System ideas, please advise.

[trig2]

I'm getting an outbuilding with a south facing single slope roof and would like to put a solar system on it for backup power. I would like it to have grid power pass through ability but would like to avoid dealing with the power company so am avoiding grid tied inverters this is just for a couple circuits not to power my entire home. My maximum load should be about 8kw. Here's what I'm looking at.

6.88kw solar from 16 qcells 430 watt panels open circuit voltage 49.3 and current 10.3a (4x in series x2 in parallel per mppt)

12kw 120/240 split phase sigineer inverter/solar charger with 2x 60a mppt max pv open circuit voltage 250

Solar Power Inverter For Home 12000 Watt 48V to 120V 240V w/ 120A MPPT Controller ETL Listed

12000 Watt Solar Power Inverter For Home |120A solar charger Pure Sine Wave 48V dc to ac 12KW 120/240Vac, off grid, emergency backup power

www.sigineer.com

4x 48v 100ah LiFePO4 sigineer batteries

Powerwall 48V 100AH 8S Li-ion Battery Pack for Energy Storage System

Model #: LFP48100 In stock in California. Free shipping to continental US only . Buy 4 pcs at a time, and get 800USD off.

www.sigineer.com

My concerns are the inverter efficiency isn't that great (peak 85%) the idle load is high at 180 watts and the harmonic distortion seems high. "THD of 7% (min 3%, max 10% under full linear loads) depending on the load connected and battery voltage." I plan on setting the inverter to swap to grid power at 50% state of charge to make the batteries last longer so that should help with the harmonic distortion as well.

I like that the ac input and output are designed to be completely separate since I have no intention to grid sell and if the power company ever asks questions that might help my case. Solar production will be double in the summer so my mini split should be able to compensate for the heat from the idle draw, and in the winter the building needs to be heated anyway. This inverter is low frequency, sigineer has 3kw high frequency inverters with better efficiency (90%) that can be paralleled and synchronized for split phase production but my understanding is low frequency inverters last longer and have better surge capability (300% for 5 seconds) so I'm not sure what the better option is. It might be nice to have the redundancy of 4 inverters if one of them dies as long as it doesn't take out the others in the process.

Thanks for your input.

 

[Abodyofscience]

If you get sun for 5 hrs the load can only on for 3 hrs.

 

[WYtreasure]

My concerns are the inverter efficiency isn't that great (peak 85%) the idle load is high at 180 watts and the harmonic distortion seems high. "THD of 7% (min 3%, max 10% under full linear loads) depending on the load connected and battery voltage." I plan on setting the inverter to swap to grid power at 50% state of charge to make the batteries last longer so that should help with the harmonic distortion as well.

Welcome to the show trig2.

Take your time and find the inverter specs that make you happy. 180 watts is a bunch of power to toss down the crapper.

At your grid connection, I understand you to be stating that power is to be traveling in one direction. Assuming I am correct, that's all well and good till something starts smoking. Then who knows which way the volts & amps are going to go. How about a ONE-WAY device between the grid and your system to keep the utility company happy?

I know little but sometimes it helps.

 

[Bvillebob]

I know 7 kw of solar panels sounds like a lot to you, but with the loads you're talking about it's not much. I have 9 kw of panels feeding a 45 kWh battery, 24v @ 1800 amp hours. My average load is around 1000 watts and a day without sun and I'm looking at 50 percent battery. That 1000 watt load x 24 hours is 24 kWh, it really adds up fast. 180 watts x 24 is over 4 kWh per day, or almost the capacity of one of your batteries just going to feed your inverter.

Look at what your daily consumption is going to be and check your numbers, you might be surprised at what your really need.

 

[trig2]

If you get sun for 5 hrs the load can only on for 3 hrs.

That's my supposed max load for sizing the inverter it definitely won't be on all the time. PVwatts has me estimated to be bottoming out at 14kwh per day in the winter (it may be a bit worse than that due to some trees) and 40+ peak in the summer (little to no shade in the summer). My demand will admittedly probably exceed my production in the winter but that's not the end of the world I will just use some grid power, and if the grid goes down I'll supplement with a kerosene heater to keep the outbuilding above freezing instead of running the mini split.

How about a ONE-WAY device between the grid and your system to keep the utility company happy?

Sounds like a good idea I'll look into it. 180w is a lot but at least in the winter when production is low it's heating the space that needs to be heated anyway.

I know 7 kw of solar panels sounds like a lot to you, but with the loads you're talking about it's not much. I have 9 kw of panels feeding a 45 kWh battery, 24v @ 1800 amp hours. My average load is around 1000 watts and a day without sun and I'm looking at 50 percent battery. That 1000 watt load x 24 hours is 24 kWh, it really adds up fast. 180 watts x 24 is over 4 kWh per day, or almost the capacity of one of your batteries just going to feed your inverter.

Look at what your daily consumption is going to be and check your numbers, you might be surprised at what your really need.

It will be close at times for sure, the primary function of they system is just backup to keep my fridge/freezers running if the grid goes down I can reduce the other loads. I just figured I might as well make use of the power in the meantime.