# vprinc:Questions about my off grid solar power system and battery options

This discussion was created from comments split from: Matching Battery Bank Size to Panels.

• Posts: 5Registered Users ✭✭
All, this thread has been very useful. I think I have my math correct but want to double check.
Planned consumption = 8 to 10 kws per day when system is used (about 25% of the year)
Panels = 24 ja 320w ~37 volt and 8.5 amps per panel configured in 6 strings of 4
charge controllers = 2 midnite 200 classics.
Battery 1100 ahrs @ 48v.

Based on the above system my array should produce (24*320)/2*4=~15kw per day so that should cover usage.
Battery 1100*48=52800 whrs. *.2 (20% DOD) = ~10.5kw which is at my high end of usage.

So my question to the forum. Is my math right?  Appreciate any and all feedback.
• Posts: 2,838Registered Users ✭✭✭✭✭
The math looks about right, but it may depend some on location and which 25% of the year. I get more than enough production in spring and summer, but not enough in fall and winter. It also depends on whether you can load-shift things like water pumping so they get done when the sun is still shining after batteries are charged.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Posts: 5Registered Users ✭✭
Sorry, should have added location. Vernon AZ. Plenty of sun even in winter.
Vprinc,

Fee free to start your own thread--If you want futher discussions. It keeps your Q&A in "your thread"... Anyway, per Estragon, the hours of sun per day by month gives you a pretty good way to estimate how much your system will generate (or how large of solar array you need).

For example, a fixed array in Phoenix AZ:

### PhoenixAverage 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 4.92 5.55 6.45 6.83 6.74 6.57 Jul Aug Sep Oct Nov Dec 6.08 5.78 6.03 5.82 5.24 4.74
If you are in Phoenix, and you have very good sun, the array set for "average" best year round production... Worst case is December:
• 10,000 WH per day * 1/0.52 off grid system performance * 1/4.74 hours of sun (Dec) = 4,057 Watt array "break even" December

Or, 24 panels * 320 Watts per panel (=7,680 watt array):

• 24 panels * 320 Watts * 0.52 off grid system eff * 4.74 Dec hours of sun = 18,930 WH per day (Dec average) ~ 18.9 kWH per day (Dec)

Your battery bank, if 2 days of storage (no sun) and 50% discharge:

• 1,110 AH * 48 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max typical discharge = 11,220 WH of storage (2 "days" with 50% max discharge)

To recharge your battery bank, rule of thumb recommendation is ~5% to 13% rate of solar charging... 5% minimum (weekend/sunny seasonal usage) to 10%+ for full time off grid:

• 1,110 AH * 59 volts charging * 1/0.77 panel+controller losses * 0.05 rate of charge = 4,214 Watt array minimum
• 1,110 AH * 59 volts charging * 1/0.77 panel+controller losses * 0.10 rate of charge = 8,429 Watt array nominal
• 1,110 AH * 59 volts charging * 1/0.77 panel+controller losses * 0.13 rate of charge = 10,957 Watt array "cost effective" maximum
Arizona has lots of sun--So your minimum charging current array is pretty near your "worst case" month of sun and loads...

And for a 1,110 AH @ 48 volt battery bank, the "recommended" maximum "useful" AC inverter size (for a flooded cell lead acid battery bank) would be around 5,500 to 11,000 Watts (more than that, is a substantial drain on the that size battery bank (basically 1kW per 100 AH of storage for a 48 volt battery bank).

The above is just a generic start about your system. At least with first guesstimates--Your system seems pretty well configured for full time off grid usage (possibly no genset needed) as you have configured.

Monitoring your battery bank with a hydrometer (logging temperature corrected specific gravity for each cell, once a month or so).

Keeping electrical connections clean and torqued.

Get a DC Current Clamp DMM (like this one from Sears, "good enough" for our needs). You can use it, for example, to measure the DC current for each string in you array and make sure each is carrying its fair share of the current (no bad connections, failed panels, etc.).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 5Registered Users ✭✭
Thank you so much Bill. You rock. I am debating forklift batteries vs NIFe. Does anyone have any experience with these 2 battery platforms. I have seen some threads but they seemed to be a bit older (2016).  Was looking for any current information.
vprinc,

What is it you want from your battery bank? Low cost, reliability, easy maintenance, long life, or all of it (not going to happen).

Well maintained Fork Lift Batteries seem to be a nice solution with >15 year life if well taken care of (kept clean and water, not discharged to dead, not sitting around with less than 75% discharged, etc.).

NiFe batteries--Seem to be very rugged, but need lots of "extra" solar power (~65% efficient batteries vs Flooded Cell Lead Acid batteries that are >~80% efficient). Plus every few years you have to dump and refill with new electrolyte (absorbs CO2 from air)--Not an easy job.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
I should also add that NiFe have a very wide range between loaded/near discharged and under charge (your battery bus voltage is very wide and, depending on how you configure your bank, charging to 64-67 VDC--That can knock many/most AC inverters "off line".

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 5Registered Users ✭✭
Thanks Bill. One last question. Not sure where to install lightning arrestors. I have seen and read things that shows it on the DC (typically at the charge controller) and AC. Any thoughts?
In general, the minimum is to install the arrestors where the solar power cables enter you home, and typically at the main AC panel (which is hopefully on the outside wall of your home. Both need a short/heavy (6 AWG minimum?) wire from arrestor to ground rod/cold water pipe...

Some folks also install arrestors on the DC output of the solar array (to help protect the solar panels too).

Lightning control is not a simple job. There are lots of issues to be aware of. Here are some links you can follow:

Off Grid Grounding Technique?
Another Question, this time about Lightning
Lightning Protection for PV Systems
Towards the end of this thread is a very nice discussion of proper generator grounding.
How Lightning Works
-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 1,026Solar Expert ✭✭✭✭
vprinc said:

Panels = 24 ja 320w ~37 volt and 8.5 amps per panel configured in 6 strings of 4
charge controllers = 2 midnite 200 classics.

So my question to the forum. Is my math right?  Appreciate any and all feedback.
Have you ran your proposed system through the Midnite string sizeing tool?  Your 72 cell panels appear to have a Voc of 46.22 volts. If your location gets cold on winter mornings you would likely be pushing into Hyper Voc. range daily. The sizing tool will be a good way to verify your configuration is workable.

2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

• Posts: 5Registered Users ✭✭
Hey Littleharbor2. I called them because the calc was so close. They said the 200 would be fine but to up the wires coming off the panels to the combiner to 8awg to help prevent the charge controllers from going into HyperVoc mode. The Midnite guys were awesome so going forward with their recommendation.
• Posts: 7,900Solar Expert ✭✭✭✭
vprinc said:
.....They said the 200 would be fine but to up the wires coming off the panels to the combiner to 8awg to help prevent the charge controllers from going into HyperVoc mode. The Midnite guys were awesome so going forward with their recommendation.
What principal are they thinking of, that increasing the Wire gauge, reduces the zero current voltage at the Charge Controller input terminals ?
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,