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diy solar

Should I change mid Project

Panhandlesolar

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Sep 12, 2020
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So I started a this DIY Grid connected project close to a year ago. Bought a 20KW DIY kit from BluesunPV in China off alibaba....Awesome deal.....

The kit included 48 400Watt panels, 2 GROWATT 10KW MPPT Inverters, all combiners, racking system, cables.....the panels will be arranged in 6 strings 8 panels each, 300V into each MPPT input.

It took awhile to get the the panels here and have for my new muti-purpose building to get built and the construction approval process, building was completed in May and received my approval to install my panel system, then I realized, I really wanted to be independent and off grid, so I put a hold on the installation process until I work out batteries and the componentry to support it. AS a side I am getting into indoor farming using vertical principles, and the multipurpose building will need more electricity for more hours of the day.

I want my house to powered by the batteries, using the Panels to charge the batteries, and in the true emergency we have a 38KW Natural Gas Generator home backup system connected to a huge 400AMP Transfer switch, yes my house has a 400AMP service, but only a 200 AMP Main Panel

Two major concerns, I do not want to perturbate the current PV install approval that is in place with the county. As it can take a while for a change. SO I will complete the install process as approved, grid tied 20KW.

I am trying to design the battery system now, I keep running the math via a few online calculators. Based on a high average of current energy usage, in the middle of the summer here in North Florida we use over 1900KWh per month, yet contrast that to 1000KWH in the full winter months. I don't mind over building to produce more. I settled on 2000KWH for my math, 2000/30 = 67KWH per day. I want to build a battery system to support at least 3 days of full battery operation and then when the batteries hit there recharge threshold the Backup Generator can cycle on and charge them until float.

As you know there is a ton of data on LiFePO batteries, I would like to build a large battery bank using the EVE 280AH 3.6V batteries as they are readily available and I have plenty of Environmental controlled space to place the bank and keep it close to the inverters and main panels. based on my fuzzy math 67kWh daily =1396Ahr requirements, so I assume I will need 5 EVE 280AHr batteries to achieve the 1396AHr, so 14S String or 3 Strings to get up to three days of Electricity. Is that right?

The more crucial question......do I need to buy new Inverters to support this plan, I got the GROWATTs very inexpensively however, there are strictly grid tie. Need to figure out to convert to Off Grid.

Any additional questions I should be concerned with.

Thank you in advance,

Jim
 
I think your math is wrong. It's not clear that you understand Wh, Ah, etc. You need to start with the kWh and work backwards into voltage and Ah.

Please make sure your bowels are empty, or you are seated over an appropriate receptacle.

67kWh daily = 67*3 = 201kWh

Assuming you're going with a 48V battery system, 201,000Wh/48V= 4187.5Ah

In order to make a 48V battery, you need 16S, so 16 280Ah cells per 48V string.

4187.5Ah / 280Ah = 15 strings.

Since it's recommended you use only 80% of the capacity of LFP, AND there's about a 15% efficiency loss in converting DC-AC, you need:

15/.8/.85 = 22 strings.

You need 16S * 22P = 352 280Ah 3.2V cells. that should only run you about $40-45K

Good luck
 
Appreciate the input, I appreciate it being direct.

Holy Crap....where did I go wrong...been using online calculators.

Based on averaging my monthly bills, worked out to about 1700KWhper month, I increased it to 2000 KWh per month for some head room, divided by 30 days= 67KWh per day usages.....I shoved that into a calculator and it dumped 1395Ah.

AH=(2000Kwh*1000)/48=41,667 monthly or 41,667/30= 1388Ah.....

So I need roughly 15 strings each being 16S, so 240 cells for 3 days.....may need to cut it down to 2 days.
 
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I think snooblee while technically correct has his head too pointy right now. I say only 15 strings since 3 days autonomy will rarely be used. So 15 x $1600. About $25,000

I don't appreciate this one bit, mister. Your words hurt my heart. One time I gave myself a buzz cut, and damned if I don't actually have a cone head. My wife warned me to never do it again. I pray I retain my full head of luscious hair well into my twilight years, or at least that I stay taller than she is...
 
This could be the key to having a $5000 battery bank rather than a $40,000 one:

"we have a 38KW Natural Gas Generator"

So when ever battery starts to get low, you could fire up the generator. Most people here use smaller generators, even downsizing from 10 kW to 5 kW.

But why exactly do you want to be off-grid? Batteries cost more than grid power, so they'll never save you money. If net metering is available, you can "store" power while the sun shines and draw when you need it. With batteries, you either do without power when PV doesn't produce enough, or you waste power from PV that you can't use or store. Better to push that surplus power into the grid for someone else to use.

"do I need to buy new Inverters to support this plan"

Depends on how these grid-tie inverters act. If they are able to reduce power production when frequency rises, they can be used together with battery inverters. I use SMA grid-tie and battery inverters together this way. Several other brands can mix and match using the same method:

 
This could be the key to having a $5000 battery bank rather than a $40,000 one:

"we have a 38KW Natural Gas Generator"

So when ever battery starts to get low, you could fire up the generator. Most people here use smaller generators, even downsizing from 10 kW to 5 kW.

But why exactly do you want to be off-grid? Batteries cost more than grid power, so they'll never save you money. If net metering is available, you can "store" power while the sun shines and draw when you need it. With batteries, you either do without power when PV doesn't produce enough, or you waste power from PV that you can't use or store. Better to push that surplus power into the grid for someone else to use.

"do I need to buy new Inverters to support this plan"

Depends on how these grid-tie inverters act. If they are able to reduce power production when frequency rises, they can be used together with battery inverters. I use SMA grid-tie and battery inverters together this way. Several other brands can mix and match using the same method:

Thanks for the insight and the link. Yes our area does do Net metering, I really want the Battery bank to eliminate the cycle time between losing grid to Generator.

Honestly never thought to cost of being of off grid just thought about wanting independence so when SHTF we are prepared. I now assume the people that do off-grid, do it out of necessity and not desire.

I need to do more research on this, I really appreciate the advice. This has been a great source of information.
 
I'm off grid presumably by necessity. The closest power pole is about 1/2 mile away, and it's with a shitty co-op that's pricey, unreliable and burning coal. I didn't bother to ask what it would take to get grid. I also like the appeal of being completely independent (as I sit here sucking down the A/C of my Phoenix Desert dwelling using almost 100kWh/day).

I'm about $13K into my off grid system (only about 30kWh/day typical). Neighbor to the West has been off grid for 20+ years. Neighbor to the East is on-grid but wants to get off grid due to the hassle of dealing with the co-op.
 
Thanks for the insight and the link. Yes our area does do Net metering, I really want the Battery bank to eliminate the cycle time between losing grid to Generator.

Honestly never thought to cost of being of off grid just thought about wanting independence so when SHTF we are prepared. I now assume the people that do off-grid, do it out of necessity and not desire.

I need to do more research on this, I really appreciate the advice. This has been a great source of information.
Keep up your research I think there are ways to reach a happy medium of SHTF safety and not breaking the bank. The most Obvious is a generator in conjunction with solar. Maybe plan 1 day Autonomy not 3 and use a generator if the sun is hidden for a prolonged time. Also jeez I use 300kw per month so yo may just have to live without AC for a day or 2
 
I'm off grid presumably by necessity. The closest power pole is about 1/2 mile away, and it's with a shitty co-op that's pricey, unreliable and burning coal. I didn't bother to ask what it would take to get grid. I also like the appeal of being completely independent (as I sit here sucking down the A/C of my Phoenix Desert dwelling using almost 100kWh/day).

I'm about $13K into my off grid system (only about 30kWh/day typical). Neighbor to the West has been off grid for 20+ years. Neighbor to the East is on-grid but wants to get off grid due to the hassle of dealing with the co-op.
I guess I just never figured batteries were so expensive, I keep reading stories of people using 14KW Leaf cells and such and they really seem to be fairly reasonable in price. maybe I should buy a couple of those....more reading on my end....fun

Our usage is high as we work from home and well I would complain about the heat in Florida, but spent plenty of time in Yuma....so you all have me beat.
 
I actually have 5X PHEV batteries. 38kWh for about $4500, but it's the same chemistry as the Leaf - WAY more scary than LFP. And it's not really any cheaper than the equivalent LFP - darn close to exactly the same price as equivalent capacity of 280Ah cells delivered. It's just what I know and had access to a year ago. Still not built!
 
Thanks for the insight and the link. Yes our area does do Net metering, I really want the Battery bank to eliminate the cycle time between losing grid to Generator.

Honestly never thought to cost of being of off grid just thought about wanting independence so when SHTF we are prepared. I now assume the people that do off-grid, do it out of necessity and not desire.

I need to do more research on this, I really appreciate the advice. This has been a great source of information.

In that case you just want a UPS.
That can be an "on-line UPS" to be entirely glitch free, but if you just want stuff like computers to stay up, having power go away for 30 to 60 milliseconds won't be a problem.

Lots of inverters have a relay inside and can pass through grid power. When grid goes down, they disconnect the relay and make their own 60 Hz power for you. If your generator comes on line with an automatic transfer switch, they'll see that and let generator run the house. Battery only has to run 5 minutes.

You can also do larger battery, PV, with generator. Generator only runs if batteries get low.

My system is half way in between. Excessive PV, small battery, no generator. Just enough battery to get through the night, but enough PV to run A/C and everything else during the day.

I have 405 Ah, 48V battery (20 kWh) lead-acid AGM, cost $5000. Could have made it 1/4 that size if I turned off all refrigerators at night. This is a good value for maintenance free, occasional power failures. I expect it to last 10 years. Other battery types offer more kWh per dollar and require maintenance, or more deep cycles at a higher up-front cost.
 
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