Selecting which battery for my system - Not sure what my math told me to get is right

ryan112ryan

Designing my system and am at the point I need to choose which batteries I want to buy.  Did my math and started shoping for options, what I landed on didn't seem right so I wanted to check here. 

Numbers:

24 volt system

8000 watt hours per day (adjusted for inefficiencies)

want 2 days autonomy

Battery temp multiplier: 1.4

Discharge limit: 50%

From what I've researched I need 1866 AH for a battery bank (does that math seem right?).  I've read it's generally best to only have two series paralleled max, 1866/2=933 ah battery.  Which seems big.... 

I found a batter Trojan SIND 6V 942 AH battery. I've never seen anyone use a battery that big.  It also has a weird industrial power connector (type 14).  I think my math is right, but something doesn't seem right at the same time.  They're also 415 lbs each!

I see three options:

1. 8 of the Trojan 6v 942ah in 2 series each battery weighs 415lbs  - Total cost $8760
   
2. 12 Crown 2v 1000ah in 2 series each battery weighs 135lbs - Total cost $8340
   
3. 20 trojan L-16 RE-B 6v 377ah in 5 series each battery weighs 118 lbs - Total cost $7320

At this point I haven't bought anything so I can go any direction, the prices are within budget, but the weight, type and connector concern me.  So what advice do people have and is my math correct?

mike95490

8,000 wh daily consumption is going to need at least 32,000 wh of battery to have a couple (4) day reserve.   Standard would be 5 days reserve, 40,000 wh
 You can discharge deeper, and have shorter battery life, or plan on running generator more frequently  - I've recently had good luck with a honda eu2000 in eco mode and an external tank, keeping batteries topped off overnight.

my 48V 800ah bank  is 38,400 wh.   You are not going to meet your needs with a 24V bank, start thinking 48v.

oil pan 4

That's what I was thinking.
Why attempt 8kwh per day on 24v?

I figure the practice limit of 12v is around 2kw, the pratical limit of 24v is about 6 to 8kw, unless you have a compelling reason to stay 24v say if your system is built into a 24v vehicle electrical system.

Then you also don't want to build your new system at the top of that limit because you will likely be left wanting more.

But you did the right thing. You asked before you bought.

You don't have any idea how many noobs who know absolutely nothing about solar, batteries and inverters buy a bunch of junk then post here after it falls way short of expectations.

My favorite are "my power bill is 2Mwh per month, every major appliance is electric and I'm cobbling together a 12v system to go off grid".

Dave Angelini

Why do you think you need 8 kWh please? 

Is this the amount you want to use from storage overnight? Some people do need large systems and many do not.

Give some more info like where, why, when, and the big one, what is this system for? Offgrid, back-up etc.