Single DER Limitation

The Microgrid Interconnect Device (MID) function is typically integrated into a single, specific, grid-forming DER - and is unable to interoperate with additional grid-forming DERs.

What is a DER?

A Distributed Energy Resource (DER) is any device that can generate, store, or manage electricity at or near where it will be used. In a home, DERs include solar panels, battery storage systems, EV chargers with vehicle-to-home (V2H) capability, and generators. A "grid-forming" DER is one that can create its own AC power independently - essentially acting as the grid for your home during an outage.

What is a MID?

A Microgrid Interconnect Device (MID), also known as a transfer switch, manages the connection between your home and the utility grid. During normal operation, it connects your home to the grid. During an outage, it safely disconnects from the grid and allows your home to be powered by local generation (battery, solar, generator, V2H).

The MID is a critical safety device - it prevents backfeeding power to the grid during an outage (which would endanger utility workers) and ensures smooth transitions between grid and island modes.

The Problem

Today, the MID function is typically integrated directly into a battery system or inverter. The Tesla Powerwall has a built-in transfer switch. This tight integration makes sense for simple systems with a single grid-forming device.

But what happens when you have multiple grid-forming DERs?

Modern Home Energy System

Homeowners increasingly want multiple backup power sources:

A Battery Energy Storage System (BESS) for daily cycling and short outages
Rooftop solar PV for daytime generation
A V2H-capable EV charger that can use the car's battery for extended backup
Maybe even a second EVSE for a second EV
A portable generator for emergencies

Each of these could potentially form a microgrid and power the home. But the integrated MID in your battery system doesn't know how to coordinate with your V2H charger or generator.

The Coordination Challenge

flowchart TB
    subgraph grid["Utility Grid"]
        G[("Grid
Connection")]
    end

    subgraph bess["Powerwall (BESS + MID)"]
        PW["Battery +
Integrated MID"]
    end

    subgraph home["Home Loads"]
        HL["Electrical
Panel"]
    end

    subgraph ders["Other DERs - Cannot Participate in Grid-Forming"]
        Solar["Solar
Inverter"]
        V2H["V2H EVSE
+ EV"]
        Gen["Generator"]
    end

    G <--> PW
    PW <--> HL
    HL --- Solar
    HL --- V2H
    HL --- Gen

    PW -.-x|"MID only knows
about Powerwall"| Solar
    PW -.-x|"Can't coordinate"| V2H
    PW -.-x|"with other DERs"| Gen

Current Situation: The MID integrated into the BESS cannot coordinate with other grid-forming DERs

Consequences

Underutilized resources: Your EV's 100kWh battery sits idle during outages because the MID doesn't know it can help
No graceful handoff: When the BESS depletes, you can't smoothly transition to generator backup
Solar curtailment issues: The BESS MID uses frequency shifting to curtail solar, causing side effects
Stranded investment: V2H capability you paid for can't be used for whole-home backup

The Growing Problem

This limitation becomes more acute as home energy systems grow more sophisticated:

V2H adoption: As more EVs and chargers support V2H/V2G, homes will have multiple potential backup sources
Battery expansion: Homeowners may want to add batteries from different manufacturers over time
Smart panels: New smart panels may include MID functionality - how do they coordinate with existing BESS?
Generator integration: Portable and permanent generators should integrate seamlessly with other DERs

Impact on Homeowners

Can't fully utilize multi-source backup systems
Vendor lock-in - must buy all DERs from same manufacturer
Side effects from workarounds like frequency shifting
No graceful degradation when primary backup depletes

How eBus Solves This

eBus recognizes that the MID MUST become a separate, distinct HEI device that can communicate and coordinate with multiple grid-forming DERs.

With eBus, a standalone MID can:

Discover all grid-forming DERs on the network via mDNS
Receive state-of-energy updates from all sources via MQTT
Coordinate which DER(s) should form the microgrid
Send curtailment commands via proper communication, not frequency hacks
Manage graceful transitions between backup sources

A market-leading BESS vendor is already shopping their third-party MID specification, and smart panels may well include MID functionality. eBus provides the standard communication layer these devices need.

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