870 watts of 12v PV.. what battery bank size??

[epsgunner]

I did read the sticky abit..

I bought 870 watts of PV so I should be at like 667 watts going into the bank on a good day (77%)

I was going to start with 3 115ah batteries and be at 345 ah of battery bank..

However after thinking about it I may run my A/C rated at 540 watts for 4-5 hours (about 54 amps per hour draw).. this would only be at peak sun time 1-4 or 1-5pm here in south TX..

So I'm thinking the draw from the bank will actually be small as the panels will be doing most of the work..

The only other things are 7 amps for 1 hour max for the water pump usage (on a switch).. and my small fridge which is about 2.5 amps per hour for 24/7..

Looks like 313 ah used per day.. but again the A/C is only in the summer months.. and only when the temp hits 85+..

I'd like 2.5 days of usefullness at only 40% out of my bank (60% left over) in case I run out of sun for a few days but its still 85+ as listed above.. (it can happen)..

How big should I make my bank?? I am thinking 600 ah or so..

[Sunking]

I bought 870 watts of PV so I should be at like 667 watts going into the bank on a good day (77%)

To optimistic, that is best case for GTI efficiency. Best case using MPPT charge controllers is 66%, and 50% for PWM

Looks like 313 ah used per day.. but again the A/C is only in the summer months..

You just answered your own question. To obtain 2.5 days of real back up the batteries need 5 days reserve. So you would need 5 x 313 AH or 1565 AH battery capacity.

This brings us back to what size panel wattage do you need to support the batteries. The answer to that is minimum is C/12 and maximum is C/8 charge current. If using MPPT charger controller the answer is pretty straight forward. The minimum charge current is 1565 AH / 12 hours = 130 amps. The maximum is 1565 / 8 hours = 195 amps.

To determine the panel wattage to deliver the charge current is again quite simple and depends on which charge controller type you use being either MPPT or PWM. I will not go into PWM because it is pointless. For MPPT the panel wattage = Charge Current x Battery Voltage. So what voltage are you running the batteries at? So run the numbers again and I bet you get a nasty SURPRIZE! What I can tell you is it will take 2, 3 or more very expensive charge 80 amp MPPT charge controllers depending on what battery voltage you run.

[epsgunner]

Just to update and get some more feedback..

I opted to get a Midnite Solar Classic 150.. and like I said I have 870 watts of panels (12v nominal.. 18VMP * 8.05 IMP rated)..

The panels will be 2 in series and 3 parallel sets into a combiner box.. (max wire run is 14 ft of 10 AWG at 8.05 amps)

For now I will be doing a 12v battery bank..

And I'd like to just get 1.5 days reserve.. so what size bank.. (figure Tojan 105RE's 220ah or the T-1275's 150ah)..

[Mike90250]

.......a 12v battery bank..

And I'd like to just get 1.5 days reserve.. so what size bank.. (figure Tojan 105RE's 220ah or the T-1275's 150ah)..

If your loads are 0, your batteries will not need to be very large at all


So first, you really need to define your loads that run off the battery, size the battery for LOADS, and then the PV gets sized to maintain the battery.

A 12V system will not be very efficient with any large loads, as much power gets wasted in internal battery and wire losses.

Running air conditioning days, while not a "load" on the batteries, diverts charge current from them.

[epsgunner]

My normal loads without the Air Conditioner..

2- 13w CFL's for 4-5 hours at night..
1- 16w laptop for 3 hours per day
1- 800w Microwave for like 15 minutes a day max.. and mostly in winter..
1- 1 amp water pump for about 1 hour/day.. on pressure and wall switch.
1- 36-80 watt ceiling fan.. 1-2 hours per day.
1- fridge 2.5 amps/hour 24/7

My hotwater is gas.. mostly cooking on gas grill during spring/summer/fall months..

[Mike90250]

My normal loads without the Air Conditioner..

2- 13w CFL's for 4-5 hours at night..
1- 16w laptop for 3 hours per day
1- 800w Microwave for like 15 minutes a day max.. and mostly in winter..
1- 1 amp water pump for about 1 hour/day.. on pressure and wall switch.
1- 36-80 watt ceiling fan.. 1-2 hours per day.
1- fridge 2.5 amps/hour 24/7

My hotwater is gas.. mostly cooking on gas grill during spring/summer/fall months..

OK, so now you do the rest of the math, Take the watts, and multiply by the hours, and come up with a total Watt Hours.
That will be your load.

2- 13w CFL's for 4-5 hours at night..

13w + 13 w = 26w x 5 hours = 130 Watt Hours

1- 16w laptop for 3 hours per day

16 x 3 = 48 WH.

[Sunking]

You need to convert your Amp Hours to watt hours.

For example you water pump uses 1 amp x 1 hour x Battery voltage = watt hours.