Project Planning

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klausfelix

New member
Joined
Jul 6, 2021
Messages
1
What I don't quite understand at this point is what size to build the packs, especially to make the system scalable,

No 1 issue is off-grid vs sending power to grid (hybrid).

When off-grid, you have to store excess power in your battery bank and then use it thru the night.

If you're grid-tie, you can just send excess to grid so battery is more of a UPS (simple backup when power goes out).

Not 2 issue - When off-grid, are you OK loosing PV power or do you insist on using it all. If you don't want to loose any power,

the battery must have a minimum size.

I'm off-grid, and I insist on using all the PV power I create.

Assuming you're thinking similarly...

let me share some key things to think about from my own operations.

The key variables are PV in, consumption out, and battery size. In my case....
- The13kw PV array generates 18,000kwh/year - max of 80kwh/day in spring/summer - which is the max the house can consume.
- Average consumption is 2,500w/hour - with max typically <= 14,000w.
- 81kwh battery bank averages 40% DOD with high of 55% in spring/summer and low of 16% in winter.

From the above we could try to estimate / extrapolate some things to help you get into the ballpark of design.

==== SPECIFICS FOR YOUR SITUATION =====
>I guess the packs need to be in 14s configuration but how many should be in parallel per pack is
14s = 'most mainstream' 48v nominal... highly recommend you go this route.
14s100p (of 3000mah/cell) would be 14,400kwh battery @ 48v (I use this https://milliamps-watts.appspot.com/ so I don't have to multiply by hand :) )

Operationally:
- By extrapolating from my experience - at a 60% average DOD on a 14.4kwh battery - you could support up to a 4kw PV array.
- Per pvwatts ( https://pvwatts.nrel.gov/pvwatts.php ) at Dallas Tx - a 4kw PV array would be 20kwh/day max in summer. That's 833w/hour * 24hrs.

Per cell....
- A PV array of 4000watts / 48v = 83a of charging input. 83a / 100cells = 830ma/cell charging. [OK]
- At 833w average consumption that's 833w/48v = 17.4a = 174ma/cell discharge. [GOOD]
- At max 5000w - 5000w/48v = 104a / 100cells = 1.04a/cell discharge. [1a / cell is hefty]
 

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
1,712
What I don't quite understand at this point is what size to build the packs, especially to make the system scalable,

No 1 issue is off-grid vs sending power to grid (hybrid).
There are 3 options (not just 2). Off-grid, grid-tie, and hybrid. Grid-tie + hybrid are both sending power to the grid. Grid-tie is battery less / sending excess power to the grid + won't work if the grid is down.

When off-grid, you have to store excess power in your battery bank and then use it thru the night.

If you're grid-tie, you can just send excess to grid so battery is more of a UPS (simple backup when power goes out).
What you say is true. However, the biggest reasons I'm off-grid are
1) To be able to live (day after day) without the grid - bigger goal than just 'a backup'.
2) To avoid Power Company interaction/requirements/permits. I didn't want them to dictate my setup at all.

Not 2 issue - When off-grid, are you OK loosing PV power or do you insist on using it all. If you don't want to loose any power,
I purposely sized my system to power about 85% of my home - which is <100%. So I only loose 'a bit' of power (maybe $60) for a few weeks in the spring when Solar is max and heating/cooling is at it's minimum. I agree that this is something to consider when designing a system.

They do make hybrid (off-grid + grid-tie) systems... I'm just not up on what to recommend.

the battery must have a minimum size.

I'm off-grid, and I insist on using all the PV power I create.
PV in vs consumption. It's possible to size a PV array to be < minimum consumption - in which case the battery is just 'a buffer' between the minute-by-minute fluctuations of MPPT tracking and steady output of an inverter and can be small.

It's also common to consume + store 'extra' in the battery for discharge a night or even over several days of cloudy / low PV input.

If you get the point where your PV input is larger than what your battery can buffer relative to your consumption - then you have excess.

You could go to extremes and make a case to have 4 x the PV you need for summer for the sake of being able to power things in winter. Or 2 x PV for what you need just to handle those few days of 114F/39C extreme heat/AC demands. In my case - power is 11c/kwh - so cheap that I'm not doing any of this to save money. Excess PV is not a crime, its just a calculation of what you want for your situation :)

After being in operation for a while now - I notice that couple of key factors dominate off-grid operations.
- The Sun up/down - 24hr cycle of PV input and consumption - set's the tone for all this
- For a home size operation, its just not practical to build 1,000kwh or 5,000kwh battery banks. Its more like 30kwh or 50kwh or 80kwh - so you can't escape the major influence of the 'daily cycle'.
- The best you can do is perhaps a 'few days' - but you can't scale up to a few months of power stored. I'd love to have 800,000 cells! :)

Assuming you're thinking similarly...

let me share some key things to think about from my own operations.

The key variables are PV in, consumption out, and battery size. In my case....
- The13kw PV array generates 18,000kwh/year - max of 80kwh/day in spring/summer - which is the max the house can consume.
- Average consumption is 2,500w/hour - with max typically <= 14,000w.
- 81kwh battery bank averages 40% DOD with high of 55% in spring/summer and low of 16% in winter.

From the above we could try to estimate / extrapolate some things to help you get into the ballpark of design.

==== SPECIFICS FOR YOUR SITUATION =====
>I guess the packs need to be in 14s configuration but how many should be in parallel per pack is
14s = 'most mainstream' 48v nominal... highly recommend you go this route.
14s100p (of 3000mah/cell) would be 14,400kwh battery @ 48v (I use this https://milliamps-watts.appspot.com/ so I don't have to multiply by hand :) )

Operationally:
- By extrapolating from my experience - at a 60% average DOD on a 14.4kwh battery - you could support up to a 4kw PV array.
- Per pvwatts ( https://pvwatts.nrel.gov/pvwatts.php ) at Dallas Tx - a 4kw PV array would be 20kwh/day max in summer. That's 833w/hour * 24hrs.

Per cell....
- A PV array of 4000watts / 48v = 83a of charging input. 83a / 100cells = 830ma/cell charging. [OK]
- At 833w average consumption that's 833w/48v = 17.4a = 174ma/cell discharge. [GOOD]
- At max 5000w - 5000w/48v = 104a / 100cells = 1.04a/cell discharge. [1a / cell is hefty]
 
Last edited:

cak

Member
Joined
Mar 14, 2021
Messages
53
I live off grid and have my whole life. Especially right now Batteries are the more expensive part of an off grid system so I would design to minimize batteries needed rather than maximize solar panel productions. As an example in my home we have very short days during the winter and no connection to the electric grid to provide power so I have lots of panels to meet the need of the shortest days and accept the tradeoff of the panels being underutilized during the summer. Long term I am working towards electrifying mowers and other tools to use some of that extra summer energy and even considering getting an air-conditioner. I also want to get an electric car that I could charge at home in the summer but charge elsewhere in the winter when I don't have road access and less solar. My goal with the batteries is to have them last about 3 days without any solar coming in and about another 4 days in rationing mode with reduced electrical usage so I could function through a whole week or dark clouds. Based on the weather in my area that covers things pretty well and worst case I run a small 1800w honda generator to keep the fridge and lights working(last year used about 10 gallons so acceptable). Just some thoughts from a lifelong off grider :)
 
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