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I need a battery backup system for my PV 10Kw system comprising 2  5KW inverters.

Can one of you advise me on best and cost effective AC/AC to add to existing PV please?

Alfie 

alfier wrote: ↑Sun Sep 05, 2021 8:24 am Can one of you advise me on best and cost effective AC/AC to add to existing PV please?

Don't you mean a AC/DC system i.e. a charger? This will require us to know what your load requirements are i.e. how long you want to run the inverters for (which depends on the load) which decides the size of the battery bank which decides the size of the charger etc.

THANK YOU for your reply. my solar panels feed 2 x 5kw DC to AC units. I would like to add batteries to allow the batteries to use the batteries during the night when only Fridge and 2 computers running dependent on cost.
I hope that's enough to help you to help me.

I was hoping for an answer as

If you only want it to run a fridge and two computers, will the batteries not cost more than you could ever save?

I expect to get ridiculed for this: my Igenix 92L small standard fridge is rated at 110kWh/annum, that is average 12.6watt/hr. A deep cycle lead acid 12v, 50amp/hr leisure battery, supplying an inverter at 80% efficiency would run the fridge for about 19hrs before it reaches 50% discharge, lower and it will have a short life.
Now the problem with compressor type fridges is they cut in and out and their start ampere can be up to 7x times their rating; my 600watt inverter with a 1200watt overload cannot consistently power up my fridge without tripping. A fridge which is absorption cooling type has a chance as it does not have a heavy compressor but at fan. My 40L Caldura at 65watt runs nicely but even if it runs 6 times an hour it will consume 11 watts much the same as my 92L.
Add 2x modern laptop computers @ say 2x30 watts, (my old laptop uses 40+) making assumption they are using continuous power these would eat the same battery down to 50% in less than 6 hours, but better if their cooling fans don't kick in.
Now to get a better time you need to consider Lithium batteries!
I installed the inverter to power my CH should there be a power outage and it works fine for that, but not the fridge and most certainly not for the freezer.
Oh and you might need pure sine wave inverters for computers.
The other problem in getting a battery down to 50% is that my inverter will give warnings as the voltage input drops and will stop at about 10.8v dc !

The batteries required would need to be the equivalent of the power you intend to use +30-50% (to prevent over discharging).

Add up the wattage of the devices you want to power and then multiply that by the number of hours you want to run them for.

E.g. Fridge ~120W, 2xPC's @ 150W each - total is 420 watts. To run them for 12 hours means 12x420 = 5040W. Add on the 50% margin and you;re looking at 7500W

Divide this number by the voltage of the batteries you plan to use (you don't state whether it is a 12V, 24V or 48V system) to get the ampere-hour capacity you need.

7.5kW of batteries is a LOT. A bank of 12V batteries would need to be rated at 625Ahr..... Given that a 'large' 12V battery is typically 110Ahr you'd need 6 of them in parallel. Not impossible, of course. But we don't know what voltage your system is, nor do we know if you have the appropriate charger for the intended batteries - they have to match.

Maybe u need to consider a new Hybred invertor and lithium batteries with deep discharge capacity the two 3.5KW batteries I have discharge down to 5% but they cost around £1k each made by a firm called Pylon

Sounds like what I want. Please send me more details.

Alfie

Try Googling Pylon batteries , sold the world over , avsilbe here on net and midsomer wholesale, one problem is they are in short supply at the moment , seen the 3000cs batteries I have at £1400 each. Think retail from Midsomer is £1300 ish with delivery in may

I have a Sofar inverter which they also sell

Got a bit lost sorry. can you give me the web site and advise how the inverter and batteries are wired in

Alfie

My fridge/freezer and freezer are powered with a three phase motor through a inverter driver and it seems to have a very small inrush. The biggest load is when it goes to defrost, so around 160 watt.