Two Conext 6848s / Lithium Bank Sizing

Really need to know your loads (Watt*Hours per day, any seasonal variations, etc.).

Also, what are your battery needs... With Lead Acid, I suggest that 2 days of storage to 50% planned discharge (i.e., 4x daily loads) is a good place to start...

When you look at Lithium (LiFePO4 or similar), they have better surge capability (i.e., FLA batteries, 100 AH @ 48 volts per 1,000 Watt inverter capability or 2x6.8kW = 1,360 AH @ 48 volt FLA batteries for your setup--Assuming you want to use near 13.6 kWatts and have surge currents up to 2x 13.6 kWatt).

With FLA batteries--They take time to charge and have limited charging current acceptance (especially during Absorb charge cycle)--And 4x daily load for full time off grid is a good place to start.

With Lithium, they charge much faster. Basically just bulk->float, little to no absorb charge time--Saves 2-6 hours of "on charge" during daylight.

So--That gets back to what is the charging range you are looking at (for FLA, 50%-100%; for Lithium around 20%-90% range of charge), and how many days of no-sun do you want (1,2,3 etc.).

Lead acid batteries being "relatively" cheap (compared to Lithium)--Making a larger bank (2 days/50% discharge) gives 2-3 days of no-sun, and enough capacity for surge current, and keeping up with daily usage in good sun (less genset usage).

With Lithium, you could can run with 1 day storage and 70% capacity usage--And recover just using solar the next day (large array, relativity few hours per day "on charge"). And always crank up the genset the next day if needed...

Just to get some numbers... Using FLA as starting point:

• 1,360 AH * 48 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max planned discharge = 13,872 WH per day
• 1,360 AH * 59 volts * 1/0.77 panel+controller deratings * 0.10 nominal rate of charge = 10,421 Watt array "nominal" (10% rate of charge)

Guessing 
http://www.solarelectricityhandbook.com/solar-irradiance.html

Orlando Florida
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 62° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.29	4.76	5.36	5.79	5.75	4.99
Jul	Aug	Sep	Oct	Nov	Dec
4.96	4.87	4.74	4.89	4.55	4.16

The "break even" array for December would be:

• 13,872 WH per day * 1/0.52 off grid AC system eff FLA * 1/4.16 hours of sun per day (Dec) = 6,412 Watt array Dec Break Even

In "real life", you have "base loads" (loads that need to run every day, such as lights, refrigerator, work computer, etc.), and you have "optional loads" (washing machine, A/C, irrigation, etc.) that you can run on sunny days and/or with a genset. The base loads, you should count on ~50% to ~65% of your predicted daily harvest for running base loads...

Anyway, 10,421 Watt array seems to be a good starting point... So, how much charging current can your Li Ion bank absorb... Such an array will produce a maximum average current (cool/clear days, not many hours a year) of:

• 10,421 Watt array * 0.77 panel+controller derating * 1/45 volts nominal minimum bank voltage = 178 Amps

So, with the above array, the Li Ion bank should take a minimum of 178 Amps charging current (check battery specifications).

My personal bug-a-boo with Lithium is if they ever catch fire, such a fire is very difficult to put out, and can release some highly toxic chemicals (Hydrofluoric acid). And I would highly suggest that a Lithium battery bank be installed in its own outbuilding--Away from people, home, possessions.

The above is just points for starting the discussion... I have made a great many guesses and SWAGS (scientific wild ass guesses) to get some back of the envelope math working.

Once you know your daily loads--It is pretty much just pressing buttons on the calculator to work out the sizes of everything. The bigger unknowns are your "safety factors"--I.e., days without sun, what if genset fails, what about a week of stormy weather, etc.).

Your 2kWatt genset is probably "too small" as a backup genset for an FLA battery bank (5% to 10% minimum rate of charge needed). But for Lithium Ion which do not have a "minimum rate of charge" specification... You are just looking at time on charge:

• Say 1,000 Watts charging current (run genset at less than 1,600 Watts to keep from overloading):
• 1,000 Watts * 1/45 Volts charging * 0.90 charger eff = 18 Amps @ 48 volt bank charging

1,000 Watts charging for a ~13,872 WH per day load => over 14 hours "on generator charge" per nominal day's usage.

It works--But you may want a genset that is capable of running your daily loads (A/C, pumping, +household loads).... But this is just one of the "fudge factors" you need to think about (running a 2kWatt Honda inverter generator for 5 days a year, and ~1,600 Watts to run a fridge/lights/laptop computer does work)--Hurricane season--Ain't that bad--And a 13 kWatt genset would probably be way overkill).

-Bill

Midnite SPDs seem to be a very good choice... The older (?) Delta and such are just a simple spark gap filled with sand. Midnite use Metal Oxide Variable Resistors (much more accurate set points):

https://www.solar-electric.com/search/?q=midnite+spd (not cheap)

This is Midnite's website for one of the models... You can review their documents and video demonstrations:

http://www.midnitesolar.com/productPhoto.php?product_ID=283&productCatName=Surge Protection Devices&productCat_ID=23&sortOrder=1&act=p

Midnite has a nice video of testing their SPDs vs Delta:

https://www.youtube.com/watch?v=vkeFYuHxJoU

Dave certainly has a lot of experience with off grid power installs (vs me--No direct experience)... In decades past, our (now retired) forum founder from NAWS--His experience over the years was the output stages of AC inverters were the most likely to be killed with nearby lighting strikes.

It is possible that the newer hardware from Schneider and others is better built/protected... but given that 120/240 VAC circuits are usually all over the home, and frequently to outbuildings/wells/etc., i.e., a big antenna, I would not not try to save money on avoiding SPDs on AC output for inverter(s) in lighting prone areas.

-Bill