Advice on System Specs, DC Loads, and Batteries in Parallel

mjp24coho · August 2008

All-

This forum has been a great resource to me - I've been reading for a while, and designing my first solar system for the past two months. I'm glad I've taken so long before buying, because each day I pick it up I make some tweaks. Anyways, I was hoping to get some feedback from you pros on my system design - I'm designing/installing it on my own, but need to challenge it. here's some background: off-grid system for a weekend cabin in S. Utah. I'll use a generator for high loads (vacuum and well pump), but want to use solar/batteries for everything else. I want to stick with 12V, since I already have a 12V booster pump to pressurize the cabin.

Here are the specs I've designed so far:

Total watts (AC) - 2200, estimated 3900 w/ surge
Total AC watt hours per day - 3,351, Total DC watt hours per day - 922
Estimated daily load (dc load with adjusted ac load) - 4,645
6.5 hrs estimated sunlight (summer)
Estimated PV array size - 950 W (5 X 190 W panels)
Estimated

battery bank - 1020 AH (4 X 255 Ah Concorde AGH 12V batteries)

Here's what else I plan on buying:

Outback MX 80 charge controller
Outback VFX2812 inverter
Midnite E-Panel
Solar Array Combiner Box
Top of pole mount
Battery cables
Battery to inverter cables
MC cables to wire PV panels to combiner box
MC cable s to wire combiner box to E-Panel

So.....here are my questions:

From what I've listed, have I spec'd out this system appropriately?
What supplies am I missing?
Can I connect all the batteries in one bank in parallel? I've read some differing

viewpoints on whether 4 batteries in parallel are too many.

I would also want to be able to utilize the E-Panel for wiring a few DC loads to run off the batteries (and not through the inverter). I have a 12V DC booster pump that pressurizes the cabin, as well as a few DC lights that I want to run directly off the batteries. In doing so, I'd like to be able to rig it so that these two loads (booster pump and DC lights) can each have their own breaker that can act as a switch. The intent is to be able to turn them each off when the cabin is not in use, which is necessary since the booster pump has a constant draw while in standby. As you can tell, I'm no electrician, but I'd like to find out if the E-Panel is wired to accomplish something like this, or if it can be configured to do it. I like the idea of having everything isolated in one box.

I know that's a lot of questions, but I've been researching this for so long, that I think I need some other eyes to look at it. Thanks in advance for any help/direction you can offer!

mike95490 · August 2008

Re: Advice on System Specs, DC Loads, and Batteries in Parallel

If you wire the 4 batteries in series, you have a 48V system, and can reduce copper wire gauge. That should count for something - Going from a 200A buss to 50A.

I do think 4 expensive batts in parallel are too many. What's the cost of a new 48V booster pump - or even a 120V one, off the inverter?

If this is ONLY a weekend cabin, I'd skip most of the solar and the big charge controller, and trickle charge over the week, and run the genset on weekends as needed. Drop 4 panels, there's $3.5K, that will buy a bit of gas.

al128 · August 2008

Re: Advice on System Specs, DC Loads, and Batteries in Parallel

your consumption figures strike me as kinda high ...

we managed to get our house's consumption down to 4.1Kwh - with a 1.8Kwh hog of 'fridge

- if I get a new fridge, we will be around 3.2Kwh

and we are a family of 4 (2 small babies and a big one - me :-) ) and believe me - we dont live like in the 1700s either

see that you can get your consumption down instead of paying through the nose - thats what I did - we went down from 7 to 4 and should be able to hit 3kwh w/ the new fridge !

the cheapest PV power is the one you dont need!!!

back on your subject (and given that your cabin uses roughly the same as our house for which I design a system currently) ... right off my pants, your PV array seems a little on the small side (both in terms of the 6+ peak sun hours that you need to cover your 4.6kw as well as for charging a 1000+ah battery bank - you might never get out of bukl-charging ).

On the other hand your inverter seems a bit big. So I personally would get 1-2 190w PVs more with the money you'd save on a smaller inverter. Also save some money getting the flexmax 60 instead of the 80 - the 60 does anything the 80 does and you dont need the extra 20A

Just for reference, here is my "mental" setup (and be warned, i am no pro either)
- 2250w (30 x 75w) PV (run as 5x6 array @60v)
- outback FlexMax 60
- 1200ah @ worth of lead'n'acid 12V (run @ 24v)
- inverter (undecided here, but prob. 1200w)

hope this helps some
cheers
al

ps: if you could - would be great to hear your rationale (which loads, PV-volts, bat-volts, etc...) - I am always eager to learn something - and we are sitting in a similar boat

mike95490 · August 2008

Re: Advice on System Specs, DC Loads, and Batteries in Parallel

I've sort of looked into high amperage chargers, to do a quick bulk charge without running a genset for 4 hours. Have you lined anything up for that yet ? Some different inverter/chargers may have different recharge capabilities

mjp24coho · August 2008

Re: Advice on System Specs, DC Loads, and Batteries in Parallel

Thanks for the input - I'm a rookie at this, so I want to make sure I get all the experienced input I can. I have all of my load calculations in excel - I've attached a screenshot of what I currently have estimated (load.jpg). I think I've pared it down as far as I can, but take a look and let me know what you think. I've also attached a screen shot of how I calculated my required PV array size and battery bank size (systems needs.jpg). Let me know if my logic seems incorrect anywhere.

I thought the Flexmax 60 maxed out at 700-800 watts, but I could be wrong. Also, from the prices I saw, I didn't find a big difference between a good 2000W inverter and a 2800W, but I could be wrong.

niel · August 2008

Re: Advice on System Specs, DC Loads, and Batteries in Parallel

wow, 9 cfls in the kitchen. it must be huge. you may be able to cut back on some of that lighting.

mike95490 · August 2008

Re: Advice on System Specs, DC Loads, and Batteries in Parallel

> I thought the Flexmax 60 maxed out at 700-800 watts

Well, that all depends on the AMPS, not the watts. It tops at 60A <I think>, and I think that's :
60A @ 12V = 720W
60A @ 24V = 1,440W
60A @ 48V = 2,880W
Change system voltage, and wattage changes

nigtomdaw · August 2008

Re: Advice on System Specs, DC Loads, and Batteries in Parallel

If your designing from scratch then take Mikes advice and My advice and go for the largest Base voltage you can it, has lots of pros and very few cons.

1. For starters Mikes previous post on Charge Controller PV Capacity
2. Lower bus amps means thinner cheaper and easier to install system wiring.
3. Higher efficiencys of Inverter Charger
4. No price hike on equipment cost.
5. Easier to construct a single string battery bank for a given size in 48 v that it is at 12 volt . Less strings means less interconnects and reduces battery imbalances.
6. Even savings on switch gear can be significant.

I thought 12 v when I started without NAWS...being around to help me eventually opted for 24v ........2 yrs+ off grid living wish Id gone 48v. I reccomend 48v systems for people living off grid unless its a lights radio and gas fridge type of thing.

mjp24coho · August 2008

Re: Advice on System Specs, DC Loads, and Batteries in Parallel

Good advice - thanks, all. I think I'll switch my designs to have it be a 24V system. I'd like to go to a 48V, but I haven't been able to find a good water booster pump to pressurize the cabin in anything other than a 12V or 24V design.

Advice on System Specs, DC Loads, and Batteries in Parallel

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