adding solar capacity for Winter suggestions

[jjhdtv]

Hello all. I have a small system for an offgrid cabin. Currently consists of a Magnum inverter ( MS402PAE), two Midnite Kid charge controllers (30A), a 750 watt East facing array, a 750 watt West facing array, DIY built 24v 360AH LifePO4 battery bank (the replacement battery is my doing, everthing else was from the previous owner professional install). This system provides all I need for the late spring, summer and fall. Not so much during the Winter months (short days). I do have a generator to charge batteries, but i do not want it to run when I'm not there (no autostart). My main goal is keeping a celluar internet security system up and running 24/7. This system takes about 4Kw a day (24 hrs). The past few winters, I typically can stretch powering these loads for about 10-14 days before batteries and inverter shut off.

Trying to size up options to help bridge the gap in the winter months. I have purchased additional panels but trying to figure out the most economical way to add to my system. The existing Magnum system also has the load center.

Looking for suggestion. I'm contimplating adding an additional Kid charge controller and an additional 800 watt of panels, or a bigger different charge controller that could handle more amps and panels. I'm also curious about using micro inverters and (AC coupling) with the Magnum to help charge the battery.
Appreciate the input.

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[MisterSandals]

This system takes about 4Kw a day (24 hrs).

4kWh per day?
4000W / 24h = 166W average? Perhaps reasonable for internet security system.

24v 360AH

24V x 360Ah = 8640Wh
Looks like about 4 days of runtime per battery charge.

a 750 watt East facing array, a 750 watt West facing array,

Do you know how many watt hours these produce on an average day?

If each array gets 1.5 hours of quality sun:
750Wh x 3h = 2250Wh (covers about half of your usage?).

To last 12 days, the panels must be producing:
(4000Wh x 12) - 8640Wh battery = 39360Wh / 12 = 3280Wh per day (720Wh less than your 4000Wh use).

I'm contimplating adding an additional Kid charge controller and an additional 800 watt of panels,

This "should" make up for your daily shortfall (720Wh) with a single hour of solar harvesting.

This is all at 100% efficiency, on paper.

 

[yodamota]

my suggestion would be to upgrade the solar power you have by at least double if not triple.
Over the summer we went from 2000w to 4000w. Its already paying for itself. during the past 2 days we had rainy completly overcast and we managed to squeeze 1.5kwh the first day and 2kwh the second day. today was more like 3kwh.

Having arrays facing different angles really helps for us. It seems the sun is more likely to be out in the afternoon on these rainy day. The sun dips under the clouds and we get an hour or so of sunlight.

I have another 2000 watts to bring in so we will be sitting at over 6kwh of solar soon :D

 

[DIYrich]

Add another charge controller for the new panels. You might be able to get by in winter months paralleling the new panels into the existing controller, but a 2nd charge controller will eliminate clipping and give you extra power the rest of the year. Micros won't be any cheaper, and would be less efficient charging the battery (double conversion of dc to ac back to dc).

 

[Supervstech]

4kWh per day?
4000W / 24h = 166W average? Perhaps reasonable for internet security system.

24V x 360Ah = 8640Wh
Looks like about 4 days of runtime per battery charge.

Do you know how many watt hours these produce on an average day?

If each array gets 1.5 hours of quality sun:
750Wh x 3h = 2250Wh (covers about half of your usage?).

To last 12 days, the panels must be producing:
(4000Wh x 12) - 8640Wh battery = 39360Wh / 12 = 3280Wh per day (720Wh less than your 4000Wh use).


This "should" make up for your daily shortfall (720Wh) with a single hour of solar harvesting.

This is all at 100% efficiency, on paper.

Please expand the math for me…

How is 8640Wh equal to 4 days of 4kWh usage?

 

[Q-Dog]

It would help to know approximately where this cabin is. Roof/panel angles too. Do you get snow on the panels? Lots of trees throwing shadows?

 

[MisterSandals]

How is 8640Wh equal to 4 days of 4kWh usage?

Ah, thanks, yes you are correct, just over 2 days, thanks (8640Wh / 4000W = 2.16).

I show my math with hopes that folks will catch my mistakes!

 

[jjhdtv]

Thanks for all the replies. I'll stick with the plan to add an additional charge controller and pannels.

Since I have an extra Midnite Kid laying around, I will try that first and see if it provides what I need this Winter. Since the Kid only handles 30A, on a 24v system, I believe I can only add about 800watts of additional panels if I use the Kid. The panels I have for the expansion are 440watt Longi brand. I think I would be ok running two panels in series at this location with the additional Kid. Final system would be running 3 Kid charge controllers all fed to the Magnum inverter, fused through the Magnum load center.

The other option would be a bigger charge controller and run 2S 2P array (I have 4 of the 440's), thus increassing the capacity 1760 watts (perhaps overkill, but I have the panels sitting around collecting dust). Suggestion on this idea in addition to a good valued bigger charge conroller would be appreciated.

My first solar project was a grid tied, critical load panel, generator backup, Hybrid Solark 12k system so I never really learned the old school stuff (e.g. Magnum inverter and separate charge controllers, etc). Unclear how many charge controllers I can safely add to the Magnum system. I'm also curious if there is any downside to mixing different brand and capacity of charge controllers.

 

[yodamota]

since you are used to midnite. What about a midnite classic? If I had started with midnite I would probably keep using midnite. Looks like great controllers

 

[MichaelK]

Thanks for all the replies. I'll stick with the plan to add an additional charge controller and pannels.

Since the Kid only handles 30A, on a 24v system, I believe I can only add about 800watts of additional panels if I use the Kid. The panels I have for the expansion are 440watt Longi brand.

Looking at your 750W array, have you ever determined what the real-world output of your panels are at noon in full sun? I would guestimate that you have not actually seen a full 30A of charging current, but more likely seeing about 25-26A? Is that correct?

I like to include a 85% fudgefactor in my solar calculations to account for real-world decreases in production, rather than what the panel can produce in an artificial test chamber. So, for the 30A Kid, the math would be (30A X 25Vcharging)/85% = 882W, so two of those Longi's would be perfect. Actually, I'd get four, and wire them into additional East/South, or maybe South/West arrays like you are already doing.

 

[DIYrich]

Growatt 3kW Stackable Off-Grid Inverter | SPF 3000TL LVM-24P

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signaturesolar.com

 

[jjhdtv]

Looking at your 750W array, have you ever determined what the real-world output of your panels are at noon in full sun? I would guestimate that you have not actually seen a full 30A of charging current, but more likely seeing about 25-26A? Is that correct?

I like to include a 85% fudgefactor in my solar calculations to account for real-world decreases in production, rather than what the panel can produce in an artificial test chamber. So, for the 30A Kid, the math would be (30A X 25Vcharging)/85% = 882W, so two of those Longi's would be perfect. Actually, I'd get four, and wire them into additional East/South, or maybe South/West arrays like you are already doing.

Trying to skimp and save the expense of a larger charge controller. As someone suggested, perhaps I should pickup a Classic 150 or FM80 and use 2 longi's facing East and 2 facing west (lined up with the existing small old panels). The extra power in the summer would be put to good use and reduce my propape bill.