Transfer switches for solar (ATS and MTS)

A transfer switch is what lets a home run on backup power without ever feeding the utility grid. In solar-plus-storage systems it creates the clean separation between grid and battery that safety and code demand. The two kinds — automatic and manual — differ in how that switch happens. Here is how each works and where it belongs.

What a transfer switch does

When the grid goes down, a backup source (battery or generator) needs to power the home's loads — but it must never energize the utility lines, which could injure line workers and damage equipment. A transfer switch guarantees this by:

• Connecting loads to either the grid or the backup source, never both.
• Physically breaking the grid connection during backup operation (open transition).
• Providing the anti-islanding separation the utility requires.

This break is why a properly islanded system is safe during an outage.

Automatic transfer switch (ATS)

An ATS senses a grid outage and switches to backup without human action:

• Detects loss of utility voltage within seconds.
• Disconnects from the grid and connects loads to the backup source.
• Reconnects to the grid automatically when utility power returns and stabilizes.

Most modern solar-plus-battery systems use an integrated ATS — often built into the hybrid inverter or a dedicated backup gateway — so backup is seamless. This is the norm for whole-home and partial-home battery backup where the homeowner wants no interruption.

Manual transfer switch (MTS)

An MTS requires a person to operate it:

• The homeowner physically flips the switch to move selected loads to backup.
• Common with portable or standby generators feeding a small set of critical circuits.
• Lower cost and simpler, but no automatic changeover.

MTS setups are typical for generator backup or budget-conscious installs where a brief manual step during an outage is acceptable.

Where it fits with storage

In a battery backup design, the transfer point defines what stays powered during an outage. Combined with load management — such as a smart electrical panel — an ATS lets the system island cleanly and shed non-essential loads so the battery lasts longer. The switch and its rating appear on the one-line diagram along with the interconnection point.

Code and listing notes

Transfer equipment must be listed for the purpose and installed per the NEC and manufacturer instructions. Reviewers confirm the anti-islanding means, the switch rating, and that the backup source cannot backfeed the utility. These details belong on the permit set — see what is in a solar permit set.

Use an ATS for automatic, seamless battery backup; an MTS where a manual generator changeover is acceptable. Confirm listings and ratings against the manufacturer documentation, and have a licensed electrician or PE review the final design. Sizing storage? See how to size a solar battery backup.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.