Adding emergency off-grid to grid-tied system?

Hi, I have 4kW grid-tied system installed by the previous house owner about 15 years ago, which is using SunnyBoy SB400US. Being in Florida I would like to have solar backup for a single room AC and a fridge in case the grid goes down (due to a hurricane for example). I have found that the newer models of SMA SunnyBoy (for example SB400TL-US) can provide what they call Secure Power Supply of 1500W without any batteries when the grid goes down. But that looks barely enough for a very tiny AC unit. In a promotional video SMA is showing they can provide power to operate a room fan and charge an iphone, which is not too impressive.

So my questions are:
1) Has anyone solved a similar problem without using batteries and how? I feel it's a waste of money to have a battery backup which is likely to be used just a couple of weeks a year or so. Instead I would rather use the solar during the day and a gasoline generator (which I already have) during a night.

2) If the batteries are the only viable option, can anyone provide some experience with a similar system? Is it effectively building a parallel off-grid system which is using the same solar panels for both grid and off-grid? That is when the grid goes down, I would manually pull a transfer switch which would transfer output from the solar into an inverter which is powering the couple of outlets to which I would plug in the AC and the fridge? The batteries would also switch from the normal operation when they are kept charged from the grid/solar when the grid is up, and change to the off-grid mode when the grid is off? What battery sizes/voltages do people recommend for this application?

In any case I would definitely prefer lead batteries (I've built an electric sports car 2000A 144V DC with both lead and lithium versions and frankly I find lithium batteries too finicky to manage - lead is much more predictable and reliable)

3) Any experience with using capacitors? If the solar inverter without batteries does not have enough surge power to start an AC unit, would adding large capacitors work?

Any help appreciated, thanks!

Modificator said:

...I have 4kW ... installed ... 15 years ago ...[want a backup system sans batteries]...

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It's tough to run a house without some batteries, but the Skybox supposedly can and the Sol-Ark can do it with very minimal batteries (you could even use lead acid, they have an interesting deal on PCCs which will supposedly last over a decade if just used for emergency use).

As your SunnyBoy is 15 years old it's probably on it's last legs so replacing it with something like a Sol-Ark or SkyBox might be a good solution for you.

Modificator said:

2) If the batteries are the only viable option....

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The more general solution with batteries is known as AC Coupling. If you're going to replace the inverter you might also look at some DC coupling solutions. As you have a SunnyBoy, there might be an advantage to using a Sunny Island, but I'm not familiar with them.

Modificator said:

[2a] What battery sizes/voltages do people recommend for this application?

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Other than the Skybox and Sol-Ark, each solution has a minimum battery size it'll require, usually hidden deep in the notes. Those systems use the battery as a buffer to handle slow switching times and surges; so it'll be a function of the array size and C-Rate.

Modificator said:

...In any case I would definitely prefer lead batteries (I've built an electric sports car 2000A 144V DC with both lead and lithium versions and frankly I find lithium batteries too finicky to manage - lead is much more predictable and reliable)

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Most people feel the reverse for LiFePO4 with a BMS.
But lithium is made for 1000s of cycles and in your emergency use case you'll only ever be using 100s over it's lifetime, so the cost of Lithium probably isn't justified anyway. You can find example calculations in the Battery FAQ that should confirm that for you.

Modificator said:

3) Any experience with using capacitors?

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Last I looked was a few months ago, Generally the $/watt-hour is still to high. But if you have them laying about they should work and you can find some discussion on them in the forums with the search tool.

@svetz - thank you very much for the information and the advice! After reading through all that material I have concluded that adding emergency solar off-grid would not be optimal for me, considering the costs and how little I would be using it. So I have decided to keep the grid-tie system for with-grid use (due to it's age and malfunctions, as you pointed out I will probably need to update it anyway - but that's a separate topic).

I've just bought a big used Generac 15kW propane generator for $1500. With addition of about $150 per week in propane for the times when the grid is down it should provide enough power for the whole-house 5 ton A/C unit, fridge and few small appliances 24/7

And will likely last 20 years or so - the same solar based results would be many times more.

Thanks again and I may be back discussing refresh of my grid-tie system

Modificator said:

... considering the costs ...

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For that DIY itch, you might be interested in something smaller: https://diysolarforum.com/threads/rockpals-300w-generator.5681/post-58529
Get a portable unit on the cheap now, add the MPPT/Panels later.

A good thought - small, inexpensive and may come in handy, thanks!