permitting, or installing.