Your consultant appears to have a good understanding of the equipment, but could relay it
differently. I would suggest Watt hours as a metric instead. Watt hours are the measure of total power in the battery bank. It's necessary to use this metric when looking at battery banks of different voltages. That's why his/her discussion in your highlighted red section doesn't seem to make sense.
Translating what your consultant said:
For a single string 1,557 AH Bank @100 Hr. rate, you will need 3 of the 4KS21P (4-KS21P) for a 12 Volt system, 6 of the 4KS21P (4-KS-21P) for a 24 Volt system and 12 of the 4KS21P (4-KS-21P) for a 48 volt system.
into watt hours, you get the following:
For a 12V string, you'll need 3 of the 4KS21P (4V X 3 batteries = 12V) and you'll get a capacity of 19,618 Watt hours based on the 100hr rate. For a 24V string, you'll need six of these batteries (4V X 6 batteries = 24V) and you'll get a capacity of 39,236 Watt hours at the 100hr rate. For a 48V string, you'll need twelve of these batteries and you'll get a capacity of 78,472 Watt hours (100hr rate).
Do you see the difference when looking at Watt hours? You can now see total capacity of the battery bank. Your devices use watts... that's a measure of power consumption. It's the measure your consultant should be using. Based on this information, we can effectively size the battery bank. But I wish to look at this differently because the calculations cannot be accurately made using an assumed 100 hour rate. Draw down rates significantly influence total battery capacity, and the above does not allow for this.
We previously calculated your daily consumption to be about 11,500 watt hours (post from 6/27/2016 12:32:27 PM). This translates into an average consumption (11,500 watt hours/24 hours) of 480 watts . That's what we use to determine the battery bank required.
We can now see what the different battery banks can supply.
The 12V pack (3 batteries in series) can't be measured at its 100 hour draw down capacity, because 19,618 watt hours [rated 1557 AH (100 hr rate) X 12.6 volts] /100 hrs is only 196.18 watts. You'll average 480 watts.
The 12V pack (3 batteries in series) can't be measured at its 72 hour draw down capacity, because 18,497 watt hours [rated 1468 AH (72 hr rate) X 12.6 volts] /72 hrs is only 256.90 watts. It's still far less than the required 480 watts.
The 12V pack (3 batteries in series) can be measured at its 20 hour draw down capacity, because 13,910 watt hours [rated 1104 AH (20 hr rate) X 12.6 volts] / 20 hrs is 695 watts. This is sufficient to supply your average draw of 480 watts.
The 24V pack (6 batteries in series) can't be measured at its 100 hour draw down capacity, because 39,236 watt hours [rated 1557 AH (100 hr rate) X 25.2 volts] /100 hrs is only 392.36 watts. You'll average 480 watts.
The 24V pack (6 batteries in series) can be measured at its 72 hour draw down capacity, because 36,994 watt hours [rated 1468 AH (72 hr rate) X 25.2 volts] /72 hrs is 513.80 watts. It can supply the required 480 watts.
The 48V pack (12 batteries in series) can be measured at its 100 hour draw down capacity, because 78,472 watt hours [rated 1557 AH (100 hr rate) X 50.4 volts] /100 hrs is 784.72 watts. The easily meets the required 480 watts.
We now know what capacities to plan on for each battery bank size (in bold above). We now halve total capacity to get usable capacity, because only 50% of the battery can be drawn down before excessive wear becomes a concern.
We can rely on the 12V (3 cell) battery bank to supply 13,910 watt hours/2 = 6,955 watt hours of usable energy to your inverter. You use an average of 480 watts, so you can expect this to power your house for 6,955 watt hours / 480 watts = 14.5 hours. Allowing for losses in the inverter and power transmission lines of 15%, your real world outcome would be about 14.5 hours X 85% = 12.3 hours. The 12V option is too small. It isn't even enough power to run your house for one day.
We can rely on the 24V (6 cell) battery bank to supply 36,994 watt hours/2 = 18,497 watt hours of usable energy to your inverter. You use an average of 480 watts, so you can expect this to power your house for 18,497 watt hours / 480 watts = 38.5 hours. Allowing for losses in the inverter and power transmission lines of 15%, your real world outcome would be about 38.5 hours X 85% = 32.7 hours. The 24V option is exceedingly trim. It will run your house for about a day and a third. This would mean trouble every time storms blow through, and during cloudy periods when you get little solar to recharge your batteries.
We can rely on the 48V (12 cell) battery bank to supply 78,472 watt hours/2 = 39,236 watt hours of usable energy to your inverter. You use an average of 480 watts, so you can expect this to power your house for 39,236 watt hours / 480 watts = 81.75 hours. Allowing for losses in the inverter and power transmission lines of 15%, your real world outcome would be about 81.75 hours X 85% = 69.5 hours. The 48V option is enough to run your house for nearly three days.
Now the actual decision on what's acceptable falls to you. The batteries should be able to supply power for the longest period you feel solar energy will be limited. For example, you might feel two days of cloud cover are the most you'll see for inclement weather; if so, the 48V 12 cell option is sufficient.
You do have the option to charge your system from other energy sources, so you don't have to consider the absolute worst case scenario when it comes to cloud cover. There's always the possibility of charging your batteries from a generator for example; a lot of
people size their systems for typical weather and then use a generator for top-offs when weather is uncommonly poor. You wouldn't want to undersize and use this strategy, but for occasional unexpected poor weather, it's a good backstop.
I must also point out that there is nearly no difference between one string of 12 cells (48V) or 2 strings of six cells (24V) when it comes to the power they'll supply you. If you still intend to use the 24V inverter, simply run two strings for your battery bank.
I should also add... it's my pleasure to help you. Do not feel guilty about asking.