Off grid and backup (ats, and grid assist charging logic)

bgflyguy

Solar Enthusiast
Warning, this is a bit of a ramble. I've been watching off-grid-garage and decided to write down some of my musings in the same vein as Andy.

I'm in the slow process of building a "small" system to run an AC in the summer, whatever else it will handle the rest of the year, and serve as a backup for when the grid goes down. I'm in the PNW, and I don't currently have an AC so whatever benefit I get from this(other than financial) will be enough.

This is more about powering as much as I can with solar and having enough for around 3 days of critical systems(lights and blowers for natural gas heat) I'm pretty good with the power limitations and building as I go. I can figure out the real size in Ah/Watts later.

I'm looking at the all-in-ones, and because of different behavior between units, I'm trying to decide what the ideal behavior would be.

I want to use solar over grid assist when I can, but I want to make sure I have enough power to get me through a grid outage. We usually get a few short periods where a tree falls on a power line and brings everything down for a while. The worst is during snow storms. Ideally solar would cover all of my use, including charging the battery enough to get through the night. But I can't always count on enough sun the next day to make that feasible. What if the power goes out at 9am before I have a chance to charge my batteries?

So, I'm thinking about only charging with grid assist charger from something like trigger at 40% and stop 50% SOC and only having the ATS kick on if the battery makes it to LVD or I'm using more capacity than my inverter can put out. Of course I would need to size the battery so that 50% SOC would meet my back up requirements. If I did that I would probably live around 75% SOC and hardly ever reach 100%. With 3 days of autonomy, that would be about 1.5* my daily usage for my solar generation. That seems like a good trade-off.

I think this logic would work with some of the Growatt units, but might be more difficult with the MPP units.

How do you guys make sure you have enough battery for backup, but also avoid charging your battery on the grid all night and not using all of your solar? I could just never use the grid and size my system for 3 days of autonomy in all conditions, but I think that would require a lot more solar and batteries than is possible. There will be significant periods without much solar to speak of. In that case I'm okay with having 3 days of battery and just running everything on the grid until the solar comes back.

I really don't want to get in a cycle of charging the batteries of off the grid to 100% all night, and then using a little bit of solar + batteries to run a load and then charging off of the grid again the next night.

Thoughts?
 

Sanwizard

Photon Sorcerer
Warning, this is a bit of a ramble. I've been watching off-grid-garage and decided to write down some of my musings in the same vein as Andy.

I'm in the slow process of building a "small" system to run an AC in the summer, whatever else it will handle the rest of the year, and serve as a backup for when the grid goes down. I'm in the PNW, and I don't currently have an AC so whatever benefit I get from this(other than financial) will be enough.

This is more about powering as much as I can with solar and having enough for around 3 days of critical systems(lights and blowers for natural gas heat) I'm pretty good with the power limitations and building as I go. I can figure out the real size in Ah/Watts later.

I'm looking at the all-in-ones, and because of different behavior between units, I'm trying to decide what the ideal behavior would be.

I want to use solar over grid assist when I can, but I want to make sure I have enough power to get me through a grid outage. We usually get a few short periods where a tree falls on a power line and brings everything down for a while. The worst is during snow storms. Ideally solar would cover all of my use, including charging the battery enough to get through the night. But I can't always count on enough sun the next day to make that feasible. What if the power goes out at 9am before I have a chance to charge my batteries?

So, I'm thinking about only charging with grid assist charger from something like trigger at 40% and stop 50% SOC and only having the ATS kick on if the battery makes it to LVD or I'm using more capacity than my inverter can put out. Of course I would need to size the battery so that 50% SOC would meet my back up requirements. If I did that I would probably live around 75% SOC and hardly ever reach 100%. With 3 days of autonomy, that would be about 1.5* my daily usage for my solar generation. That seems like a good trade-off.

I think this logic would work with some of the Growatt units, but might be more difficult with the MPP units.

How do you guys make sure you have enough battery for backup, but also avoid charging your battery on the grid all night and not using all of your solar? I could just never use the grid and size my system for 3 days of autonomy in all conditions, but I think that would require a lot more solar and batteries than is possible. There will be significant periods without much solar to speak of. In that case I'm okay with having 3 days of battery and just running everything on the grid until the solar comes back.

I really don't want to get in a cycle of charging the batteries of off the grid to 100% all night, and then using a little bit of solar + batteries to run a load and then charging off of the grid again the next night.

Thoughts?
Try to get an idea of what your daily load looks like first. I used the Emporia Vue, and its working great. The cheapest version is about $60, and has ct sensors that wrap around your main panel input from the grid. Takes about 10min to install. Just google it.
Once you have your load info, you can better calculate your needs. I call this a data driven approach to sizing. Better to not just guess.
Next, find an inverter that fits your loads, add 30% for growth, and uses 48V. Then build a 16s battery using the info on this site. You can always add more battery later to scale.
 

bgflyguy

Solar Enthusiast
Yeah, but that still doesnt help figure out when to use the grid to charge or run a load. if I just run until its empty and charge with whatever is available. I'll be running on the grid and using the solar to keep my batteries topped off. That's not ideal.

Whether I'm using 1watt or 100kw the conversation is the same.
 

XO4001

New Member
If it was me I would run the solar as much as possible and when there is a power out and the batteries are low I charge them with a generator.
 

bgflyguy

Solar Enthusiast
If it was me I would run the solar as much as possible and when there is a power out and the batteries are low I charge them with a generator.
That's probably cheaper than buying enough batteries to have a 3 day reserve, but I think I can do this without a generator.
 

Sanwizard

Photon Sorcerer
Yeah, but that still doesnt help figure out when to use the grid to charge or run a load. if I just run until its empty and charge with whatever is available. I'll be running on the grid and using the solar to keep my batteries topped off. That's not ideal.

Whether I'm using 1watt or 100kw the conversation is the same.
My inverter runs my loads on solar first, then battery, then grid as a last resort. If running on battery, and the sun comes out, the entire load may be on PV. It charges the battery only up to the voltage I program in. I can use the grid to charge also. I can limit the charge current also.
I have it set to ONLY charge on PV for now, even though it can use the grid to supplement the batteries and PV. ( I have enough solar to top off the batteries by about noon).
You just need a flexible inverter. I use two PIP MPP Solar LV6548's in parallel.
 

Sanwizard

Photon Sorcerer
How are you guys converting from voltage cutoffs in the inverter to SOC with LFP?
If I understand you correctly, I set the voltage cutoff in my BMS to 48V, and recover voltage to 50V. I then set the voltage cutoff in the inverter to 50V, charge to 55.5V, and float to 54V.
That keeps the cells around 3.4V max, and I never get in the knees of the cell volts. I use a Victron 500amp shunt to monitor SOC. (It keeps a log too).
I am also almost to the point of getting my Raspberry Pi working with the inverter to get some cool monitoring with Grafana. (Thanks DMI inc and JBlance!)
 
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