What Is a Microgrid Interconnect Device? The Switch Behind Battery Backup

Any battery system that keeps your lights on in a blackout has one of these, and most homeowners never learn its name.

Short answer: a microgrid interconnect device (MID) is the automatic switch between your home and the utility in a battery or solar-plus-battery system. When the grid fails, it opens. Your house becomes a small, isolated "microgrid" that the battery can power without backfeeding the utility lines. When the grid comes back and settles, it closes again. The NEC requires one for any connection between a microgrid and the utility (NEC 705.70).

What counts as a "microgrid" in a house?

In the NEC, a microgrid is a system that can run connected to the utility or on its own. A home with a battery, with or without solar, fits that definition.

Normally the system runs in parallel with the grid. In an outage it runs as an island, cut off from the utility and supplying your loads by itself. NEC 705.50 describes these two operating modes.

The MID is the device that moves the house between them. The NEC defines it as a device that lets a microgrid separate from, and reconnect to, its primary power source, which for a house is the utility.

The separation is the whole point. Grid-tied inverters are required to stop when the grid disappears; why solar shuts off in a blackout covers the reason. Once the MID has physically disconnected the house, nothing connects your wiring to the utility lines, so the battery inverter can safely take over.

What happens in an outage

1. The grid fails. The system's controls detect that utility voltage is gone or out of range.
2. The MID opens, disconnecting your home from the utility.
3. The battery inverter takes over and supplies the backed-up circuits on its own.
4. Solar restarts inside the island, if the system supports it, and can recharge the battery during daylight.
5. The grid returns. The controls wait for it to stabilize, match it, and close the MID. The house is back on utility power.

Manufacturers describe the transition as seamless. In practice most equipment rides through it, but zero interruption isn't guaranteed. If you have sensitive equipment, ask for the rated transfer time.

Where is the MID installed?

Where the MID sits determines what stays powered:

The meter-socket version is a specialized meter collar, and like any collar it needs the utility's approval before it goes in.

The built-in version is common on hybrid inverters. An informational note in NEC 705.70 points out that MID functionality is often built into an inverter or energy storage system instead of being a separate box.

Choosing between whole-home and partial backup depends on battery capacity and power output, not on the MID. See essential loads vs whole-home battery backup.

Examples of the product category

Battery makers build the MID into their own equipment, and each maker describes it in its own terms:

• Tesla Backup Gateway and Backup Switch. Tesla says both control the Powerwall system's connection to the grid and detect outages automatically. The Backup Switch installs in the meter socket. See Tesla Backup Switch vs Backup Gateway.
• Enphase IQ System Controller. Enphase's datasheet says it provides MID functionality for IQ Battery systems.
• FranklinWH aGate. FranklinWH lists a microgrid interconnect device among the aGate's functions.

The battery system's own controls coordinate the MID, so it is normally part of that brand's system and not a part you can swap between brands. Features and compatibility change between product generations, so check the current documentation with your installer.

MID vs transfer switch vs interlock

All three keep a home power source off the utility lines. They do it in different ways:

The practical difference in an outage: with a standby generator, the automatic transfer switch leaves the house dark while the engine starts and stabilizes. A battery inverter is already running, so the switchover is far quicker.

A generator transfer switch isn't a stand-in for a MID. NEC 705.70 requires the MID to be listed or field labeled for that application. For the generator side, see transfer switch explained.

Not every home needs a grid-tied battery and a MID. A power station with a 120/240 V L14-30 outlet, like the EcoFlow DELTA Pro 3, can back up chosen circuits without fuel by feeding a manual transfer switch just as a portable generator would: it never runs in parallel with the grid, and the switch connects the utility or the station, never both.

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The Reliance kit takes that same L14-30 connection through its inlet box and cord. Like any transfer switch, it's an electrician install, usually with a permit.

How to tell whether your system has one

You can check these without opening anything:

• Look at the equipment near the meter and panel. A gateway, system controller, or backup switch from the battery maker, or a ring between the meter and its socket, is the MID or holds it.
• Read the placard. NEC 705.10 requires a permanent plaque or directory at the service equipment that identifies the power sources and where their disconnects are.
• Check the app. Battery apps typically show whether the system is on-grid or off-grid. Seeing "off-grid" during an outage means the MID has opened.
• Ask for the single-line diagram. It shows where the MID is and which circuits are backed up.

Leave the rest alone. Don't open the enclosure, pull the meter, or use any service override on the device. The meter socket belongs to the utility, and the conductors feeding a MID usually have no overcurrent protection except the utility's.

Why is this electrician work?

A MID sits at the service: in the meter socket, ahead of the main breaker, or as the main breaker itself. At that point the utility's full available fault current is present, and nothing upstream on your side will trip. Installing one takes a permit, an inspection, and the utility's interconnection approval. Meter-socket devices also need the utility to accept that specific product.

It's also a design decision, not only an installation. Where the MID goes sets which circuits are backed up, whether solar can keep running during an outage, and whether main panel work is needed.

Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• NEC Article 100. Defines the microgrid interconnect device in the 2023 edition onward. Earlier editions defined it in Article 705.
• NEC 705.70. MID requirements: required for any connection between a microgrid and the utility, listed or field labeled for the application, and overcurrent protection from all sources. Numbered 705.170 in the 2017 edition.
• NEC 705.50. Microgrid operation in interactive and island modes.
• NEC 705.40. Loss of primary source.
• NEC 705.10. Directory identifying the power sources.
• NEC 702.5. Transfer equipment for generators, for comparison.
• NEC Article 706. Energy storage systems.
• IEEE 1547 / UL 1741. Interconnection and anti-islanding requirements for inverters.

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• Tesla Backup Switch vs Backup Gateway
• Critical loads panel explained
• Home battery installation requirements
• Size your storage: home battery sizing calculator

More in our solar and storage guides.