Can I use mains as backup for off-grid solar system?

Question

I want to run 6 freezers/fridges (10kWh/day) with a 4kW array of solar panels, 4x 300Ah 12v LiFePO4 batteries, and a 2kW 12V Projecta Intelli-Wave Pure Sine Inverter.

Instead of using a generator as backup for consecutive cloudy or rainy days, can I just use household mains instead? Ideally it would be nice to just somehow plug my inverter into an outlet permanently, and have the mains power automatically take over if batteries fall below a certain charge (e.g. 30%). Does such a connection exist?

Australian sun is supposed to give 8-20kWh daily for 4kW of solar panels. And batteries can power all the freezers for a day without sun. But sometimes we like to power other things as well, or we might get a week or two of rain and it's too much hassle to start up the generator all the time.

Australia has 240V 10A household outlets as standard.

Answer 1

Instead of using a generator as backup for consecutive cloudy or rainy days, can I just use household mains instead? Ideally it would be nice to just somehow plug my inverter into an outlet permanently, and have the mains power automatically take over if batteries fall below a certain charge (e.g. 30%). Does such a connection exist?

Yes, that feature is already on the inverter you chose. You simply need to hook up the wires according to the instructions.

Consult with the manufacturer whether the sequencing will work the way you want, and that it is compatible with lithium batteries, and will know what 30% is on a lithium battery.

I'm honestly not a big fan of this unit, since it has an RCD and many modes that require manual intervention to clear. It does not seem fit to run unattended for long periods. You will need refrigerator alarms.

I do not agree with the idea of "all in one" combo devices that combine the function of charge controller, battery charger, inverter, transfer switch, And every other darn thing. They are more difficult to find, they tend to do everything in a mediocre manner so your choice is limited. It takes ages to find the perfect unit that does all the things the way you want them done. Suppliers are very limited, takes weeks to arrive... and then later, it breaks. Now you are entirely down, because you put all your eggs in one basket. It's more difficult to troubleshoot with so many functions rolled into one. You find the unit is discontinued, and you must start the product search all over again while you're down. It takes weeks to arrive... and then it fails suddenly.

I recommend a discrete charge controller (perhaps two, one per solar panel bank), a separate battery charger fed from AC mains (the better way to do a "transfer switch"), separate inverter. Now if one goes down, only that one has down - if you lose the charge controller you can still charge batteries off mains. Seeking a replacment is simple and wide variety of products will do.

Answer 2

What you want to do is definitely possible, but the particular inverter you've chosen might be the best choice. What you need is actually multiple different functions: a transfer switch to automatically change from your own power to the grid power, a charge controller to control the charge to the batteries, and an inverter to convert from DC to AC. The device you've chosen includes a transfer switch and inverter, but not a charge controller.

The reason you want a charge controller is that maintaining the charge on the batteries depends on their chemistry. Ordinary lead-acid batteries (as used in vehicles) are durable and simple, but the LiFePO4 batteries you mention have much higher charge density, meaning that they can store more energy per size and weight. The cost of this is that the batteries are more complex to charge and maintain, so a charge controller takes care of this for you and makes sure that the batteries don't discharge too much, don't overcharge, and don't overheat.

You can't simply wire the batteries and solar panels directly together because running battery current through the solar panels would likely destroy the panels and/or damage the batteries.

For a project like this, I'd be inclined to talk with a local dealer who has installed such systems before to make sure that all of the pieces are sized appropriately, that they work effectively together, that they are safely designed and installed and that they accomplish your goals cost effectively.