Design a 24v or 48v system

The typical voltage "limiting" hardware are solar panels (Vmp-array voltage), MPPT vs PWM charge controller (you have MPPT--Good), AC inverter input bus voltage (yours is 24 volts--Need different inverter if want 48 volts), and batteries for bank (your 8x 6 volt batteries will work fine for 12/24/48 volt bus).

If you plan on going for larger single system in the future--A 48 volt inverter + battery bank will give you more "headroom" (the Outback MPPT controller will work on 12/24/48 volts--But the size a supported 48 volt battery bus is 2x the wattage of a 24 volt bus)...

• 80 amps * 29 volts charging * 1/0.77 panel+controller derating = 3,013 Watt typical cost effective array @ 24 volt bus
• 80 amps * 58 volts charging * 1/0.77 panel+controller derating = 6,026 Watt typical cost effective array @ 48 volt bus

 Also, depends on the size of your battery bank... I suggest about 800 AH is as large as you want to go with a bank... If 800 AH at 24 volts, it would be a 400 AH @ 48 volts (same amount of energy storage -- Power=Voltage*Current).

Looking at your AC inverter (features, efficiency, Tare Losses, "search mode", remote panel, etc.) tend to help make the decision of what AC inverter (and sometimes battery voltage) limit your choices. Large Wattage inverters on a higher voltage battery bank tend to "waste" power power when driving relatively small AC loads (i.e., if the inverter draws 40 Watts * 24 hours per day just being "on", that is more power than your one of your freezers).

-Bill

Inverter-chargers can be very nice... They can be highly programmable (compared to the simple Iota type). And the inverter can include its own internal transfer switch... Run from battery, start genset, transfer from inverter/battery power to genset AC power, limit input AC power to genset rating, vary charging current based on Genset Max - AC loads = Battery current.

Generator support mode, you can run your 1,800 Watt genset and inverter-charger with generator support can add "battery power" to AC output (genset 1,800 Watts max, AC load max load = 1,800 W genset + 3,000 Watt from battery = 4,800 Watt total AC load support--Until battery goes dead, fuel runs out, or AC load drops back to less than 1,800 Watts--Then 1,800 Watt Gen - 500 W AC load = 1,300 Watt available for battery charging from genset (all numbers made up--Just example of function).

Or, you can keep the simple AC inverter, and parallel your solar charger + AC genset charger to single battery bank. Will work fine.

Argument for separate AC inverter and AC charger--If one fails, the other still works. With Inverter-Charger, if "one fails", both Invert/Charge functions are dead until repaired or replaced.

Your system is close to supporting 3,300 WH per day (my suggested aim point for a near normal electrical existence for a full off grid cabin/very energy efficient home (using propane/wood/etc. for heating/hot water/cooking/etc.). So, using your number:

• 3,300 WH per day * 1/0.52 off grid AC system eff * 1/3.1 hours winter sun = 2,047 Watt array "winter break even"

Now... You do should break your loads into base loads (need to run every day, like fridge, LED lighting, cell phone charger, etc.). And then the optional loads that you run when the sun is out (washing machine, irrigation pumping, power tools, vacuum cleaner, etc.).

Base loads should make up around 50% to 65% of your total solar power for a "reasonable" build and forget (basic maintenance) system (vs one you have to look at the hours of sun, loads, etc. every day and figure out what you are going to "run" that day). Of course, if you have a genset--This is more the issue of fuel costs, noise, etc. to run a genset vs buying more panels.

So, you can look at your base load harvest as:

• 2,047 W array * 0.52 AC system eff * 3.1 hours of (winter) sun * 0.65 base load fudge factor = 2,145 WH of base load support (winter)
• 2,047 W array * 0.52 AC system eff * 4.9 hours of sun (summer average) = 5,216 Watt*Hour per day summer available energy

Lots of ways of slicing and dicing these numbers... It really depends on what works best for you (more panels, less genset fuel. Or fewer panels, smaller battery bank, only 9 month or less "sunny season" usage--No need to over design system--Bring a small genset+fuel for a week of winter playtime).

I try to use the math to help you decide where best to spend your money and a reasonably accurate prediction (in solar if I am within 10% of predicted output--That is pretty much "dead on" accurate for solar).

Your thoughts?

-Bill

This is where we are hitting another "gotcha" with the standard 140-150 volts max MPPT controllers, and the getting to be common higher voltage solar panels (Vmp~36 volts and higher).

Guessing you are around Mckenzie TN, with a minimum winter temperature of 22F... You would use an Outback String Sizing Tool (download and run on your computer):

http://www.outbackpower.com/resources/technical-support/string-sizing-tool

But as a rough estimate, using Midnite's online tool (because I am running Chromebook here):

http://www.midnitesolar.com/sizingTool/index.php

You will be very close to 148 volts Voc-cold at 22F... The FM 80 has a Vmax input of 150 VDC, and a maximum startup voltage of 145 VDC... Downloading the Outback tool and putting in all the actual numbers for your panels (Voc, Vmp, Isc, Imp, etc.) is really needed here--And how cold it actually gets in your region.

If you pick 24 volt battery bank and only 2x panels in series... You will be fine. 3x panels in series--Close to maximum input voltage for the FM 80 in January.

If you pick a 48 volt battery bank and 2x panels in series--It will work, but not be optimum (MPPT controllers need a higher input voltage to work "optimally"). And with 3x panels in series, you are pretty close to maximum input voltage in cold weather.

There is nothing "wrong" with your panels, or even your Outback controller--It is just the nature of the beast and evolution of the solar panels over time, where they are installed, etc.

-Bill