Richa: Re-configuring/Adding to my off grid system--Questions

Welcome to the forum Richa,

You are sort of working the problem from the middle. And while we can answer the question you have asked, it may result in a less than optimum system for you.

The simple answer is... Depending on which brand/model of MPPT solar charge controller, the larger units are in the 60-80 Amp output range. Your 151.2 Amps would take 2-3 of these charge controllers connected to their own solar arrays (you cannot share panels between MPPT controllers) and paralleled to the battery bus. So, if you want 151.2 Amps charging, then the basic answer is:

• 151.2 amps * 14.5 volts charging * 1/0.77 solar panel+controller deratings = 2,847 Watt solar array

Note--Realistically, the "answers" for solar are accurate to about 10%... So you defined array--Anything within 10% of 2,847 Watts is an "exact answer" as far as solar is concerned (I used your digits so you can follow the math).

Next--You need to pick your solar panels--Or use the ones you have. With an MPPT solar charge controller, you want, at least, Vmp-array >~ 25 volts for proper MPPT controller operation (based on solar panel temperature deratings, charging a 12 volt battery bank @ ~15 volts, and such). And if you get higher end, moderate solar array maximum input voltage, your Vmp-array ~ 100 Volts maximum (assuming you are in a region with freezing/sub zero temperatures--Vs on a Caribbean Island).

Your solar panels are probably Vmp~17.5 volts, so the maximum series string would be:

• 100 VDC max array Vmp-std-temp / 17.5 volts = 5.71 ~ 5 panels in series

So your 6x 100 Watt strings are "on the edge" for exceeding maximum array input voltage for a "mid voltage" MPPT controller. If you are in a warmer climate, you could run a 6x100 string * 2 strings for 1,200 Watts:

• 1,200 Watt array * 0.77 solar panel derating * 1/14.5 volts charging nominal = 64 Amps

So, cost effective wise, if you run 6x100 Watt panels in series (2x parallel strings for 1,200 Watts), you would need a minimum MPPT controller rated for 64 Amps @ 12 volts (if cost is important, you can probably get away with 60 Amp controller and not lose much current).

The second 5-6x panel string 140 Watt panels, you probably again have the issue of 5 or 6 in series, and would get a second solar charge controller (6x140=840 Watt array)...

Obviously, if you want your 151.2 amps peak charging current, then you need more panels to get to ~2,847 Watt array. Adding more panels to the second controller, getting a third controller, etc...

The above sort of answers your question... I have some suggestions/concern about your configuration--A common issue trying to build a system with components you already have (In no particular order):

• AIMs controller--Hopefully it is a TSW/PSW inverter (true/pure sine wave) and not a MSW (Modified Square Wave). MSW inverters tend to be hard on their loads (induction motors, electronic power supplies, etc.).
• AIMs controller--Not the most reliable brand
• 3,500 Watt AC inverter--A very large Wattage AC inverter for a 12 volt battery bank. That needs very short cabling and very heavy/expensive copper wires and fuses/breakers. Normally, I would suggest a maximum 12 VDC AC inverter somewhere in the 1,200 to 1,800 Watt maximum "practicable" range.
• With your 6x 6 volt @ 420 AH -- I would be suggesting a 24 volt battery bank (and inverter-charger/solar charging system). That gives you 1/2 the operational and charging current--However, 6x 6 volt batteries does not divide evenly into 24 volts--4x6 or 8x6 would be the nearest choices using your existing battery bank.
• I highly suggest designing a system to support your loads. Loads=>battery bank. Battery bank=>solar panels, Hours of Sun per day (your location)=>solar panels. Loads => AC inverter specifications.
• With many lower end AC inverters, they can have very high "tare losses" (energy required just to turn on the AC inverter). 
• If you run a 24 volt battery bank @ 630 Amps (different battery AH capacity--Such as 8x 6 volt @ 215 AH "golf cart" batteries, you could use a single MPPT controller rated >~ 75.5 amps (=151.2/2). There are several MPPT charge controllers that are 80+ amp output into 24 volts.

I could not find a 3,5 kWatt AIMS, but there is a 4,000 Watt AIMS 12 volt TSW inverter-charger:

https://www.aimscorp.net/4000-Watt-12-Volt-Pure-Sine-Inverter-Charger.html

• Nominal Input Voltage: 12.0Vdc
• Minimum Start Voltage: 10.0Vdc
• Low Battery Alarm: 10.5Vdc-11.0Vdc
• Low Battery Trip: 10.0Vdc-10.5Vdc
• High Voltage Shutdown: 16.0Vdc
• Low Battery Voltage Restart: 13.3Vdc
• Idle Consumption: < 3.47 amps DC
• Power Saver Mode Idle Consumption: <0.53 amps DC

Energy consumed by just the Inverter-charger

• 3.47 amps * 12 volts = 42 Watts just turned on
• 42 Watts * 24 hours per day = 1,008 WH per day
• 1,008 WH per day / 12 volts = 84 Amp*Hours per day

And if you assume that you use 25% of battery bank AH capacity per day and 50% of capacity:

• 1,260 AH * 12 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max battery discharge = 3,213 WH per day "average" daily load suggested

And if your inverter consumes 1,008 WH per day if run 24 hours per day (your usage may be different):

• 3,213 WH daily load from bank - 1,008 WH tare losses = 2,205 WH per average day (2 days storage, 50% max discharge)

Almost 1/3 of your daily harvest would be used to run the 4 kWatt inverter 24 hours per day.

Running your inverter anywhere near 2,000 Watts would use your daily load in ~1 hour (or ~2 hours if you run one day, and take a couple days to recharge, or only run the inverter when the sun is up, etc.).

Anyway... Lots of issues and questions here. Your actual power needs are probably different than my (relatively) worst case usage assumptions.

Your questions/corrections to my stuff above?

-Bill

Generally, the recommended rate of charge for a flooded cell lead acid battery bank is 5% minimum (standby, weekend, summer system usage). And 10%-13%-20% for full time off grid. The math would look something like this:

• 1,290 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,215 Watt array minimum
• 1,290 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,429 Watt array nominal
• 1,290 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 3,158 Watt array "typical cost effective maximum"
• 1,290 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.20 rate of charge = 4,858 Watt array "more or less max usable"

In general, with a fixed solar array--Discharging an FLA battery bank to 75% state of charge, will be rechargable to ~100% state of charge in one day (with enough sun).

So, the amount of power drawn from battery bank:

• 1,290 AH * 12 volts * 0.85 AC inverter * 0.25 discharge = 3,290 Watt*Hours (AC inverter 120 VAC output from battery bank)

Then fixed array facing south, and a random New Mexico city:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Roswell
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 57° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.80	5.28	6.32	6.57	6.43	6.12
Jul	Aug	Sep	Oct	Nov	Dec
5.92	5.74	5.82	5.74	5.14	4.54

This is a "good amount" of sun during winter... December 4.45 Hours:

• 3,290 WH per day * 1/0.52 off grid AC system eff * 1/4.45 Hours of sun per day (Dec Average) = 1,422 Watt array December "Break Even"

Note that solar power is, of course, variable.... For example, the 3,290 Watt array harvest in December... I would suggest that your "base load usage" (loads you must run like Lights, some water pumping, computer, cell charger, etc.) should be around 50-65% of "predicted solar harvest". And if you have a sunny/clear day (week ahead), then use your optional loads too (washer, irrigation watering, etc.).

And, the reason for 25% battery usage--There are usually enough hours of sun in a day to recharge the FLA battery bank. For 50% and deeper discharge, your battery charging time (something like 2-5 hours "bulk charging" and ~4-6 hours of "absorb charging"--The sun is not on the Array enough hours in a day (sunrise to sunset). Generally, a deeply discharged FLA battery is going to need at least 2 days to fully recharge.

-Bill