I000 watt partially off-grid system

zoneblue

0.77 is our rule of thumb for the combined real world panel plus controller effciency loss. Its (i think) got by mulitplying 95% (CC) and 80% (PV heat related loss).

For the wire size, this is always a job for the voltage drop calculator: eg:

=1000W inverter, means peak current of 1000W/12V/0.85 = 98A.
= cable is 3m, thus 0 AWG (50mm2) gives you a satisfactory 0.2V lost in the cable.
http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.3224&voltage=12&phase=dc&noofconductor=1&distance=3&distanceunit=meters&amperes=98&x=36&y=12

Have a play with the calculator and try to find other solutiions...hint... like a shorter cable.

C means capacity (i guess), so 0.1C means 1/10 of the AH rating of the battery. Thus teh ideal charge rate for 300Ah is 30A.
For the inverter you dont want to routinely exceed a 0.25C discharge rate, otherwise the voltage will sag and the inverter may drop off line.

Just also beware teh direction that your rounding takes you... smaller battery plus bigger inverter might lead you into somethign like this:

220Ah battery 1000W inverter, discharge rate of
= 1000W /12V /0.85
= (thus) 98A/220Ah = 0.45C
= (also) (assuming it will run at all) inverter run time from 100% SOC (state of charge) to 25% SOC at 1000W = 1/0.45 = 2.2hours til dead flat battery

Fun fun.

mike95490

bigbrovar said:

Also what size of wire is recommended for connecting two 12v 220AH batteries in parallel.

Why not use a pair of 6V 200ah golf cart batteries ?   Made for deep cycle use. wire in series, simple
if you can't get golf cart batteries, then this link explains parallel wiring pretty good,
http://www.smartgauge.co.uk/batt_con.html

bigbrovar

Hi guys. Just wanted to give update on my project. Marriage and terrible inflation in my country forced me to put the off-grid project on an hiatus. However, I did a presentation with my wife and thanks to deplorable power situation in my local county (average of 3 hours of grid power per week) she agreed to the completion of the project.

I want to thank everyone here for holding my hands. I have learned a whole lot. I would appreciate if you folks can go through my revised plan for the project and leave comments and suggestions.

My power need and run times: 
Home Theater = 90w for 5 hours = 450 (1800-23:00)
Lighting Points (LED) = 25w for 4 hours = 100w (19:00 - 23:00)
Standing Fan = 50w for 9 hours = 450w (00-04:00,19:00-23;59)
Freezer = 140w for 4 hours 600w   (Startup power of 1500w) (23:01-03:00)

Total power = 1600 / 0.85 (Inverter efficiency) = 1882w 

Inverter: 
12v
Power 850w 
Efficiency 85%
Surge Factor 5:1

PV 240w x2 (Connected in series) 

CC 30A mppt 

Battery 2 220ah @C20 connected in parallel.

Plan issues and questions

The above is what I have and can afford. If I have more cash to spend I would of changed some things but I have to work with that I have.

from the above. My discharge rate is set to about 15ah max (for 9 hours use daily) and I plan not to use more than 160ah in a day. This I hope would put me below the C20 discharge rate and closer to C30. Would this have any impact (Positive or Negative) on my battery life? 

When I calculate wiring and PV efficiency I hope to get at least 30a from the PV at Pick sun hours (wiring loss is calculated to be no more than 3-5% between PV and CC)

My charge rate is estimated to max at 7% or battery capacity at between 30-28ah at pick hours down to about 14ah. Would this affect the performance of my batteries? which total to about 440AH@C20. 

would appreciate if you guys could have a look at my calculations if it makes any sense.

zoneblue

Home Theater = 90w for 5 hours = 450 (1800-23:00)
Lighting Points (LED) = 25w for 4 hours = 100w (19:00 - 23:00)
Standing Fan = 50w for 9 hours = 450w (00-04:00,19:00-23;59)
Freezer = 140w for 4 hours 600w   (Startup power of 1500w) (23:01-03:00)
Total power = 1600 / 0.85 (Inverter efficiency) = 1882w

Mind your units.
90W x 5hr is 450Wh/d (watt hours per day).
Total daily consumption of 1560Wh/d. (AC).

The freezer, if it was grid powered for the rest of the day, ie it is down to temperature, will cycle and may use less energy as a result, however its surge remains problematic for your inverter. Reread the entire thread for further considerations.

bigbrovar

Thanks. I have concerns about the surge of the freezer too. But giving the 850w Inverter has a surge factor of 5:1 and can handle more than 5 times it's load... So fingers crossed.

BB.

The 850 Watt inverter is probably not enough to start the freezer (let alone freezer+loads). If you leave the freezer on grid power, you probably will be just fine (don't put new/unfrozen food in freezer until grid power is restored to avoid defrosting the rest of the freezer during power outage).

What solar panels do you have (Vmp and Imp ratings). Those are probably Vmp~30 volt panel--and using those with a PWM type charge controller on a 12 volt battery bank.

And what batteries are you using? 2x 12 volt @ 220 AH or 2x 6 volt @ 220 AH? (typical "golf cart" battery is 6 volt @ ~220 AH).

-Bill

bigbrovar

BB. said:
The 850 Watt inverter is probably not enough to  start the freezer (let alone freezer+loads).

Even if the Inverter has a surge factor of 5:1? My watt meter showed the freezer normal current is about 140w and startup current (when completely defrost can be as high as 1500w) while usual startup current is about 450w. 

What solar panels do you have (Vmp and Imp ratings). Those are probably Vmp~30 volt panel--and using those with a PWM type charge controller on a 12 volt battery bank.

The controller is mppt and the VMP is 30.5 I intend to connect them in series the mppt controller is rated 30A.

If you leave the freezer on grid power, you probably will be just fine (don't put new/unfrozen food in freezer until grid power is restored to avoid defrosting the rest of the freezer during power outage).

That won't work because I am almost completely off grid. If I get 10 hours of grid power in a week, I would throw a bbq party. The power situation in my town is zero tbh. hence why it is important to find a way to run the freezer on the system. Its the only thing that would make all the money I spent on this system worth it  

And what batteries are you using? 2x 12 volt @ 220 AH or 2x 6 volt @ 220 AH? (typical "golf cart" battery is 6 volt @ ~220 AH).

I am using 2 12v 240AH @C20 (Connected in parallel). They are Flooded Lead Acid. The tubular type.

BB.

Sorry--Yes, you said MPPT controller (not PWM). Assuming the controller will support the voltages needed--It should be fine. Note, that controller on a 12 volt battery bank will support (in my humble opinion) about 565 Watt array (cost effective maximum). So, if you ever decide to add more panels, you will need a larger or second charge controller.

I have not heard of an inverter that supports 5x surge--Typically, 2x is the maximum for a "good" inverter (many seem to support less). Note that 5x surge is a lot of current:

• 850 Watts * 5x surge * 1/0.85 AC inverter eff * 1/10.5 low voltage cutoff = 476 Amps surge @ 12 volts

A typical flooded cell lead acid battery will reliably support a surge current of around:

• 440 AH * 1/2.5 surge rating for battery = 176 Amps
• 176 Amps * 12 volts * 0.85 inverter eff = 1,795 Watts AC surge maximum recommended

So, between trying to wire up a 12 volt battery to support ~176 to 476 Amps surge current with less than 1.0 volt drop, and the limits of a flooded cell deep cycle lead acid storage battery, I would be surprised if you would get 5x surge current... If you do all correctly, and the inverter support ~2x surge current--It is possible that it will start your freezer.

OK, some basic off grid math. If your system supports 2 days of "no sun", and 50% maximum battery discharge (longer life):

• 440 AH * 12 volts * 1/2 days storage * 0.50 maximum discharge * 0.85 AC inverter eff = 1,122 Watt*Hours per day

You can support upwards of 1 day of storage and 50% discharge for ~2,244 WH per day--The maximum I would suggest. Your daily loads of 1,560 Watt*Hours per day is a relatively "heavy" load for this size battery bank.

A 480 Watt solar array would need hours of sun per day minimum of:

• 1,560 Watt*Hours per day * 1/0.52 off grid system eff * 1/480 Watt array = 6.25 hours of sun per day minimum
  

That is really not enough solar array except for very sunny regions during the summer.

My suggestion is a larger battery bank and I would suggest 24 volts, and 2x larger solar array. Unless you can get several hours per day of AC utility power with a battery charger (on a daily basis).

As vtMaps says, a fridge/freezer is typically a "defining" load for off grid power systems. And usually what take a "small system" to a "medium size power system".

-Bill

mike95490

Unless you have a $400 peak hold amps meter, you will not be able to accurately measure peak AC starting current,  A killl-a-watt style meter only samples about 1x per second.

bigbrovar

Hi guys. just want to say my project is now live.  It came on last saturday 23-05-2016. the final setup (for now are)

• 240w  * 2 panels connected in series.
• 30a MPPT CC
• 12v 220ah (@c20) *2 connected in parallel
• 850w Pure sine wave inverter (12v)

Wiring:

• PV to CC = Distance is 20 meters 10mm2 flexible copper wires)
• CC to battery less than 1 meter 10mm2 copper wires
• Inverter to battery 16mm2 

Regarding the torny issue of running the inverter on my 175L freezer (with starting power of close to (1000w - 1500w).. well from my test.. it worked ... kinda.. when I plug the freezer first time. the inverter turned off due to over load. But after turning it on like 3 min after it came on and ran.. for hours. Same time happened again when I turned on the freezer it took 3 tries before the inverter was able to power on the freezer.

what could be the consequence of continuing to power the freezer with this inverter (would it damage the freezer / inverter) long run?
times are hard in my country but I might be able to squeeze some cash to procure a 1200w inverter. would appreciate inputs from you guys.

N.B I was able to power the freezer to run on solar after the battery went into floating charge.  pretty happy with the progress so far.

BB.

I don't remember if you have had this suggestion--Try different extension cords... Short/lightweight and/or heavier/longer cord.

Basically the extension cord can add enough resistance to reduce surge current, but lengthen starting time--May be enough to allow your inverter to reliably start the freezer.

I would not like to see your freezer/inverter in this cycle... It is stressing components (there are over heat/lock PTC in the freezer motor). Generally they are not designed for operating many times per day.

-Bill

westbranch

When looking at inverters you should assess the standby rate/load in amps.  I ended up choosing a Cotek 1500w , surge 3000w, over the 2000w which  has about 2 times t.he standby rate of the 1500w .  Both were pure or TRUE sine wave,.... hth
PS the 15) runs our fridge,no problem

bigbrovar

I have decided to upgrade to a 1200w inverter. Now am considering whether to stick to a 12v system or go 24v. I know the standard view is that 24v is more efficient and all that. But my thinking is that I might get better overall harvest from my 2 240w panels (series connected) with an MPPT controller than I would if I have a 24v system. 

example, as early as 6:30 when the sun is warming up my panels already generating about 17-20v enough to start charging the batteries (although at a lower current of about 2amps, 4amps) and same thing happens around between 17:30 to 18:30. My concern is that changing to a 24v system might mean my battery would only start to charge once there is enough sun to push total panel voltage to 30v, 35v (usually around 08:00am) and stops charging earlier.

are my concerns valid?

vtmaps

bigbrovar said:

are my concerns valid?

No.  Two of those panels in series is PERFECT for a 24 volt system with an MPPT controller. 

The Voc (open circuit voltage) of your panels is the same at dawn as it is at noon.  The panel current (amps) is higher at noon than at dawn.  Because Power (watts) = Volts X Amps, there is more power at noon than dawn.

At dawn, the panel voltage quickly reaches Voc and the controller wakes up and tries to draw power from the panels.  The controller becomes a load on the panels.  At dawn the panels cannot produce much current (amps) and the load causes the panel voltage to sag... sort of like when you start the car and the headlights dim.

The MPPT controller can vary the load it presents to the panels... as the load varies, the controller searches for the load that results in the highest power (watts). 

For example, your panel has a Voc of about 38 volts.  Of course, with an open circuit there is no current flow, thus 38 volts X 0 amps = 0 watts.  When you put a short circuit across your panel, you get Isc (short circuit current), which is about 8.4 amps for your panels (at noon, much lower at dawn).  Of course, with a short circuit the voltage of the panels is 0.  Thus 0 volts X 8.4 amps = 0 watts. 

So you see that at the extremes of open circuit and short circuit ( no load and infinite load), there is no power.  Somewhere between those extremes is maximum power.

To find the maximum power of your panel, the controller will load the panel so that the voltage sags to Vmp (voltage at max power) and the current will be Imp (current at max power).  Thus, for your panel, 30 volts X 8 amps = 240 watts.

Meanwhile, on the battery side of the controller, it's just about watts.  The more watts the controller can harvest, the more current (amps) that it can push into the batteries.   At dawn, there are not many watts to be harvested and the current into the battery will be low.

--vtMaps

bigbrovar

Thanks alot @vtmaps. I ordered a 24v Inverter following your advise.

 I would like to use the opportunity to thank everyone who has contributed to this thread. to say I have learned a lot is an understatement. Honestly you guys have brought immense joy and happiness to my home. When I started this I knew almost nothing about this whole solar thingie. But right now I can now generate my own electricity and bring some happiness to my home. 

This project was borne out of survival and necessity, power situation in my part of the world is abysmal (understatement of the year) we get power like 3 hours in a week if we are lucky. I had to rely on the unstainable petro generators which is not always available due to some fuel scarcity situation in my country. For the past 3 days now, for the first time. I have been producing clean, *free* energy that powers my home and meets my basic needs. I would never have done and if I had it would never have been successful without the help of you guys. 

if there is a higher power out there (God, Allah, Jehovah, Buddha, the Force) may it reward you.

I think it is a good time to make this thread as solved. The goal has been achieved. I will open a new thread if and when I have any other issue.