What is battery storage readiness (ESS-ready) and which ADUs need it?

Battery storage readiness — often called ESS-ready (Energy Storage System ready) — means the electrical design includes the panel configuration, conduit, and reserved space needed to add a home battery later without tearing into finished walls. A growing number of new detached ADU projects in California are subject to ESS-ready provisions under the applicable energy code, and the requirements are still evolving.

What ESS-ready provisions typically include

Where the energy code requires it, the plans generally need to show:

• A dedicated subpanel — sometimes called the "emergency" or "backup" panel — fed by at least a 60A circuit from the main panel.
• At least four branch circuits identified on the panel schedule as "backed-up" circuits (the loads the battery would carry during an outage).
• Conduit routed from the main panel to the planned battery installation location.
• Reserved breaker space for the future battery connection.

The intent is that a homeowner can add a battery system later as a relatively simple connection rather than a major rewire.

How ESS-ready relates to solar

ESS-ready provisions frequently travel alongside solar requirements, because batteries and PV are designed to work together. For qualifying newly constructed detached ADUs, the energy code may require solar PV (or, where not required, solar-ready conduit). When both apply, the electrical plan coordinates:

1. Solar PV interconnection point and conductor routing.
2. The backup/emergency subpanel and its backed-up circuits.
3. Conduit pathways for both the PV system and the future battery.
4. Panel space and labeling for all of the above.

Which ADUs actually need it

This is squarely a "verify for your specific project" item. Applicability depends on whether the ADU is newly constructed and detached, the conditioned floor area, the climate zone, and the energy-code version in effect — and local jurisdictions can layer additional requirements. Garage conversions and attached additions often follow different rules than ground-up detached units.

Requirements vary by jurisdiction and energy-code cycle — confirm what ESS-ready and solar provisions apply to your specific project with your local building department.

When these provisions apply, they must appear on the electrical plans; our electrical plan sets incorporate the backup panel, backed-up circuits, and conduit, and our Title 24 reports document the energy-code basis. See the complete MEP guide for how solar, storage, and panel design fit together.