have an on grid system looking to add an inverter for off grid AC output

[edun]

Hello All,
I'm new to this forum and am looking for advice for an off grid scenario without batteries. I have a Solar Power on grid system installed. It works great and I have no problems with it. I understand how an off grid system uses batteries to provide power when the solar panels are not producing enough or no power. I do not want to mess with batteries or need to provide 24x7 power. During a long utility power outage I would like to run my freezer and refrigerator when the panels are producing electricity and the local utility is off line. I'm thinking of putting a manual transfer switch which will divert DC power from feeding my on grid inverter and feed a second off grid inverter that will provide 120 and 240 voltage to a couple duplex and a 240 recepticals not tied to my house's system so no possibility of back-feeding power to the utility during an outage. I would also need the 2nd inverter to shutdown or to have power cut off on the AC output side when current and voltage are below a specific set point. Don't want to fry my freezer or refrigerator's motors. Does anyone have experience with such a system or know where I can find expertise to design one?
Thanks
EDun

[Naptown]

It will not work without batteries.
You would need to add batteries, a sunny Island or equivalent.
It will not run with a generator creating an artificial grid either
Sorry but that's how it is.
The reason is you need the batteries to buffer the PV for a cloud or increased load momentarily.
And the battery bank will need to be huge if you want to run stuff like refrigeration.
Buy a generator instead. If you have natural gas it will be the most cost effective solution and will run more of the house for longer than batteries ever could.

[Sunking]

Not going to work, you have to have batteries or even better a generator.

[edun]

Off grid AC output

It will not work without batteries.
You would need to add batteries, a sunny Island or equivalent.
It will not run with a generator creating an artificial grid either
Sorry but that's how it is.
The reason is you need the batteries to buffer the PV for a cloud or increased load momentarily.
And the battery bank will need to be huge if you want to run stuff like refrigeration.
Buy a generator instead. If you have natural gas it will be the most cost effective solution and will run more of the house for longer than batteries ever could.

I understand why you say batteries are needed as a buffer for the PV system. The use of batteries in this case is to prevent drawing more power than the system produces at a given point in time. This could also be done by disconnecting the load. This then requires something to be done to prevent the system from cycling on and off rapidly as the sun goes down and the system gets close to producing power at the disconnect setting. A solution would be to set the disconnect at say the max load for my freezer 1,500 watts and not reconnect until the power was 25% greater than the disconnect. In this case 1,875 watts. A time delay could also be used to not reconnect to the load until the pv system was generating power above 1,875 watts say for 5 minutes. So a cloud passes by and the disconnect is tripped. When the cloud clears and the power level is above the reconnect threshold for 5 minutes AC power is restored.

Note I have a generator. Just looking for a creative solution to use my PV system during a long term power outage to keep my freezer cold without adding batteries to the system.

[Naptown]

You are lacking in the theory of this
PV panels are a current source not a voltage source.
As soon as you disconnect a load the voltage will rise meaning more watts. This will short cycle whatever is on the circuit and destroy your compressors in a very short time. the inverter you wish to use (and I am assuming it is a grid tie) will disconnect as soon as the grid is lost. Even with a transfer switch the inverter will not work without a stable grid. This means a hybrid inverter and batteries.
You may not like the answer but it is a fact. What you want to do is not going to work. Well let me take that back it could work given enough time effort and money UL certification etc. More than what you can get from a forum. And certainly more than you wish to spend.

[Sunking]

As Rich eluded to there is a huge difference between a grid tied inverter and battery inverter.

Again solar panels are current sources, and GTI's are current devices that must operate into the very low impedance of the grid. Battery inverters are voltage devices who operate into very high impedance load devices compared to the grid. All GTI's are designed to disconnect and shut down when the grid low impedance is lost. If not the voltage output would go to infinity and the inverter will blow up. On the other end of the scale if a battery inverter were connected to the grid low impedance would short it out and blow up. They are opposite devices.

[Beanyboy57]

As Rich eluded to there is a huge difference between a grid tied inverter and battery inverter.

Again solar panels are current sources, and GTI's are current devices that must operate into the very low impedance of the grid. Battery inverters are voltage devices who operate into very high impedance load devices compared to the grid. All GTI's are designed to disconnect and shut down when the grid low impedance is lost. If not the voltage output would go to infinity and the inverter will blow up. On the other end of the scale if a battery inverter were connected to the grid low impedance would short it out and blow up. They are opposite devices.

Thanks for that info. I guess there are many of us that are looking to use less fossil fuels and we see solar panels as the answer, however as you point out, there is a lot that we don't know about electricity and the complex distribution systems that it requires.

[edun]

thanks

You are lacking in the theory of this
PV panels are a current source not a voltage source.
As soon as you disconnect a load the voltage will rise meaning more watts. This will short cycle whatever is on the circuit and destroy your compressors in a very short time. the inverter you wish to use (and I am assuming it is a grid tie) will disconnect as soon as the grid is lost. Even with a transfer switch the inverter will not work without a stable grid. This means a hybrid inverter and batteries.
You may not like the answer but it is a fact. What you want to do is not going to work. Well let me take that back it could work given enough time effort and money UL certification etc. More than what you can get from a forum. And certainly more than you wish to spend.

Rich this helps clarify the issues around doing what I want to do - Edun

[edun]

Thanks

As Rich eluded to there is a huge difference between a grid tied inverter and battery inverter.

Again solar panels are current sources, and GTI's are current devices that must operate into the very low impedance of the grid. Battery inverters are voltage devices who operate into very high impedance load devices compared to the grid. All GTI's are designed to disconnect and shut down when the grid low impedance is lost. If not the voltage output would go to infinity and the inverter will blow up. On the other end of the scale if a battery inverter were connected to the grid low impedance would short it out and blow up. They are opposite devices.

Thanks SUNK more good info Edun