New Configuration

Re: New Configuration

Welcome to the forum Brian.

For an AGM battery (and any lead acid battery)--Spending lots of time below ~75% state of charge will sulfate the battery and cause it to loose capacity (and eventually not store enough energy for your needs). Spending even a few days below 75% state of charge is not a good thing.

For a "nice" balanced offgrid system--We like to start with a 5% to 13% of rated battery bank capacity with a 0.77 derating of the solar panels+charge controller... For your bank:

• 900 AH * 14.5 volts charging * 1/0.77 derating * 0.05 charge = 847 watts minimum
• 900 AH * 14.5 volts charging * 1/0.77 derating * 0.13 charge =2,203 watts cost effective maximum

Note: You have a very large battery bank and, ideally, you want a pretty large solar array to "keep it happy".

If, all you wanted to do was keep the bank floating (properly charged while stored), you could go down to 1% rate of charge with solar... This would be, for example, a weekend cabin that is only used during a couple seasons with a generator to recharge during the day, and use the batteries at night. The solar array would be just to keep the batteries floating/charged during non-use. (or could be for emergency backup power system+genset too).

MPPT controllers--If you are in sub freezing weather, you might gain 10-15% extra power from an MPPT controller over a PWM controller. But, this is usually not the best reason for purchasing a MPPT controller.

MPPT controllers are great for running the solar array Vmp>>Vbatt-charging... This allows you to put the array farther from the battery bank and keep the wire gauge smaller. Running a large array with Vmp=17volts with >10' of wire can mean you are in the 8-2 awg, or even larger, guage wire from the solar array.

Before you buy any more hardware, we need to define your needs and system requirements first... Then pick the supporting hardware.

So--do you have any daily usage figures (Amp*Hours or Watt*Hours of load per day?). How much sun do you get (where is the installation)?

For example, assuming you get at least 4 hours a day of "full noon time equivalent sun" (sort of an OK estimate for 9 months of the year for many locations)... We could guess that with 850 watts of panels you could use a maximum of:

• 850 watts * 0.52 derating * 4 hours of sun per day = 1,768 WH per day
• 1,768 WH / 12 volt battery bank = 147 AH per day (or a bit more)

-Bill

Re: New Configuration

You are very welcome Brian.

The k-a-w meter also has kWH logging... It would read something like 1.1 kWH over the 5 hour period (assuming 5hours * 220 watts = 1,100 WH = 1.1 kWH).

For Example:

• 220 watts * 5 hours * 1/0.52 derating * 1/4 hours of sun = 529 watts minimum of solar panels

Of course, you should never plan on using 100% of your usable power every day... But that gives you a very rough idea of what you are looking at.

-Bill