formula for number of panels vs number of batteries

System

I have 4 x 130 watt panels and I live in northern Arkansas. Is there a formula to give me the optimum number of 12 volt batteries in this location?

I don't have all the panels I need or want, but in the meantime I want the battery bank to be adequate so that I am not wasting sun hours or not being fully charged.

Thanks,
Jim

mike95490

Re: formula for number of panels vs number of batteries

You first figure your loads, and size your batteries to that.

Then you need at least 5% of your battery capacity to recharge, up to about 15%, unless you want to spend more $ on PV. The charge controller throttles back excess PV capacity.

Better ?

nigtomdaw

Re: formula for number of panels vs number of batteries

Neil knows this better than me but the rule of thumb if I remember right is between 3% and 6% of battery capacity to match the charging ability of your PV array

ie if you have a 12v 1000 ah battery bank then a PV array producing between 30 and 60 amps would be in your target zone.

If that makes sense.

I have a 2100 amp hour 24v battery bank and I have seen a max of 68 amps from my 2.4 kw array which sees me just inside the 3% window, I need about 4 hours of full sun to replace my daily draw, I use about 6-8 kw hrs daily

If i have this wrong I apologise and await correction and enlightement from the clever boys.

Obviously as Mike says loads are an inportant factor but I think this was what you were asking

Cariboocoot

Re: formula for number of panels vs number of batteries

If you want to get an idea of how many kW/hrs per day you could "harvest" where you are there's the PV Watts program:

http://www.pvwatts.org/

But basically you plan your batteries according to your loads and the get the necessary # of panels to provide sufficient charging to keep them up.

Doing it "backwards" based on your panels @ 70% efficiency you could handle 600 Amp/hrs at most, and 300 would be better. Check my system info in my sig and you'll see what I mean.

System2

Re: formula for number of panels vs number of batteries

Thanks for the reply, y'all.

I calculated loads before I started, but then realized I couldn't afford to cover them all in my first investment. So I bought 4 x 130 panels, controller, etc. I now have 6 x 6 volt Trojans, which means total of 675 amps, if I'm not mistaken.

Going by the readings I get on the meter I guess I'm ok with what I have. I regularly see between 20 and 32 amps going in. The loads I connect now take the voltage down to 12.1 or so by nightfall, then I disconnect them. Then when it gets back to 12.6 the next day, I reconnect them again.

I am understanding this right?

So adding more panels and batteries in the future, I need to stay between 3 and 6 percent of amp capacity in the battery bank?

Thanks,
Jim

Cariboocoot

Re: formula for number of panels vs number of batteries

The standard minimum charge current is 5% of the Amp/hr rating. For 675 Amp/hrs of batteries that's 33.75 Amps. It would be better to achieve 10% or 67.5 Amps. At 20-32 Amps you will be chronically under-charging your batteries and they will fail prematurely.

I suggest you separate your batteries into 3 banks and 'rotate' them to keep them all in good state of charge until you can afford more panels. It's a pain, but it beats having sulphation eat away the Amp/hr capacity.

BB.

Re: formula for number of panels vs number of batteries

The typical rule of thumb is 5% to 13% of the 20 Hour Rating of the battery bank.

You did not tell us if this was a 6v, 12 or 36 volt bank--so working backwards:

4x130 watts = 520 watts

520 watts / 32 amps = 16.25 volts... So probably a 12 volt bank

3 strings * 2 series batteries each string => 675 amp*hour (assuming your AH calculations are correct you have 225 AH rated batteries)

675 AH * 0.05 = 33.75 Amps

So--you are at the bottom end of the PV rating for your system--no problem there.

You could easily have (13%/5%)*520W=1,325 watts of solar panels and still be good for your system.

Currently, I am not sure that you are treating your batteries very well right now... You need a hydrometer to see how far you are drawing them down.

I don't know if you are measuring 12.1 volts under load or during resting conditions (3 hours of no load/no charge). And the 12.6 volts is that during charging, or again after resting... It looks like you may not be quite filling charging your battery bank and going down to 50% capacity. OK--but sort of hard service for the bank (voltages need to be exact, and temperature corrected).

Take a look at the Battery FAQ and see what you think.

-Bill

crewzer

Re: formula for number of panels vs number of batteries

Is there a formula…?

There are “rules of thumb” (ROT's) and formulas that may help you.

Matching Load and Autonomy to Battery Bank

Start with determining your net daily energy load. A 1 kWh/day net load will require 100Ah from a 12 V battery bank assuming 85% inverter efficiency. One ROT is to size your battery bank and for three days of autonomy with a 50% depth of discharge, or ~16%/day. 100 Ah/day x 3 days / 50% = a 12 V battery bank rated at ~600 Ah.

Matching Array to Battery Bank

Another ROT is that a balanced system with a 12 V battery bank has a PV array rated at 1 W STC for every 1 Ah of 12 V battery bank capacity. This translates into a charging current of ~6% of battery Ah capacity. Accordingly, your 520 W STC array should work nicely with a 12 V battery bank rated at ~520 Ah, and the array should be compatible with a 12 V battery bank rated at up to ~600 Ah (~5% charge current).

Matching Array to Local Environment

How many Ah your array can deliver is a function of array STC Imp spec and local insolation. Your array is rated for ~30 A STC Imp (check the specs), and your northern AR location between Little Rock and Springfield, MO means that your system with a clean, unshaded, and south-facing array tilted up at local latitude (~36 degrees?) will receive an annual average ~5 hours/day of “full” Sun.

5 hrs/day x 30 A = 150 Ah/day gross. Assuming battery coulombic efficiency (Ah out / Ah in) of ~90%and inverter efficiency of 85%, your average net Ah availability will be 150 Ah/day x 90% x 85% = ~115 Ah, or ~1.38 kWh/day. This is more than the 1 kWh/day target above, so that’s good!

This assumes a series (PWM) controller. An MPPT controller will realistically deliver between ~0% (in the summer) and ~20% (in the winter) more energy.

However, average summer month days will deliver higher insolation, and winter days lower. Likewise, sunny days will be higher, and rainy/cloudy days lower. Check these links for average daily insolation data for each month:

http://rredc.nrel.gov/solar/pubs/redbook/PDFs/AR.PDF
http://rredc.nrel.gov/solar/pubs/redbook/PDFs/MO.PDF

You may need to seasonally adjust your load accordingly.

HTH,
Jim / crewzer

bryanl

Re: formula for number of panels vs number of batteries

A simple rule of thumb is a watt of solar panel for every pound of battery. That'll get you close enough to start with for small systems.

mshen11

Re: formula for number of panels vs number of batteries

[Start with determining your net daily energy load.]

that seems like the first assumption in the calculation. what if youre not that rich and your net daily is too big but you still want a system. then how would you do your calcuations, give a budget of $x.

and what is there a recommended way to expand?

crewzer

Re: formula for number of panels vs number of batteries

what if youre not that rich and your net daily is too big but you still want a system. then how would you do your calcuations, give a budget of $x.

Well, you could run the calculations “backwards.” For example, if you have enough $$ to buy, say, 260 W STC (~17.3 Vmp x ~15 A Imp) of PV, a series (PWM) controller, a 12 V x 260 Ah battery bank, and a 300 W TSW inverter, and you have 5 hrs/day of “full” Sun available (check PV array alignment and local data), then you might be able to do something like this:

12 V nominal x 15 A x 5 hrs/day x 90% battery coulombic efficiency (Ah out / Ah in) x 90% typical Morningstar SureSine 300 inverter efficiency (@ 50W - 150 W output) = ~729 Wh/day net energy available.

and what is there a recommended way to expand?

Expanding a battery-based system can be a challenge, because you generally do not want to mix new batteries with old ones.

One expansion strategy might be to start with a slightly "oversized" battery bank, and then add more PV later. Also be sure the controller and inverter are initially sized to handle the larger system.

Another strategy is to assume that you'll kill your first set of batteries. Don't worry, as you won't have been the first to do so. In this case, just build and enjoy a "starter" system. When your ready to expand, sell the original controller and inverter, buy more PV modules along with a new controller and inverter, recycle the old batteries and buy new ones, and then enjoy your new larger system.

A third option is to keep the original array, controller, and inverter, buy more PV and other controller and inverter, and then hook both "old" and "new" systems to a new, larger battery bank.

HTH,
Jim / crewzer