Off Grid Solar design

Welcome to the forum gregapex,

To answer your questions... Yes, AC inverters can be upwards of 90-95% efficient--But that is in their "sweet spot" of power draw. If you draw less power (i.e., a 3,000 Watt inverter supplying a few hundred Watts, or a 3,000 Watt inverter supplying 3,000 Watts, the efficiency is usually less, and can be much less). An example of a typical inverter (with a handy graph):
https://www.solar-electric.com/lib/wind-sun/SureSine.pdf

I use 85% for a generic installation where your power usage is not controlled (i.e., sometimes heavy loads, sometimes light loads).

For example a 3,000 Watt inverter may draw around 20-40 Watts just "turned on". Those "tare losses" can be significant in a small system (say you have a refrigerator. You have the inverter running 24x7, but the fridge itself draws 120 Watts for 15 minutes and 0 watts for 15 minutes--A 50% duty cycle).

Regarding battery AH calculations--Typically we start with 2 days of storage and 50% planned discharge as a good mix. 3 days can be done, but as you see, it forces a significantly larger battery bank and larger solar array if you are trying for 10-13% rate of charge.

Since you are trying for "golf cart" batteries for your first bank--Which is a good idea for your "training bank"--The batteries will last around 3-5 years with good maintenance--And they are "cheap enough" that if you "murder your first bank" (not unusual for new users--Over discharging and/or under charging is a typical way battery banks are damaged/ruined), it is a good method to learn if your system plans are a good match for your energy needs and local conditions (sun, weather, etc.).

I am not a solar installer--But where / how does California have a 3 day storage requirement for sizing a battery bank?

Note that a 2 day + 50% maximum "planned" discharge does let you, on occasion, use 3 days of storage (i.e., 100%, 3x days at 25% per day, to 25% state of charge). It is not great to take a lead acid bank down that far (suggest not going below 20% SoC)--But if you charge as soon as you can (next sunny day, fire up genset), it does work. I.e., a battery bank that is sized 4x daily loads vs 6x daily loads (2d/.5 vs 3d/0.5 discharge).

For a lead acid battery bank, the cycling life seems to be pretty much based on Amp*Hours of cycling... I.e., a battery rated for 200 cycles at 20% state of discharge will last 400 cycles at 40% SoD, and 800 cycles at 80% SoD. Or a bank 2x larger will last ~2x longer. Money wise, it is a wash.

And taking a lead acid battery down below 20% SoC, the cells themselves are not "perfectly balanced"--Not all cells are at 20% SoC, some may be a 25% SoC and others at 10% SoC (equalization charging should bring all cells to 100% SoC--But EQ charging is hard on a Flooded Cell Lead Acid battery bank too--So you do not do it often--Typically once a month or every few months as needed--Checking electrolyte specific gravity and when cells differ by ~0.030 SG units more more, typically time for an EQ Charge).

For most rechargeable chemistry batteries--Taking them to zero and "reverse charging" a cell (taking it "dead" or zero SoC, then continued discharge actually "reverses cell voltage") is generally a cell/battery killer.

Do remember too that a typical battery rating may be "end of life" when battery capacity falls to 80% of new battery capacity. But I have seen some Lithium Ion systems rate battery life to 50% of capacity--Watch for the Marking Number Games. A battery at 50% capacity due to cycle/aging life (or possible mfg fault)--May not be covered by warranty if the company calls >50% capacity still "good". Part of the deratings/over sizing of solar array allows the system to still meet your energy needs towards the end of battery bank service life.

The formulas for calculating the size of the solar array are based on The average battery bank charging voltage with a derating for Warm/Hot solar panel voltage. Vmp (voltage maximum power) falls as the panels get hot. P=V*I and as V falls, P falls too.

The 77% derating assumes Vmp falls to ~81% of Vmp-standard-test-conditions and 5% charge controller efficiency. 0.81*0.95=0.77 (or 77%) overall panel+charger "actual typical" max harvest. You can also derate for dirty/dusty panels and such too (some folks here use 75% as derating).

Whatever numbers you use, I try to avoid over estimating the losses in the system--It just drives the price of batteries, panels, equipment higher... Being that solar panels are historically cheap these days and batteries are historically expensive--Defaulting to a larger solar array and a (somewhat smaller) battery bank usually gives a nice balance for system life and system costs.

My suggested math for solar panels assuming 13% minimum rate of charge:

• 617 AH * 58 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 6,041 Watt array @ 13% rate of charge

Regarding the 250 feet from array to home... You want to send electrical power at the highest working voltage you can. That can be sending 120/240 VAC or sending Vmp-array--Which ever is higher. This keeps cable size smaller (lower current, less voltage drop). Sending (for example) 12 volts at high current is pretty much cost prohibitive.

You have choices to make here... For most people, having the battery bank and major system components near the home is easier to maintain vs having to walk 250 feet to a remote battery shed/AC inverter/genset installation.

For charge controllers, you have a choice between ~150 VDC max panel voltage (works out to around Vmp-array-STD rated of ~100 VDC) or other "high voltage" solar charge controllers (around 600 VDC max panel voltage, or ~400 VDC Vmp-array-STD). The 600 Volt contorllers are a nice solution but these controllers tend to be much more expensive. Doing several paper designs (before you buy hardware) is a good place to start (and learn about these interrelated issues).

Midnite Solar has a nice array calculator for their MPPT solar charge controllers. Depending on how cold it gets (Vmp and Voc rise as panel temperatures fall).

Example of cable sizing for 250 feet with a maximum of 3% voltage (power) loss (you can have >3% loss on solar array--Your choice) on 150 VDC controller:

6071 Watt array / 260 Watt panels (all Standard panel ratrings) = 23.35 panels or ~24 panels.
31 volt panels: 3x 31 Vmp panels = 93 volts Vmp array (150 Volt max controller for cold winters)
24 panels / 3 panels per string = 8 parallel strings
8 strings * 8.82 amps Imp per string = 70.6 Amps Imp-array

Using a generic voltage drop calculator and playing with numbers for 3% max voltage drop @ 250 feet @ 70.6 amps @ 93 VDC Vmp-array

https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=11&necconduit=pvc&necpf=0.99&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=93&phase=dc&noofconductor=1&distance=250&distanceunit=feet&amperes=70.56&x=59&y=24&ctype=nec

Results for 3/0 copper cable
Voltage drop: 2.72
Voltage drop percentage: 2.93%
Voltage at the end: 90.28

Now the same thing with 600 VDC controller:

6071 Watt array / 260 Watt panels (all Standard panel ratrings) = 23.35 panels or ~24 panels.
400 Volt Vmp-array / 31 Vmp panels = 12.9 panels per string or ~13 panels in series
13 panels * 31 volts Vmp = 403 volts Vmp-array-std
2x 13 panel strings * 8.82 amps per string = 17.64 Amps Imp-array

Voltage drop calculator: 3% max drop @ 250 feet @ 17.64 Amps @ 403 volts:

https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=2&necconduit=pvc&necpf=0.99&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=403&phase=dc&noofconductor=1&distance=250&distanceunit=feet&amperes=17.64&x=50&y=10&ctype=nec

Result for 10 AWG cable on high voltage controller...

Voltage drop: 10.48
Voltage drop percentage: 2.60%
Voltage at the end: 392.52

Much rather have 250 feet of 10 AWG copper vs 3/0 cable cost wise...

You also have to check the controller to ensure that Vmp-array-hot>minimum voltage and that Voc-array-cold is under max controller input:

For example a Midnite MPPT controller, their continuations:

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

For 600 volt controllers, there are Schneider, Midnite, and Morning Star controllers and others (I am sure):

https://www.solar-electric.com/xaxwmp80amp6.html

https://www.solar-electric.com/midnite-barcelona-200amp-mppt-charge-controller.html

https://www.solar-electric.com/morningstar-ts-mppt-600v-48-db-solar-charge-controller.html

Unfortunately, only Midnite "lower voltage" controllers have a handy web page to do these array/temperature calculations "auto-magically".

I will stop here for the moment... Lots of questions and answers (sort of).

Your thoughts?

-Bill

Guessing you are somewhere around Auburn California--$2,500 for 4 "golf cart" batteries? Are these Lithium or something else? FLA GC batteries should be less than $200 each...

Updated 10/7/2022 BB:
I blew this one... Greg is taking about 16 battery bank (8 series x 2 parallel strings). $2,500 is certainly not out of the ball park.

"Calibrating now for the 4000KWH "... Assume typo for 4,000 WH per day (not kWH).

Yes, even with that much drop (say 34 volts @ 35 Amps)--That will cost you a lot of lost energy (heating wiring):

• P=V*I= 34 volts drop * 35 amps current = 1,190 Watts of "lost energy"
• 16 panels * 260 Watts per panel = 4,160 Watt STD array
  
• 1,190 Watts lost / 4,160 Watt array = 0.29 = ~29% loss of solar energy (at full power)

We try for 3% max drop of array voltage if possible... 30% drop is a lot.

Yes, you can use Aluminum Cable... You need roughly 2 AWG "heavier" Aluminum cable vs Copper for the same resistance/voltage drop.

Placing the "electrical stuff" (charge controller, battery bank, AC inverter) at your home is usually the preferred solution (close to home for maintenance/inspection). With "high voltage" line from array to charge controller.

I suggest a separate solar "shed" for everything. There is always a possibility of fire--And burning batteries (Lead Acid, not great, FLA will vent electrolyte and hydrogen/oxygen during charging--Lithium Ion can be highly toxic if there is ever a fire).

You may wish to revsit the array to charger solar wiring. I think that is too much drop.

We have not talked about your harvest... Just a quick sanity check. Sacramento Ca, fixed array facing south:

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

Sacramento
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 51° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.52	4.36	5.72	6.32	6.64	6.91
Jul	Aug	Sep	Oct	Nov	Dec
6.95	6.95	6.64	5.86	4.23	3.41

• 4,100 Watt array * 0.52 off grid system eff * 3.41 December = 7,377 Watt*hours December long term daily average
  
• 4,000 WH per day planned usage / 7,377 WH per day average December harvest = 0.54 = 54% of predicted harvest

With a 4,000 WH per day Average load, that is not too bad (if you don't lose 29% to array wiring run). That will leave you extra energy for a few days of bad weather--Can always crank up a genset if needed.

-Bill

Good evening Greg,

I am not in the solar business / do not work for anyone (more or less retired).

Our host Northern Arizona Wind & Sun I suggest because they are good folks and also have committed to starting and continuing funding/support for this forum.

https://solar-electric.com/

I am purely a volunteer here (as is is pretty much everyone else). So I can, if appropriate, give links to NAWS and their products (they are out of Flagstaff Arizona). They have never given me (and before me) any directions regarding editorial control except to keep it business/family friendly and post the good/bad/ugly regarding solar power.

NAWS does have engineers on staff to help with designs and hardware selection, and can even build out/configure systems for ease of installation (i.e., wire up charge controller, AC inverter, support gear and test before shipment)--A nice service.

Regarding your location, because I am a moderator here, I can see folks' IP Addresses... And with a quick lookup can (usually) guess the rough region folks are posting from. For example of an IP lookup I usually use:

https://www.iplocation.net/

I try to not recommend a brand/model of whatever... I am not in the solar business so I cannot really give you "useful" recommendations other than what I have read about here and other places.

I was just observing that 6 volt @ 2xx AH capacity are very common to choose for solar because they are relatively cheap and rugged. They will not last much more than 3-5 years--But are great "training batteries" to get started (more of us have "murdered" batteries first time with off grid power).

Costco, Walmart, your local battery distribute are all great places to start looking. Flooded Cell Lead Acid "DEEP CYCLE" batteries can be a cost effective starting point.

I am very sorry Greg, I blew this one... Your are taking about 16 battery bank (8 series x 2 parallel strings). $2,500 is certainly not out of the ball park. Sometimes my eyes just glaze over trying to do technically detailed replies with multiple different posters (and I do need new glasses too).

• FLA--Flooded Cell lead acid (cheap, rugged, need to check cells/add distilled water once a month or so, works in sub freezing conditions--usually least expensive solution)
• AGM--Absorb Glass Matt (sealed battery, no water checking/adding, better surge current, lower self discharge, only "gas" near end of life--Mid price range)
• Li Ion (LiFePO4)--Almost the "perfect battery" (sealed, low voltage drop, high current charging/discharging, light weight, compact, can be expensive, should have a BMS battery monitor system, work well hot weather, cannot cycle near or below freezing--expensive to purchase but long cycle life can be least expensive long term)

Agree it is nice to have "integrated" solutions are nice. Schneider (formerly Xantrex) offers some top of the line integrated equipment.

As with anything--Higher end, more features/options/configurations, the chances are that having somebody to help design/engineer/etc. the system may be very helpful.

https://www.solar-electric.com/schneider-conext-865-1034-mppt-solar-charge-controller.html (high voltage MPPT charge controller)
https://www.solar-electric.com/schneider-electric-xw-pro-6848-21-inverter.html (120/240 VAC AC inverter-charger)

Again--While I like and respect our host and their people. the links above are just suggested starting points for further research.

I am not in the business, and what you "need", your price sensitively, expectations for service/support/upgrades/etc. in the future are all a complex dance.

There are lots of low cost unis out there today too... Many have some pretty smart features--But you may run into a lack of support down the road.

And we just "jumped" into the middle of your design and thought processes... There are lots of other issues and stumbling blocks just waiting to trip you up.

I personally suggest:

1. Measure and review your power needs. Aiming for a (smaller) off grid power system at 3,300 WH per day (very efficient off grid home and near normal "modern electrical life"). May not achieve 3,300 Watt*Hours per day, but worth trying for smaller home/cabin
2. Review your loads and see if any can be reduced (modern fridge vs 20 year old that works fine), LED lighting, solar friendly well/water pumping, etc. and reduce your loads as much as practicable for your needs.
3. Once you have nailed down your loads--Then define battery bank capacity (we did that above) and AC inverter "losses")
4. Next define solar array based on first rate of charge--10%-13%+ typical; and second based on daily loads and amount of sun per day, by season if appropriate).
5. Once you have "sized" the system, start selecting products that meet your needs.
6. At this point, depending on your desires/interest/abilities--You may wish to involve a knowledgeable consultant/engineer/retailer to help suggest products and possible configurations/trade-offs. You have a good idea of your energy needs, and how the basic parts fit together. You will recognize if somebody is trying to give you a snow job at this point.
7. Figure out how the devices connect together and interact. There are limitations that make selection of the solar panels/array configuration based on Controller input requirements/min and max voltage/current/your local high and solar temperatures/etc.
8. After you have done the above... You may want to try some other choices--Such as different battery chemistry, different brand/models of controllers, look at their costs/function you need/long term life and support, etc. And see which design best fits your goals.
9. Now you are ready to buy hardware.

I will add another thing to think about... As we get older (I am on the "wrong side" of 65), and with you being blind, how much you can do and maintaining the system as we age... Moving around 1,000+ lbs of batteries every 5-7 years sounds OK when we are younger... As we get older, these maintenance chores can become more challenging. Vendors come and go, outside service/contractors come and go--This is a long term commitment that can tax anybody's abilities.

For people looking at off grid--I do frequently ask why and how much would bringing power cost you. Off Grid system are not cheap, and, I would suggest, do not add much value to the property (future sales). Having power lines to the property (even if not cheap), can increase the value of the property (i.e., $20,000 for OG system vs $20,000 for power lines). Yes power is not cheap, and especially in California, utility power is probably getting more expensive and "rarer" (i.e., stopping service to "remote customers", blackouts during summer from loads or wild fires, etc.).

Since I am not in the business of selling solar equipment--I at least want to make sure that if you "go off grid solar", it is the correct decision for you.

Sincerely,
-Bill