Solar critical outlet backup design feedback please...

[keymaster]

I have a 12.8 kWH grid tie solar system on the roof currently. It doesn't provide any power in the event of power outage. I need to provide power for a few appliances via a stand alone solar setup. I would like to run them full time from solar / batteries unless the bank goes below a set voltage and then switch to conventional power. Usually power outages are only a few hours here. Load would be a 25 cubic ft household Fridge, a deep freezer, fish tank with 1 pump, and a laptop / monitor. I think it's about 500 watts when everything is running, but the fridge won't run constantly.

I have purchased the following.
2 ZNShineSolar 450 watt solar panels
1 2 panel roof racking system
1 MPPT solar charge controller (80amp)
1 100 ah 12v Renogy battery
1 200 ah 12v Renogy battery
1 1000w inverter (generic brand, not pure sine wave) looking for recommendations for replacement
50ft 10 awg solar extension wire
10ft 10 awg solar wire
2 40 amp DC circuit breakers. One between the panels and the MPPT controller and the other between the controller and the batteries
1 50 amp fuse between the batteries and the inverter

I have the panels mounted on a shed, wires ran to the basement and connected to the MPPT controller, inverter and batteries. I think the next step is to purchase a Automated Transfer Switch and pull the wires from the breaker box in the basement and route them to the ATS, pull power to the ATS and connect it to the battery bank.

What ATS recommendations would you have, is there anything wrong with my initial design?

 

[MichaelK]

Couple of problems already.

You cannot pair different batteries of widely differing amphour capacity.

For a large refrigerator, a freezer, and the inverter itself along with the other plugins, I'd expect you to need ~4kWh per day. Only 900W of solar is likely to supply that only the summer months with very long days, and the sun high in the sky. In winter, the system is likely to die. I would get at least one more panel, but most likely two is better. Then they could be wired 2S2P into your charge controller.

Do NOT use anything other than sine-wave for anything with an electric motor. Both the frig and freezer will be put at risk with a MSW inverter.

I would look at upgrading to at least 24V to try to run what you want. Get rid of the 100Ah battery and get a second 200Ah one and put them in series for 24V. Then get a good sine-wave 24V inverter to power them. Samlex makes a good quality high-frequency (low surge) 2000W inverter that has quite low idle draw. I would consider though only 200Ah at 24V to be rather low. My own 24V system has ~600Ah battery. I have a Schneider Conext 4024 for my own.

Please don't buy anything else though until you've fleshed out a functional plan. What you've already assembled is not.

 

[keymaster]

Thank you for taking time to give feedback. I didn't know about mixing batteries. 1st lesson learned. I will remove the 100ah and replace it with a 200 to match and move to series wiring them for 24v.

I appreciate the recommendation on Samlex inverter.

My goal is to have this system function as a blended backup for these critical things in the event of a power outage and not fully off grid. Do you have any recommendations for an ATS?

Thanks again for taking time to help me better understand the DIY solar world....

 

[timselectric]

You cannot pair different batteries of widely differing amphour capacity.

Not in series. But parallel is fine.
So if going for a 24v system, it's a no.
If going with a 12v system, no problem.

 

[timselectric]

Actually 24v would work. You would just be limited to 100ah.

 

[MichaelK]

Not in series. But parallel is fine.
So if going for a 24v system, it's a no.
If going with a 12v system, no problem.

Do you have any kind of reference that describes that in more detail? Everything I've ever read about pairing batteries has always indicated that they must match, both in serial or in parallel?

 

[50ShadesOfDirt]

It's good to have the design work done before acquiring components; it's not too late to start down the design path, at the end of which you can better determine the path forward with what you now have on hand, or adjust.

Design steps:

1. Go here, and enter in each appliance's values (watts, hours/day you want to run it, etc.):
https://unboundsolar.com/solar-information/offgrid-calculator
2. Go here, using numbers from above, and fiddle with various entries/components, and you'll see in real-time what your system component (inverter, mppt, panel) sizing is:
https://www.altestore.com/store/calculators/off_grid_calculator/

There are many similar website pages/calculators, but these two pages should help you get through most of the necessary calculations. This helps you quickly decide if you can do what you want to do, and you can vary component choices for what-if scenarios.

You are desiring an ATS solution; unless well-qualified to DIY (to code, etc), you probably will get to deal with AHJ/codes/electricians ...

You might want to consider building a separate system that powers identified critical things; run them on the solar gear full-time (reducing grid usage), and have automation refill the battery-bank from the grid side (this is not a direct tie, so OK). Little to no AHJ involvement, as it is a separate system, depending on who installs it (you or an installer).

Hope this helps ...

 

[timselectric]

Do you have any kind of reference that describes that in more detail? Everything I've ever read about pairing batteries has always indicated that they must match, both in serial or in parallel?

 

[timselectric]

That is taking it to the extreme.
But, it easily proves it.

 

[keymaster]

thank you for all the feed back. I got caught up in the "ready, fire, aim". I appreciate the links to the configuration tools and I am going to do a complete redesign based on those tools. .

thanks again.

 

[Tomthumb62]

Do you have any kind of reference that describes that in more detail? Everything I've ever read about pairing batteries has always indicated that they must match, both in serial or in parallel?

The "must be matched even in parallel" from my limited understanding applied much more to lead-acid systems than lithium. Also, aging. Putting new lead-acid batteries into a 5-year old system is a no-go. But not really a problem with lithium or not so much that it matters a whole lot, depending on how hard the original batteries have been cycled.