Trying to size stand-alone off-grid solar system for 12V consumption.

usao · January 2017

Basically, I need a continueous feed of 16A @ 12V for an stand alone purpose. Im trying to figure out the details. Here's what I have figured so-far.
1) Approx 200W continuous for 24/7 operation = 4.8 KW/day. Let's go with 5KW/day consumption.
2) Running with a constant load of 16A, I would need a 20A charge controller. Ive heard that MPPT is the best, these are going on eBay for around $10 on the cheap end for a CMPT02 unit. No inverter needed as the consumption is 12V.
3) Solar panels would need to supply 5KW/day into batteries. Assuming 5-hours sun/day (Phoenix, AZ) on average, that means I need the panels to generate 1KW/hr to reach 5KW/day???
4) Batteries... Assuming I want to cover 1 full day of operation, that means I need 12V @ 16A * 24H (384AH). Assuming that I don't want to go below 50%, then I would need 768AH or better, correct?

Overall, am I on the right track or have a screwed up the math?

dennis461 · January 2017

This is actually a fairly large system you are going to need.
Plug your numbers in here...
https://www.batterystuff.com/kb/tools/solar-calculator.html

CMPT02 is not a good choice, it's ampere rating is too low(the one I looked at) and it is not a MPPT

usao · January 2017

Thanks, I like that it shows you all the calculations. Ill save that site for reference.
As for the controller, I was looking at cheap, and the eBay offer said it's MPPT (eBay item number: 131360246544).
Guess you can't count on the Chinese to be accurate, they are trying to push stuff out quickly...

newestnewbie · January 2017

I am just past that stage, so thought I chime in?
1) First call is to, more than, try to reduce your daily power requirement well below 5kW when you are offgrid! That's "insane", but typical initially.
2) Now, assuming you disregard 1) (???), and you really want to go with 12V because your distance to cover is really short(RV!), then:
3) For 5kW/day, you'd effectively need to draw 5.8kW, because at 12V expect 86% inverter efficiency (always "err" on the safe side)
4) For long life battery, expect 30% discharge max, thus 5814W / 0.3 = 19380W. This @ 12V = 1615Ah battery capacity required! That's quite a number of batteries(!) which for your desired 12V, you will parallel-connect.
5) Next, you need to generate more than the 5.8kW/day, because there are also inefficiencies before the solar power even reaches the battery! If you have TOP panels and TOP charge controller and TOP short distance between, then expect to require to generate from your panels 5.8kW / 0.9 = 6.45kW/day! That's quite a number of panels(!) which almost certainly requires you to be stationary (won't fit on the largest RV at today's PV efficiencies).

Then if that still doesn't frighten you and your bank account(??), note that you will need lots of (costly) safety equipment on top: breakers, switches, fuses, thick cables,... - which actually is my own question about that I wanted to post... lol

I trust the above helps you to reconsider...

usao · January 2017

My load is a 12V load and draws a fixed current 24/7, can't change those starting points.
No inverter involved, DC only.
I was going with 50% discharge, but from Reading at the Rolls site, 30% is a much better number.
I was thinking along the lines of 6x of the 2V batteries.

just starting · January 2017

384 amps @ 12 volts per day or 192 amps at 24 v or 96amps at 48 v per day * 4 for the battery bank. Makes A 10k $ system

mike95490 · January 2017

I have about a 7Kwh daily load, and could not meet that with a 12V system.
5Kwh load / 24 hrs = 208w average draw, So it could be met with a massively parallel 12V system or one built up out of 2V, 1,000ah cells

littleharbor2 · January 2017

usao said:

Thanks, I like that it shows you all the calculations. Ill save that site for reference.
As for the controller, I was looking at cheap, and the eBay offer said it's MPPT (eBay item number: 131360246544).
Guess you can't count on the Chinese to be accurate, they are trying to push stuff out quickly...

Funny, just this morning I sent a very direct message to one of the larger solar controller venders (China based) on eBay. I made it very clear that they are breaking the law by advertising these junk controllers as MPPT. I might ad this isn't the first time I have done this. I doubt it has much effect on this situation but they need to know the only people buying these are unknowing novices and we know their bulls*#t

BB. · January 2017

Lead Acid batteries take time to fully/properly recharge. There are not "enough hours of sun" in a day to fully recharge a flooded cell battery bank from 50%.

AGM may be a bit better, or possibly LiFePO4 would recharge much more quickly (lead acid batteries have an "absorb" phase from ~80% to 90%+ charge that takes a few hours). As said above, planning on 25% to 30% discharge per day would be a better plan (i.e., battery bank is 4x larger than daily load).

Is the load all year round, or just during "non-winter" time (i.e., only when you have lots of sun)? Where (roughly nearest major city) will the system be installed.

-Bill

westbranch · January 2017

And you have to know that that 12V device will get between ~ 11.0 V to ~17.0 V or more during the day if you are charging during the day, because of the function of the charge controller... the voltage range needs to be tolerated... it's not 'just' 12V..!

usao · January 2017

@mike95490, my load is 5KW total, not per day. It adds up to 5KW * 24 = 120KW/day.
If I draw 120KW/day from batteries, I need to replace that in about 5 hours of sun.
120KW/5h = 24KW/hr. So, I would need about 100 of the 250W panels.
If I assume that 5KW @ 12V is approx 400A, then I would need 9600 AH / 30% depth-of-discharge, or a total of 32KAH capacity.
This is sounding like it's not feasable.

Photowhit · January 2017

Correct!

mcgivor · January 2017

.usao said

usao said:

Basically, I need a continueous feed of 16A @ 12V for an stand alone purpose. Im trying to figure out the details. Here's what I have figured so-far.
1) Approx 200W continuous for 24/7 operation = 4.8 KW/day. Let's go with 5KW/day consumption.
2) Running with a constant load of 16A, I would need a 20A charge controller. Ive heard that MPPT is the best, these are going on eBay for around $10 on the cheap end for a CMPT02 unit. No inverter needed as the consumption is 12V.
3) Solar panels would need to supply 5KW/day into batteries. Assuming 5-hours sun/day (Phoenix, AZ) on average, that means I need the panels to generate 1KW/hr to reach 5KW/day???
4) Batteries... Assuming I want to cover 1 full day of operation, that means I need 12V @ 16A * 24H (384AH). Assuming that I don't want to go below 50%, then I would need 768AH or better, correct?

Overall, am I on the right track or have a screwed up the math?

@mike95490, my load is 5KW total, not per day. It adds up to 5KW * 24 = 120KW/day.
If I draw 120KW/day from batteries, I need to replace that in about 5 hours of sun.
120KW/5h = 24KW/hr. So, I would need about 100 of the 250W panels.
If I assume that 5KW @ 12V is approx 400A, then I would need 9600 AH / 30% depth-of-discharge, or a total of 32KAH capacity.
This is sounding like it's not feasable.

So, I'm a little confused , which one is your load? Seems to have increased somewhat.

usao · January 2017

The initial post stated 5KW/day (which was in error).
This is why I wanted to go through the math and make sure I was doing things correctly.
I have a 5KW static load. Im drawing approx 30A@ 220V going a pair of 2880W IBM 12V power supplies... The target equipment is all 12V.
My bullet-item 1 was incorrect. I mixed up daily load with instantaneous load.
I wanted to see if I could get off the AC circuit and go strictly on DC to avoid grid charges for the fixed load. Im paying $0.09/KWH, which works out to around $10.80 per day.
I wanted to see if I could go with DC battery/solar to avoid this charge, but it looks like it's going to be way too expensive. It would take YEARS to make up the purchase costs.

mcgivor · January 2017

With that size of load, grid tie would be the logical solution, if it is an option, supplementing day time use.

Raj174 · January 2017

Sounds like your running some very powerful bit miners. Something close to 20 TH/s, hopefully, since their using that much power.
I believe that even if you could manage to put such a system together it would not work, because as westbranch said, the battery voltage will range between 11 and 17 volts and would not be acceptable to most equipment that needs a regulated 12 volt supply. A good idea. I had considered it myself, however it would only work on an existing system as an opportunity load. And even at that, the extra power produced would be quite limited.

Estragon · January 2017

No way you can beat 9c/kwh with a solar/battery solution. Overall cost for that is going to run on the order of 10x that.

Depending on location etc you MIGHT be able to offset some daytime grid power with solar sort of cost effectively.

Trying to size stand-alone off-grid solar system for 12V consumption.

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