Need Help on Deciding Solar PV Expansion

GoneSquatchin

Hi all,

First time poster here...

Last spring I installed a Renogy 100W Starter kit with a 12v 215ah GC Battery Bank at our in-progress off-grid cabin. The system worked great for our long weekend stays throughout the summer and during hunting season. I have all the lights (LEDs) and a few outlets connected via a Xantrex ProWatt SW 600 inverter. I also have a trimetric battery monitor setup.

Normally, I don't draw the batteries down any more than 80% (One time during construction, the insulators had every light in the place turned on and some of the lights were incandescent and the batteries got down to 75%). I have a generator(s) for the well pump, coffee maker, and vacuum cleaner. So when the generator is running, I have a transfer switch and I turn off the inverter.

However, I understand that my 100W panel is not enough for charging the 215ah battery bank correctly. (I say "correctly" because since we only use it for 2 or 3 days at a time, the batteries are always at 100% by the next time we get to the cabin). I would also feel more comfortable if I could be putting more amps into the system during the day that could be directly usable in the cabin (radio, tv, etc.) than I currently am getting.

So... Option 1: I could add a 2nd Renogy 100w panel (on Amazon for $150) (Series or Parallel?) and I would have to upgrade my charge controller to a 15 or 20 amp (PWM or MPPT?) (current CC is a 10a PWM).

or... Option 2: I have found a local supplier of a GT Topoint 250w panel for $200 and I would upgrade to an MPPT charge controller (thinking the new Rogue 20a model for $225). If I pick this option, I will bring the 100w panel home for a small emergency backup system that I'd like to setup.

So I need opinions on what the best route to go is. Ex., which charge controller if I add another Renogy panel. (It seems a "good" pwm controller is around $100. Not especially cheap. Why not just pay the additional $125 for the MPPT). Also, the 2 Renogy Panel option gives me a fail-safe if something dies. I can always revert to 1 panel on the existing 10a pwm controller in a pinch. Option 2 does not give me a fail-safe unless I buy another panel and/or another CC

One other possible consideration is that this is a remote cabin that I only get to a dozen or so times a year (Spring thru late November). So theft may be a concern... Although, we're only talking a few hundred dollars difference between the 2 options and I do have insurance...

Looking forward to everyone's thoughts.

Thanks,

Jeff

BB.

Welcome to the forum Jeff.

I will take a stab at answering your questions below:

GoneSquatchin wrote: »

Last spring I installed a Renogy 100W Starter kit with a 12v 215ah GC Battery Bank at our in-progress off-grid cabin. The system worked great for our long weekend stays throughout the summer and during hunting season. I have all the lights (LEDs) and a few outlets connected via a Xantrex ProWatt SW 600 inverter. I also have a trimetric battery monitor setup.

For a pair of golf cart batteries, roughly the recommended solar array size would be:

• 14.5 volts charging * 215 AH * 1/0.77 panel+controller derating * 0.05 rate of charge = 202 Watt array minimum (weekend/seasonal use)
• 14.5 volts charging * 215 AH * 1/0.77 panel+controller derating * 0.10 rate of charge = 405 Watt array nominal (full time use)
• 14.5 volts charging * 215 AH * 1/0.77 panel+controller derating * 0.13 rate of charge = 526 Watt array "cost effective" maximum

So--For longer battery life, I would suggest that you add to your array to a minimum of ~200 Watt total worth of panels. For many deep cycle batteries, they really recommend 10% minimum rate of charge but many people do get away with 5% for a weekend cabin.

Normally, I don't draw the batteries down any more than 80% (One time during construction, the insulators had every light in the place turned on and some of the lights were incandescent and the batteries got down to 75%). I have a generator(s) for the well pump, coffee maker, and vacuum cleaner. So when the generator is running, I have a transfer switch and I turn off the inverter.

Especially if you get your array in the 202 to 405 Watt array size--You can certainly discharge down towards 50% state of charge and not feel "guilty". The batteries are there to be used (and reducing generator noise/fumes/cost of fuel is nice too).

The batteries will not last quite as long (75% state of charge vs 50% state of charge, the batteries will cycle ~1/2 as many times)--But for a seasonal cabin, your batteries will probably die of "age" (3-5 years is an average golf cart battery life) anyway.

However, I understand that my 100W panel is not enough for charging the 215ah battery bank correctly. (I say "correctly" because since we only use it for 2 or 3 days at a time, the batteries are always at 100% by the next time we get to the cabin). I would also feel more comfortable if I could be putting more amps into the system during the day that could be directly usable in the cabin (radio, tv, etc.) than I currently am getting.

If you can get an estimate of the Amp*Hours (at 12 volts) and/or Watt*Hours (typically at 120 VAC) you need will help too. For a typical battery bank, 2 days of storage and 50% maximum discharge (for longer life), gives us:

• 215 AH * 1/2 days storage * 0.50 maximum discharge = 53.75 AH per day @ 12 volts
• 12 volts * 215 AH * 9.85 AC inverter eff * 1/2 days storage * 0.50 maximum discharge = 548 Watt*Hours @ 120 VAC per day

Of course, if some of your power is used during the day (Radio, laptop computer, etc.)--The solar panels can "help" with that (i.e., battery is only "used" at night and/or during bad weather).

So... Option 1: I could add a 2nd Renogy 100w panel (on Amazon for $150) (Series or Parallel?) and I would have to upgrade my charge controller to a 15 or 20 amp (PWM or MPPT?) (current CC is a 10a PWM).

If the solar array is close to the charge controller+battery bank (i.e., you only have a ~10 foot run of solar array wiring), a pair of 100 Watt panels in parallel with a 15-20 amp PWM controller is "good enough" (no reason to waste money for weekend cabin, especially if there is a security issue).

or... Option 2: I have found a local supplier of a GT Topoint 250w panel for $200 and I would upgrade to an MPPT charge controller (thinking the new Rogue 20a model for $225). If I pick this option, I will bring the 100w panel home for a small emergency backup system that I'd like to setup.

If the solar array is a fair distance from the cabin, then a higher voltage Array + MPPT charge controller will make sense. Also, as the solar array gets larger, the "GT Panels" (not "12 volt" or Vmp~17.5 volt solar panels) can be much cheaper and when paired with the more expensive MPPT charge controller, you can still save money (and add in the savings in copper wiring costs from array to charge controller).

So I need opinions on what the best route to go is. Ex., which charge controller if I add another Renogy panel. (It seems a "good" pwm controller is around $100. Not especially cheap. Why not just pay the additional $125 for the MPPT). Also, the 2 Renogy Panel option gives me a fail-safe if something dies. I can always revert to 1 panel on the existing 10a pwm controller in a pinch. Option 2 does not give me a fail-safe unless I buy another panel and/or another CC

MPPT controllers can even cost much more than $125 (US designed/supplied). For smaller systems, PWM can be just fine unless you want "bells and whistles" (automatic equalization, remote battery temperature sensor, computer interface/logging, high voltage Vmp-array for longer wire runs/higher powered systems).

One other possible consideration is that this is a remote cabin that I only get to a dozen or so times a year (Spring thru late November). So theft may be a concern... Although, we're only talking a few hundred dollars difference between the 2 options and I do have insurance...

Back to the loads--If you are happy with 100 Watts of solar array--You will probably be ecstatic with 200 Watts. No need to "over size" the system unless your loads require it.

Get a Kill-a-Watt meter to measure 120 VAC loads. For smaller DC loads, a DC AH/WH meter, and/or an AC/DC Current Clamp multi-meter. (links are suggested starting points for your reading/research).

You could also get a simple AC battery charger (a ~10 to 20 to 45 amp @ 12 VDC unit would be fine for your present bank). A Honda eu2000i should drive a 40 amp 12 volt Iota charger (possibly 45 amp). The Honda eu1000i would top out on a 15-20 amp charger (again @ 12 volts).

-Bill

BB.

I should add a calculation for how much a 200 Watt array would generate. Using PV Watt for Williamsport, with a fixed array tilted to ~41 degrees from horizontal:


Month
Solar Radiation
(kWh/m²/day)


1
3.06


2
3.61


3
4.50


4
4.60


5
5.15


6
5.29


7
5.38


8
5.19


9
4.61


10
3.77


11
2.58


12
2.35


Year
4.18



Toss out the bottom three months (assume using generator when needed), that would make February at the "break even" month (may or may not need genset):

• 200 Watts * 0.52 end to end AC system efficiency * 3.61 hours of sun = 375 Watt*Hours per day (February average) for 120 VAC power
• 200 Watts * 0.61 end to end DC system efficiency * 3.61 hours of sun = 440 Watt*Hours per day for DC power
• 440 WH per day * 1/12 volts battery bank = 36.7 AH @ 12 volts per day (again February average)

So you can get an idea of how much power you can obtain from your solar array/system.

-Bill

GoneSquatchin

Bill,

Thanks so much! That's great info!

I did forget to say that my array is about 15' from panel to charge controller and another 4' to the batteries. I'm currently using 10 gauge wire. I think I'm OK with that distance if I parallel 2 12v panels?

I do admit that when I get down to the low 80's SOC, I start to get worried and fire up the generator :-) I do have a Schumacher 20a battery charger that I've used a few times. But, I guess I've never had the batteries discharged enough to see any more than the 2a slow charge from the charger. Otherwise, I'm happy with the performance of the system. So I agree, I'll probably be much happier with 200+ watts of panels.

Regarding charge controllers... I see a good Morningstar 15a PWM is around $100. (As opposed to any old PWM from Amazon for between $30-$50 and even some Tracer MPPT models being as low as $75) With the Rogue 20a being $225, I kinda wonder if the Morningstar PWM controllers are worth the money? Any suggestions on a good PWM that doesn't cost so much? I also kind of like the Rogue for the option of doing a manual equalization periodically. I assume that will extend the life of my batteries?

I guess the real question is... With 200-250 watts of panels, how much will an MPPT help me compared to a PWM? A subquestion would be, some Tracer MPPT controllers are the same or cheaper than the Morningstar PWM controllers, so why not just go MPPT regardless... (unless there are much cheaper PWM controllers that are considered reputable?)

Thanks again,

Jeff

BB.

GoneSquatchin wrote: »

I did forget to say that my array is about 15' from panel to charge controller and another 4' to the batteries. I'm currently using 10 gauge wire. I think I'm OK with that distance if I parallel 2 12v panels?

For a "12 volt" solar array, you would want a maximum of ~0.2 to 0.5 volts of drop in the wiring from the array to the battery bank:

http://www.calculator.net/voltage-drop-calculator.html

100 Watt / 17.5 volts Vmp = ~5.7 Amps Imp

And 15 feet one way run with 10 AWG wire:

Voltage drop: 0.17
Voltage drop percentage: 0.97%
Voltage at the end: 17.33

So, you are OK there.

The next issue is from the charge controller to the battery bank... You would want about 0.05 to 0.10 volt maximum drop. Normally, using heavy/short cable from charge controller to battery bank is best. This ensures that the battery is quickly/fully charged with as much solar energy as is available. More wiring drop means lost energy (due to resistance) and also the battery being charged t a lower voltage than is really ideal (slower charging, wasted sun).

10 AWG, 5.7 amps, 4 feet:

Voltage drop: 0.046
Voltage drop percentage: 0.32%
Voltage at the end: 14.454

Also, you are OK there (with the relatively low current of a 100 watt array).

I do admit that when I get down to the low 80's SOC, I start to get worried and fire up the generator :-) I do have a Schumacher 20a battery charger that I've used a few times. But, I guess I've never had the batteries discharged enough to see any more than the 2a slow charge from the charger. Otherwise, I'm happy with the performance of the system. So I agree, I'll probably be much happier with 200+ watts of panels.

It is hard to not worry--But batteries, they are there for you to use. And they are a "consumable" item--Just like gasoline and a generator. Generally, if you treat them relatively well, they will give you good service for the money.

Regarding charge controllers... I see a good Morningstar 15a PWM is around $100. (As opposed to any old PWM from Amazon for between $30-$50 and even some Tracer MPPT models being as low as $75) With the Rogue 20a being $225, I kinda wonder if the Morningstar PWM controllers are worth the money? Any suggestions on a good PWM that doesn't cost so much? I also kind of like the Rogue for the option of doing a manual equalization periodically. I assume that will extend the life of my batteries?

It is a trade off... If the charge controller goes bad, then you either kill your batteries (couple hundred dollars of loss) or have a long drive back to town to pickup/order a new charge controller. Some do not give very fast/accurate charging.

The inexpensive charge controllers--Some work pretty well, and other do not. I will let others here who have more experience than I regarding what they would suggest in the low cost range.

I guess the real question is... With 200-250 watts of panels, how much will an MPPT help me compared to a PWM?

For a small well designed system, MPPT will give you something like ~10-15% more energy in sub freezing weather. In warm weather/climates, do not buy an MPPT for better/more efficient charging.

Buy and MPPT because:

• you have a large array with long cable runs (run higher Vmp-array)
• you can get large format (>200 watt panels) cheaper than you can get ~140 Watt "12 volt panels" (cheap panels+expensive MPPT << expensive panels+cheap charge controller)
• You want the features of the more expensive/complex MPPT controller (logging, communications, auto equalization, etc.).

In general, less than 400 Watt system, generally a PWM controller will be just fine.

For >800 Watt systems, generally, you would want to look at MPPT.

A subquestion would be, some Tracer MPPT controllers are the same or cheaper than the Morningstar PWM controllers, so why not just go MPPT regardless... (unless there are much cheaper PWM controllers that are considered reputable?)

Branding seems to be highly variable. I have seen people here very happy with their Tracer... But things change so much/fast/counterfeit goods it is difficult to be sure.

But even the higher end US companies have had their problems too... But usually it is easier to get help if they are local (in country).

Jeff, plug the following into Google and you can search for Tracer MPPT and read the various threads/posts about their product:

• site:forum.solar-electric.com tracer mppt

The "site:" tag limits searches to a specific website (ours in this case).

-Bill

GoneSquatchin

Bill,

I really appreciate all your help! So, first I was leaning towards the single GT panel and MPPT. But, now you've got me leaning back towards adding another 100w panel and a larger PWM controller (I just wish the 12v panels were cheaper). I would see some below freezing weather in late October and November during hunting season where the MPPT may be beneficial. But, usually we would be out all day hunting and not using any power anyway during the day. Only concern with that setup is equalization: I see that the Morningstar Units do a short 1-2 hour equalization every 28 days. (For that matter, so does the cheap controller that came with the Renogy kit). Should I be concerned about this monthly equalization during the winter when I'm not there to check on the water levels?

Thanks for the tip on the google search. I guess the Reliability concerns about the Tracer may keep me from relying on it unattended. And in my case, there is no local town to run to for replacement parts. I would have to live on generator and order new parts

Thanks again,

Jeff

BB.

I would not worry about equalization... 1-2 hours once a month should not be a problem. Just monitor water level in your flooded cell batteries (if they have to be filled every month or the plates are exposed, then they may be getting over charged).

For very cold climates, there is one nagging problem. The charge controller will increase the battery charging set points by 0.5 to ~1 volt or so. And equalization could be 15.5 to 16+ volts for some industrial type batteries.

Many 12 volt AC inverters have ~15.0 volts to 15.5 volts as their over voltage shutdown limit. There are a few inverters that will run >16 volts... So depending on your usage and temperatures, this could be an issue.

However, if in cold weather, you are out hunting during the day and using power at night--It should not be a big issue (high battery voltage only when they sun is shining bright and sub freezing battery bank).

-Bill

GoneSquatchin

Hi everyone,

Just thought I'd follow up... (I'll follow up in my other thread too)

I ended up picking up (2) 255W Solar World Panels for $300. Don't know if I'll use both at the cabin or not, but I figured the price was too good and I'll probably setup a small backup system at home too. So the extra panel won't go to waste

Now to look for a mppt controller...

Thank you for all you assistance!

Jeff

westbranch

if only one panel look at Morningstar's line http://www.solar-electric.com/inverters-controllers-accessories/chco/mochco.html