Off-Grid Design... newB first draft :)

n0eggs

Hello Solar World!

Welcome to my first attempt at putting together a system! All comments welcome - I'm ready to take my lumps!

BB.

Welcome to the forum.

Where (roughly) will the system be installed?

How much energy per day Watt*Hours per day by season?

Emergency backup power? Weekend system, full time off grid home?

First warning, you need to put 2 to 3 panels on series for Vmp-array in the 40 to 100 VDC range. Your panels are probably in the Vmp 30 volt range. To low for charging a 24 volt bank (need Vmp 35+ volts for Vmp-array)

I am not on a computer right now, but your wiring is probably too small of awg for the batteries and inverter.

Bill

n0eggs

THANKS Bill!

I'll be on the coast of southern Baja California Sur in Mexico, expecting full-time off-grid usage, but with very minimal draw needs (no hair driers, or other heavy type appliances - ac, microwave, etc...).

I also plan on putting in a second pure 24v system with no inverter to power a water pump and separate fridge/freezer units.

You are correct on the panels, as they are rated for 32.25V - so, you're saying that rather than having six parallel panels, I should maybe consider wiring them in three parallel groups of two in series?

I got the inverer wiring specs direct from Cotek which list a 4 gauge as long as the length is kept under 6ft - benefits in bumping it up a size or two?

Estragon

Need more info on the panels. What is a "Renogy 31 20watt 24v" panel (Isc, Voc, Vmp)?

The battery bank looks like it's proposed ~700amp-hour at 24v? If so, it can likely take 100a or more charging. A decent 3500w generator can handle that (though a power factor corrected AC charger would help), but a 30a charger will take a long time and the generator wouldn't be running very efficiently unless running other loads while charging.

I think Bill is right saying inverter and battery wiring is light. For the inverter, controller, and charger, breaker and wire sizes should be given in maker manuals. I'd use recommended breaker sizes, and upsize wire to the biggest that physically fits. Battery interconnects should be in the 2/0 - 4/0 range, and as short as practical IMHO. You're essentially constructing a 700ah 24v battery, which is a potentially huge current source, so you want to minimize the resistance in wiring and connections.

The wire from panel combiner to controller may be heavier than needed. Making some assumptions about panels (~250ish watt 37ish Voc) putting them in two strings of three would be [email protected] - ish. You'd need to use actual numbers and distance with a voltage drop calculator, but likely a 6ga or so would work. The 0000ga as shown in the diagram is heavy, expensive stuff. With two strings, pv breakers and combiner box would be optional.

Having separate inverter and charger can work (my 12v system does), but having a combined inverter/charger can be simpler (eg built in AC transfer switch), and sometimes better value (eg more flexibility in charging, PF corrected, etc).

It all starts with loads and needs though. Answers to Bill's questions would help us.

BB.

OK, I think we can help here.... If your Vmp is less than 33 volts per panel (check with the Outback string sizing tool--Warmer climates, the Vmp-cold does not go very high). I have not used Outback's (they may require you to install a Java app/tool to run the software)... Basically, Hot panels, Vmp-array-hot drops voltage (you should really be >~40+ volts at the charge controller minimum for 24 volt battery bank) and less than Vmp-array-cold~140 VDC maximum (sub freezing panels can get 10-20% higher voltage than "hot panels").

http://www.outbackpower.com/downloads/string_sizing_tool/string_tool_instruc.pdf

Say you can put three panels in series (Vmp-array ~ 96 VDC). That gives you Imp-array of (guessing: 320 watts / 32.25 volts = Imp~9.48 amps, or 2xImp-array~19.75 amps). The minimum AWG cable I would suggest using NEC table and a 1.25x derating:
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

• 19.7 amps Imp * 1.25 NEC derating = 24.6 amps or ~ 25 amps minimum branch circuit rating
• NEC table (depends on Copper or Aluminum, ambient temperatures, insulation type but ~10 AWG wire minimum (read note at bottom of chart--12 AWG is rated for 20 amps minimum for some reason, not 25 amps).

And then based on voltage drop... Typically 3% to 1% voltage drop for your array is "cost effective". 75 feet one way run (some calculators use round trip wire length), 96 volts nominal, 19.7 amps nominal:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=96&phase=dc&noofconductor=1&distance=75&distanceunit=feet&amperes=19.7&x=65&y=10

10 AWG:
Voltage drop: 2.95
Voltage drop percentage: 3.07%
Voltage at the end: 93.05

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=1.296&voltage=96&phase=dc&noofconductor=1&distance=75&distanceunit=feet&amperes=19.7&x=70&y=17

6 AWG:
Voltage drop: 1.17
Voltage drop percentage: 1.22%
Voltage at the end: 94.83

So--For longer distances, you should really have higher Vmp-array voltage, and 3x panels in series here would reduce your copper wiring to much more practical size ($$$) and since you only have 2 parallel strings, you really do not need the combiner (for debugging and servicing, you may want the combiner or get an AC/DC Current Clamp DMM (Digital Muli-Meter for $60-$120 or so. Very easy to measure DC current):

http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P (good enough for our needs)

I will continue on in a little bit--Some other things to take care of now (famous warning--Do not purchase any hardware until you have a workable paper schematic--There are a lot of questions still to resolve).

-Bill

BB.

A 2,000 Watt @ 24 volt inverter should have a branch circuit and breaker rated for:

• 2,000 Watts * 1/0.85 inverter eff * 1/21 volts battery cutoff voltage * 1.25 NEC derating = 140 Amp minimum

Your 225 Amp breaker is OK (48 VDC Rated?).

Using a basic NEC table:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

And you need somewhere around 1 to 2/0 cable based on 150 Amp fuse/current draw.

If you are really going to use a 225 Amp breaker, 4/0 is the minimum suggested wire size. You can go smaller (NEC is pretty conservative)--But it is not bad to follow NEC for a safe and reliable system.

If you where going to go with 4 AWG cable, 140 amp draw, with a 1.0 voltage drop:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=24&phase=dc&noofconductor=1&distance=15&distanceunit=feet&amperes=140&x=57&y=17

15 feet total (one way run) from battery bank to AC inverter of 4 AWG wiring

Voltage drop: 1.04
Voltage drop percentage: 4.35%
Voltage at the end: 22.96

Your battery bank of 4x 6 volt @ 428 AH (20 hour rate) * 2 parallel strings = 24 volts @ 856 AH battery bank.

That is a pretty "hefty" size bank... At around 800 AH, I like to suggest looking at the next higher voltage (48 volt battery bank in your case).

Your bank at 10% rate of charge would be 85.6 amps nominal recommended for good battery life. Your solar array (5% minimum, 10% nominal, 13% typical solar cost effective maximum) sized for this bank, would be around:

• 29.0 volts charging * 856 AH bank * 1/0.77 panel+controller losses * 0.05 rate of charge = 1,612 Watt array minimum
  
• 29.0 volts charging * 856 AH bank * 1/0.77 panel+controller losses * 0.10 rate of charge = 3,224 Watt array nominal
  
• 29.0 volts charging * 856 AH bank * 1/0.77 panel+controller losses * 0.13 rate of charge = 4,191 Watt array "cost effective maxim

Note that the Outback is limited to 80 amps charging... And your battery at 10% rate of charge is ~86 amp charging. Your are at the limit for a single controller. if you want more solar, 2x controllers would be needed. Or, you change to a 48 volt battery bank (P=V*I -- double bank voltage, double maximum solar array wattage).

Also, for your system... I like to use the NEC derating of 1.25x for wiring and breaker (80 amps * 1.25 = 100 amps branch circuit and breaker ratings)--Note a few companies have different rating systems. The typical 80 amp breaker will not trip at 80% of rating and will trip (minutes to hours) at 100% of rating. Outback and Midnight have some slightly different breaker ratings (for some of their breakers).

And sizing the AC inverter... Typically for a 24 volt battery bank, the maximum suggested inverter would be 500 Watts per 100 AH, or ~4.28 kWatt for your AC inverter maximum (2,000 watt inverter is well under that maximum rating).

If you do not use much energy, perhaps you can use just a 4x battery bank (single string). That would save on the solar array and cost of batteries.... Large battery banks do not help a lot--Especially in sunny regions.

On full time off grid system, we suggest 2 days of storage and 50% maximum discharge... Your battery bank would support (nominally):

• 856 AH * 24 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 4,366 Watt*Hours per day storage/no sun usage

I did not find any "hours of sun" listings for Baja California--But if we assume 5 hours minimum as a good number for a dry southern climate. A 10% solar panel off grid system would supply (and here is a link that you may find a city near with similar weather):

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

• 3,224 Watt array * 0.52 off grid system eff * 5.0 hours of sun minimum = 8,382 Watt*Hours per day (probably at least 9+ months of the
  

Anyway--A start.

-Bill

n0eggs

Really appreciate the feedback Bill & Estragon

A few of my original considerations:

1. I was leaning toward over-sizing the battery bank to establish more than enough reserve, keep discharge rates low and increase battery life... defaulting to generator charging if/when the array fell behind

2. Rough guesstimate of <5000w per day usage

3. I would need to match panel voltage with bank voltage
             
New ideas based on comments:

- Change panel configuration to improve charging, lessen need for combiner box, expensive/heavy wiring and array size
- Possibly go with a single string of smaller AH, higher grade batteries (Rolls S-500EX as an example) and downgrade inverter to 1500
- Bump up AWG for bank, inverter, charger and controller

Need to fire up my windows laptop to run the outback tool... I'm on a chromebox 

I'm certainly not tied to the Renogy panels (they seemed to be a decent product with a competitive price per watt) but here's some additional specs on them:

Maximum Power at STC: 300W	Maximum System Voltage: 1000VDC
Optimum Operating Voltage (Vmp): 32.25V	Open Circuit Voltage (Voc): 39.82V
Optimum Operating Current (Imp): 9.33A	Short-Circuit Current (Isc): 9.78A

BB.

The 2 days of storage/50% max discharge (4x daily no-sun load) seems to be pretty optimum unless you need something special (3 days or so of no-sun storage, occasional deeper discharge, high starting current load).

If you average loads are smaller, use a smaller AC inverter (less tare losses, cheaper hardware).

If you need more power (a couple weekends during summer of power tools/etc.), then, many times a cheaper (or your backup) genset and a few gallons of fuel is the noise and smoke.

You can use other brand of string sizing tools. For example, use Midnite's tool. Set panel information and minimum (and maximum) local temperatures, and it will give you Vmp-array-hot and Vmp-array-cold for different panel configurations--That is really all you need.

http://www.midnitesolar.com/sizingTool/index.php

-Bill

Estragon

At 5kw/day budget usage, 2days reserve for no sun, and keeping discharge at 50%, you'd want 5kw x 2 days ÷ 50% = 20kw storage. Your original s550 at ~425ah x 48v = 20400, just about right. S550ex at ~350ah x 48v = 16800, a bit low but may work okay.

In real world use in most climates, it's pretty rare to get little to no pv charge for two days. At higher latitudes, short winter days make it more likely - in Baja probably less so. Also, some of your 5kw budget will likely use "surplus" pv power available when the batteries are full or nearly so.

All that said, for a first bank, I'd be inclined to go for the cheaper $/ah bank, and would definitely suggest a single 48v string instead of 2x24v parallel strings.

n0eggs

Just doing some searching for separate battery chargers to handle a 48v bank - doesn't look like there are many options out there, mostly finding charger/inverter combos in the 4000w neighborhood... maybe stick with a 24v bank?

Estragon

Is there a reason you prefer separate inverter and charger?

mike95490

Estragon said:

.....In real world use in most climates, it's pretty rare to get little to no pv charge for two days.......

Well, there is northern california,  many times in winter, 4-10 days, heavy cloud cover and really gloomy days, I get 300-400 watt hours total harvest from 5Kw of PV.    Time to get another jug of diesel

mike95490

n0eggs said:

Just doing some searching for separate battery chargers to handle a 48v bank - doesn't look like there are many options out there, mostly finding charger/inverter combos in the 4000w neighborhood... maybe stick with a 24v bank?

When your loads become more than 2kw, the wiring problems with heavy enough wire to keep voltage loss low enough for the inverter to not crash on you, start becoming tough.  And the 2nd charge controller, because 1 is not enough at the lower voltages.
 IOTA makes a decent one, and then you can look for golf cart chargers that run off 240V.   forklift chargers need 3 phase or 480V generally.

Estragon

> @mike95490 said:
> Estragon said:
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> .....In real world use in most climates, it's pretty rare to get little to no pv charge for two days.......
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> Well, there is northern california,  many times in winter, 4-10 days, heavy cloud cover and really gloomy days, I get 300-400 watt hours total harvest from 5Kw of PV.    Time to get another jug of diesel

Yup - usually get that in late fall/early winter here. Short days plus thick clouds = not much production. This year it went on for weeks :-(

n0eggs

Really appreciate everyone's comments. Here is my attempt at a revision: