Grid tied with offgrid backup

Welcome to the forum Woodzy,

I am guessing that you do not have GT Solar power agreement with your utility--So are limiting feeding energy back to the grid to zero Watts (or as close as practical)?

And, yes, you can connect a GT Solar inverter to an Off Grid AC Pure/True Sine Wave inverter (sine wave to ensure that the GT Inverter "qualifies the 240 VAC 60 Hz Off Grid inverter's output (Must be held within specifications for 5 minutes--Using GT inverter defaults).

And, in its simplest terms, most Off Grid AC inverters can be "back driven" by a GT Inverter and actually share the AC loads and even recharge the battery bank.

Issues include... The OG Inverter frequency must be accurate to 60 Hz +/- 0.5 or possibly 1.0 Hz (North America) and the AC Output between roughly 212 and 264 VAC. If those values are not "true" for 5 minutes, the timer will reset to Zero and start over again.

In the "olden days", the AC Coupling of GT and OG inverters was new and not really supported (SMA was an early/first company to implement in an engineering function from what little I know). And since the OG inverter had no "idea" or "control" for AC Coupled battery charging, the battery bank could be over charged/over voltaged over time... A simple fix was to have a separate controller monitoring the battery bank and either dump excess power to a resistor bank (dump controller), or put a relay in line with the GT inverter and cut the AC lines to the GT inverter--And turn it back on when the batteries needed charging again.

Obviously, the OG inverter needs to be as large or larger than the energy feedback from the GT inverter.

The next step was for the OG inverter to monitor the battery bank state of charge... When full, the OG inverter would go to something like 61/59 Hz and "knock" the GT inverter off line (for at least 5 minutes). In engineering terms, this is known as a Bang Bang controller (full on, then full off, then full on again).

The current method is for the OG inverter to vary its AC frequency between 60 Hz +/- 0 Hz for 100% GT inverter output, and to 60 Hz +/- 1.0 Hz (or some other smallish number) to vary from 100% to 0% GT inverter output--Giving feedback from the OG inverter to the GT inverter (proportional control, not Bang Bang).

You end up with some version of Bang Bang vs proportional control depending on the abilities of the GT and OG inverters. Newer/more expensive models have better feedback... Older/cheaper models have simple or no feedback).

And--In North America, you have the issue of 120 vs 120/240 VAC split phase vs 240 VAC single phase... Most larger GT inverters in North America are 240 VAC... And most smaller OG inverters are 120 VAC... The larger OG inverters are typiically 120/240 VAC split phase (which should work fine with a 240 VAC GT inverter).

And depending on the AC coupled GT inverter feedback (BB or PC), in your case if sharing between Utility Power and OG Inverter power--The programming of the GT inverter may not be the same between the two setups... To a degree, you might be OK with proportional control, but you cannot exceed the grid's safety requirements.

There are also Hybrid AC inverters... They do both Off Grid + battery bank (if the grid is down), and GT operation when the grid is up and there is excess solar power that can be backfed into the utility... This is what is done today for many modern/larger AC inverters (Schneider, Outback, Magnum (I think), possibly SMA and some others).

The modern hybrid AC inverters are really neat and have lots of programmability... And a Hybrid Inverter does not really cost more to build than a standard Off Grid inverter with AC and Generator support (other than lots of software engineering). You connect the Utility/Grid power to AC1, a backup genset to AC2, battery bus to DC input, and solar+solar charger to the battery bus. The devices are all networked together so they share information and functional control of the various pieces (battery charging, GT feedback, off grid battery backup, AC Generator control, etc.).

These hybrid inverters are not simple devices... Requires a lot of reading on the user/installer's part. And even now, sometimes there are software limitations and bugs that make things a bit more difficult.

Most people go with the Hybrid Inverter route... The AC Couple does work--But can bring a load of issues/complexity with it (what happens when a failure occurs, which mfg to call for help, will they talk to a user installer, etc.).

The above is about all I know... Details matter here. Exact brand/model and your expectations, etc. Some features like zero grid energy feedback (many folks have utilities that do not allow GT Solar Inverter connections and some don't want Battery Backed Hybrid inverters where you could buy cheap power overnight and sell it back to the utility during expensive afternoon Time of Use Rates).

You may be surprised at the number of "issues" you can run across when trying to do AC coupling/Hybrid inverter functions...

One thing I always suggest is to look at how much utility power costs you vs battery based OG inverter power... Batteries last typically around 3-7 years (more expensive batteries can last 15+ years)... If you figure out the cost of battery replacement, initial system installation costs, service, and cost of new controllers (electronics) every 10+ years--The costs for battery backed/hybrid inverter power can easily be in the $0.40+ per kWH range (back of the envelope calculations I did here a few years ago)... And even in California, our time of use rates are around $0.25 to $0.40 or so... And hybrid inverters are somewhat of a "wash"... If you want/need battery backed AC power--They do work and give you some money back selling to the utility... But cycling your battery bank at night does wear the batteries. And you have to look at their replacement every XXXX cycles or Y years to see if it make economic sense...

GT Inverters are historically cheap, and so are solar panels ($/Watt)... So generating GT Solar power is probably easily under $0.15 per kWH (or less)... So GT Solar can save money... However, utilities really do not like GT solar (or at least their accountants don't like GT Solar) and if you have a utility that does not allow GT solar (slowly becoming more wide spread)--A zero feedback GT or Hybrid system is your only choice.

Over the 15 years of so I have had GT Solar, my utility has slowly been changing the rules (rate plans, cost and time periods for peak power). At some point, it may become uneconomic to install--Or flat out illegal (Hawaii and Nevada now?) for new installs... The game keeps changing.

-Bill

There should be some Hybrid inverters that can be programmed to not charge from the grid (and feed zero Watts to the grid)--However, you need to find a good vendor and somebody with experience configuring... Not everyone has been sucessful from what little I have read...

This is beyond my knowledge. Perhaps somebody here can give you some recommendations... But it could be "costly" (hardware, consulting, etc.)....

Good luck,
-Bill