FAQ
TL;DR: Two 1 kW AC sources won’t naturally share 1.0 kW and 0.2 kW unless synchronized; “easier to do with DC than with AC.” Use sync’d inverter, anti‑islanding, and load diversion for priority solar with zero export. [Elektroda, Rick, post #21681289]
Why it matters: This FAQ helps builders and facility engineers design safe, zero‑export solar that tops up from grid only when needed.
Quick Facts
- Unsynchronized AC sources can’t cleanly current‑share; rectify, filter, and re‑invert or synchronize phases for safe combining. [Elektroda, Rick, post #21681289]
- Grid‑tie inverters lock to grid voltage/phase and require bidirectional metering and anti‑islanding protection. [Elektroda, Rick, post #21681292]
- Zero‑export strategy: divert surplus to local loads (e.g., hot‑water, battery) using a controller like an I‑Boost. [Elektroda, Conrad Mannering, #21681297]
- To keep solar running during outages, use a contactor to isolate the grid and an inverter capable of islanded operation. [Elektroda, David, post #21681294]
- Priority scheme: CT measures flow; PLC/MCU opens on grid loss and tops up from grid only when solar is insufficient. [Elektroda, Abdul Rasheed, #21681296]
Can I just parallel two AC sources to share load up to 2 kW?
Not safely unless they are synchronized in voltage, frequency, and phase. Otherwise, circulating currents and instability occur. A practical approach is rectifying both to DC, filtering, then using an inverter to supply a single AC bus. Sharing then becomes controlled, not accidental. “Easier to do with DC than with AC.” [Elektroda, Rick, post #21681289]
What is a grid‑tie inverter and why does it matter here?
A grid‑tie inverter converts PV DC to AC that matches the grid’s voltage and phase, powering local loads first and exporting surplus. Utilities use bidirectional meters, and anti‑islanding prevents backfeed during outages to protect workers. This aligns with a priority‑to‑solar design, minus export if you add diversion. [Elektroda, Rick, post #21681292]
How do I stop exporting solar while still maximizing self‑consumption?
Use a zero‑export/diverter controller with a current transformer. When it detects export, it routes surplus to a resistive load (e.g., water heater) or a storage battery, keeping export at or near zero while preserving grid top‑up when PV is low. [Elektroda, Conrad Mannering, #21681297]
Can the system keep running when the grid goes down?
Yes, if you add grid isolation and an inverter that supports standalone (islanded) operation. A contactor disconnects the grid on loss‑of‑mains, and the inverter continues serving local loads from solar and storage. Without isolation, anti‑islanding will trip and stop production. [Elektroda, David, post #21681294]
How do I prioritize solar and only top up from the grid?
Three steps:
- Measure building import/export with a CT.
- Use a PLC/MCU to modulate local loads or enable grid input only when PV < load.
- Open a contactor on grid loss; keep inverter supplying the local island. [Elektroda, Abdul Rasheed, #21681296]
What if I need asymmetric sharing, like 1.0 kW from source A and 0.2 kW from source B?
Direct AC paralleling won’t hold that ratio. Convert to DC and use a single inverter, or use one synchronized inverter as the master and supplement from the other side via controlled DC or diversion. Equal sharing is the natural outcome without active control. [Elektroda, Rick, post #21681289]
Is there a real‑world sizing example for zero‑export top‑up?
Yes. The thread discusses a 30 kW DC building load with 25 kW solar available. The control scheme draws the missing 5 kW from the grid to meet demand, keeping solar fully utilized without export. This illustrates the top‑up concept at medium scale. [Elektroda, Abdul Rasheed, #21681290]
What is anti‑islanding, in plain terms?
Anti‑islanding is protection that forces a grid‑tie inverter to stop output during a grid outage. It prevents backfeeding the public network, which could injure utility crews. Any system that continues during outages must positively isolate from the grid first. [Elektroda, Rick, post #21681292]
Could a simple contactor and MCU handle priority switching?
Yes. Sense current flow, prefer solar, and enable grid only for the deficit. On detecting zero grid, open the contactor to form an islanded microgrid. Pair this logic with an inverter designed to operate without the grid present. [Elektroda, Abdul Rasheed, #21681296]
What’s a solar diverter (like I‑Boost) and where does it fit?
A diverter monitors export and instantly routes excess PV to local thermal storage, such as a hot‑water tank. It raises self‑consumption and enforces zero‑export limits. Think of it as an automatic priority load that soaks up surplus. [Elektroda, Conrad Mannering, #21681297]
What failure edge case should I plan for?
Plan for grid loss while PV is producing. Without fast isolation, the inverter’s anti‑islanding will shut down generation to avoid energizing lines, causing a sudden loss of power. Proper contactor control prevents backfeed and keeps local supply online. [Elektroda, Rick, post #21681292]
Why not just feed excess to the grid and get credit?
You can, but policies may pay less for exported energy than the retail import price. The thread also notes concerns about local grid stress from aggregate exports. Zero‑export with diversion maximizes onsite use while respecting constraints. [Elektroda, Abdul Rasheed, #21681295]
Is AC combining ever preferable to DC combining here?
Not for fine‑grained sharing. AC combining needs precise phase sync and tends toward equal sharing. DC combining with one robust inverter gives deterministic control over contribution and simplifies priority logic. “Easier to do with DC than with AC.” [Elektroda, Rick, post #21681289]
Do I need batteries for this approach?
Not strictly. You can maximize self‑consumption with diversion alone. Adding a battery increases resilience and time‑shifts energy for outages or evening peaks, and some inverters can sync seamlessly when grid returns. [Elektroda, Conrad Mannering, #21681297]
What components form a basic zero‑export, priority‑solar stack?
Core pieces: PV array, inverter capable of islanding, grid isolation contactor, CT for import/export sensing, PLC/MCU for logic, and an optional diverter or storage. This stack enforces solar‑first, grid‑top‑up, and safe islanding. [Elektroda, David, post #21681294]
