what is the main design difference between a grid tied inverter and off grid inverter?
1- What is the main difference between a grid tied inverter and an off grid inverter in respect of their internal design characteristic? 2- How can a grid tied inverter be integrated in an off grid system?
1) Completely different designs. Grid tied inverters use the grid as a voltage, frequency, and phase reference and will take all the energy available from the PV array and send it to the grid. An off grid inverter has to generate its own voltage and frequency. It also has to monitor the AC load so it only makes as much AC power as is required even if the PV array has more power available.
2) AC coupled system such as the Sunny Island.
To answer Question 2, you can integrate a grid-tied inverter with an off-grid system through coupling the grid-tied inverter with storage. One of our favorite components for coupled systems is the Xantrex 6048. It's highly versatile, able to power grid-tied, off-grid, or backup systems.
The XW6048 is an inverter/charger, meaning it inverts power bidirectionally between AC and DC to charge and drain batteries. The unit can accept a PV generation source in both a DC-coupled architecture (an architecture whereby PV goes to a charge controller, on to storage, then out to load through the inverter/charger) and AC-coupled architecture (where PV goes through a dedicated solar inverter, then on to load through the XW, while the XW allows the batteries to pull from and push to the AC service).
A DC-coupled system is ostensibily gentler on the batteries and will yield more storage charge cycles, so is best for heavy storage users, while the AC-coupled architecture is more efficient and best suited for full-time grid-tied systems.
Lots of exciting things coming in the near-term around intelligent storage too!