Updating system and Voltage throughput

Welcome to the forum GP,

I always like to design a "balanced" system... To do that, normally we figure out the loads (Amp*Hours @ xx Volts or Watt*Hours per day), size the battery bank to support that, then size the solar array to keep the battery bank "happy" and sized on location (hours of sun per day).

Don't have loads, so start with the battery bank. 8x 6 volt @ 230AH golf cart batteries. 

First, generally size the battery bank to supply 2 days of power and 50% maximum discharge... Can do 1 day of storage and 50% discharge for a weekend/sunny weather cabin.

• 2x 6 volt series string = 12 volt battery bank
• 4x  12 volt series strings of 230 AH = 920 AH @ 12 volt battery bank
• 920 AH * 1/2 days * 0.50 max planned discharged (for longer life) = 230 AH per day @ 12 volts
• 230 AH * 12 volts = 2,760 WH per day

That is a good size battery bank... Technically, when you get over ~80 AH battery bank, you may want to look at 24 (or even 48) volt battery bank. The question is--120 VAC inverter for your loads what you need--Or do you have a lot of existing 12 volt loads that you need to power? (i.e., can you justify a 24 volt input AC inverter for the cabin?)

There are some interesting things you can do with 24 Volt battery bank... For example, there are a fair number of 24 volt "RV" type water pumps (replace a 12 volt pump) and you can get a good selection of 24 VDC capable LED lamps, and even USB chargers--And leave the 120 VAC for the "rest of the system" (modern 120 VAC energy star rated appliances are almost as efficient as their DC counterparts). And let you only run the AC inverter when needed (inverters draw something like 6-20+ Watts just turned on with no loads--Tare losses).

Note that the MPPT solar charge controller (like you have) array size supported is directly related to the battery bank voltage... The controller will output 80 amps (in your case) at 12, 24, or 48 volts... Which means that you can, for example, manage a 2x larger array at 24 volts vs 12 volts with the same controller... (AC inverters are "captive" to there base battery input voltage--No 12/24/48 capable AC inverters out there (in the mass market) that I have seen.

Your present system is almost what I suggest as an "aim point" for a very efficient near normal off grid existence in a conservation minded family/home (3,300 WH per day for Lights, full size 120 VAC fridge, clothes washer, water pump, laptop computer, cell charger)...

If you are ever planning on a larger system (full time residence)--You could probably justify 48 VDC too...

Anyway, back to "keeping your battery bank happy". Generally suggest 5% minimum rate of charge for weekend/non-winter cabin. 10%-13% for full time off grid. And >13%-20% or so for winter harvest (oversize solar array to save genset runtime).

• 920 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 866 Watt array minimum
• 920 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,732 Watt array "nominal"
• 920 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 2,252 watt array "typical" cost effective maximum

As you can see--You don't want to "oversize" the battery bank vs your loads--You need more charging capabilities to meet the bank's charging requirements (and/or more genset time for charge/EQ the bank when needed).

Re-configuring your 8 batteries as 2 series * 4 parallel (12 volts @ 920 AH), or 4 series * 2 parallel (24 volts @ 460 AH), or 8 series * 1 parallel (48 volts @ 230 AH)--Same energy storage, just battery bus voltage changes, and as you get higher voltage, the lower DC battery bus current (and smaller AWG wiring you can use). For example, the above 5/10/13% calculations give identical results with the same 8x Golf Cart Bank.

Next, sizing the battery bank based on your daily loads and location... Lets use 2,760 WH per day somewhere near Denver Co, with a south facing fixed array:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Denver
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 50° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.27	4.85	5.58	5.62	5.81	5.90
Jul	Aug	Sep	Oct	Nov	Dec
5.72	5.60	5.87	5.60	4.52	4.12

For the moment, assume you are not there in winter (Nov thru Jan)--Pick February at 4.85 hours of sun (no shade from trees, buildings, deep in valley), the array would be:

• 2,760 WH per day * 1/0/.52 off grid AC system eff * 1/4.85 (Feb) break even hours of sun per day = 1,094 Watt array February "break even" harvest

With your present battery bank and charge controller:

• 80 amps * 14.5 volts charging * 1/0.77 panel+array deratings = 1,506 Watt array "cost effective" maximum for your 80 Amp controller on a 12 volt battery bank
• 866 Watt array minimum suggested array (5% rate of charge)
• 1,094 Watt array "break even" February (long term daily average) harvest
• 1,506 Watt array "12 volt suggested max" array

And for your present battery bank, the maximum suggested AC inverter wattage would be (roughly)

• 250 Watts * 1/100 AH (@ 12 volts) * 920 AH (@ 12 volt) battery bank = 2,300 Watt max AC inverter for your present bank
• Suggest around 1,800 Watts max for a 12 volt battery bank (basically copper wiring AWG/Costs/Voltage drop design issues)
• 2,300 Watt inverter / 2 = 1,150 Watt "nice size" inverter for your bank
• 2,720 Watt*Hours per day / 5 hours per night (example) of AC power = 544 Watt "average" draw (5 hours per night from existing battery bank)

Sorry for all the math and "backing up" and asking these questions/modeling of your system... Suggest you figure out where you are at today and where you want to head with your 'upgraded" system--Then design the system to support those needs.

Cheaper to do "paper designs" first and figure these issues out first--Then look at the hards?are designs next (array/solar panels, charge controller, AC inverter, battery bank)--Again paper designs before buying hardware.

Your thoughts/corrections to my guesses?

-Bill

Note: I had a typo in my previous post... Suggest a maximum AH size for battery bank is ~800 AH or larger to step up to next higher bus voltage (not 80 AH).

-Bill