Need Help on Deciding Solar PV Expansion

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