Excessive panels???

SolarWombat

Hi all,

I recently purchased four (pricey) Surrette batteries, and am hoping for advice as to whether adding more panels to keep them happy is overkill.

My off-grid system:
546 amp/hr batteries @ 24 volts
Max daily usage 2000 watts
Outback MX-60 controller
Xantrex DR-2424 inverter
850 watts panels

Currently my system is floating between 2 & 3 pm when I'm home and noon when I'm not. This is with max sun this time of year and not using dc swamp cooler yet. I'm considering adding another 300 watts of panels, which would be an increase of 36%. The online calculators suggest that adding more panels is unnecessary; however, my priorities are to keep the batteries between 70-100%, and to use a generator as little as possible. My experience with previous learner batteries (crowns, 450 amp/hr @ 24 volt) was that I couldn't keep them fully charged, but then they were rather haggard by the end.

As I see it, the only negative is the cost of the panels, and the positives are a quick recovery after a high consumption night or a period of clouds.

Any advice is greatly appreciated.

BB.

Re: Excessive panels???

The normal starting point is 5% to 13% rate of charge:

• 546 amp*hr batteries * 29 volts * 1/0.77 charger derating * 0.05 rate of charge = 1,028 watts minimum solar array
• 546 amp*hr batteries * 29 volts * 1/0.77 charger derating * 0.10 rate of charge = 2,056 watts "good sized" solar array
• 546 amp*hr batteries * 29 volts * 1/0.77 charger derating * 0.13 rate of charge = 2,673 watts ~cost effective maximum solar array

You typed that you used "2,000 watts per day"--I assume 2,000 Watt*Hours per day?

Using PV Watts for Flagstaff Arizona and 850+300 watts of solar panels (1,150 watts or 1.15 kW) and 0.52 overall system derating (fixed array):

"Station Identification"
"City:","Flagstaff"
"State:","Arizona"
"Lat (deg N):", 35.13
"Long (deg W):", 111.67
"Elev (m): ", 2135
"PV System Specifications"
"DC Rating:"," 1.1 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.6 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 35.1"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 5.19, 96, 8.16
2, 5.92, 98, 8.33
3, 6.27, 114, 9.69
4, 6.44, 111, 9.44
5, 6.56, 114, 9.69
6, 6.61, 106, 9.01
7, 5.95, 98, 8.33
8, 5.54, 91, 7.74
9, 6.59, 108, 9.18
10, 6.19, 107, 9.10
11, 5.47, 96, 8.16
12, 5.07, 94, 7.99
"Year", 5.98, 1233, 104.81

You live in a very sunny area--so assuming worst case (20 year) average power is December of 94 kWH per month:

• 94 kWH per month / 30 days per month = 3.13 kWH per day = 3,130 WH per day for December.

So--You look to be OK if you add another ~300 watts of panels to meet the minimum charging requirements.

Battery bank wise, we normally recommend ~1-3 days of storage with 50% maximum discharge.

Picking a "large" battery bank at 3 days of storage with 2,000 WH per day (0.85 inverter efficiency) and 50% maximum discharge of battery (for longer life):

• 2,000 WH * 1/0.85 inverter eff * 3 days * 1/0.50 max discharge * 1/24 volt bank = 588 AH @ 24 volts

You appear to be right at the recommended maximum size of battery bank--If your loads remain low and you don't often have multiple days of bad weather, you could possibly go back to 2 days of storage:

• 2,000 WH * 1/0.85 inverter eff * 2 days * 1/0.50 max discharge * 1/24 volt bank = 392 AH @ 24 volts

So--next time, you might want to consider a smaller battery bank (will charge quicker with your smaller array).

-Bill

SolarWombat

Re: Excessive panels???

Okay, thanks a bunch, that helps tremendously. Yes, I meant watt hours.

petertearai

Re: Excessive panels???

No one ever regretted getting more pv.

Cariboocoot

Re: Excessive panels???

petertearai wrote: »

No one ever regretted getting more pv.

Except when the bill comes in.
And I don't mean if Mr. B. drops by for coffee.

techntrek

Re: Excessive panels???

petertearai wrote: »

No one ever regretted getting more pv.

Until the credit card bill comes in.

Dave Angelini

Re: Excessive panels???

Or, the bill comes in for fuel in an area that does not require a generator!

petertearai

Re: Excessive panels???

Should have said I never regretted buying more pv.And will get more as $ permit.

arkieoscar

Re: Excessive panels???

546 amp*hr batteries * 29 volts * 1/0.77 charger derating * 0.13 rate of charge = 2,673 watts ~cost effective maximum solar array
BB-Can you explain to me why you use 29 volts to calculate this? Is there a situation where this battery would ever draw 85 amps at 29 volts?

Cariboocoot

Re: Excessive panels???

arkieoscar wrote: »

546 amp*hr batteries * 29 volts * 1/0.77 charger derating * 0.13 rate of charge = 2,673 watts ~cost effective maximum solar array
BB-Can you explain to me why you use 29 volts to calculate this? Is there a situation where this battery would ever draw 85 amps at 29 volts?

2673 Watts of panels won't produce 85 Amps @ 29 Volts. More like 71, which is 13% of the 546 Amp hours. 29 Volts is the high charge Voltage one might expect in such a system. "24 Volt" systems usually charge at 28.8 nominal, but cold batteries need a bit more V just as hot batteries need less. Bill is being very precise here. Usually you just see basic calculations to "get you in the ballpark". That would look like this:

54.6 Amps @ 28.8 Volts = 1572 Watts, less 77% derating = 2042 Watt array.

It gets a bit different if you push the charge rate to maximum and factor in cold charge Voltage. EQ Voltage is another issue, but usually simple to achieve as you don't need full current.

If your panels are at high altitude they'll put out more and you can use a smaller array. If you live somewhere that the panels will be hot they'll put out less and you may need a larger array. The differences between a basic calculation and the systems specs fine-tuned for a particular site can be large. I think perhaps we don't stress that enough.

BB.

Re: Excessive panels???

Using 29 volts (or 28.8, etc.) is just reflecting some of the losses in the system...

Lead Acid Batteries are nearly 100% efficient when looking at Amp*Hours... Use 100 Amp*Hours for your loads, it will take ~100 Amp*Hour to recharge your battery bank.

However, when looking at Power or Watt*Hours, batteries are less efficient. Remember the equation for power is:

• Power = Volts * Amps
• Energy (in Watt*Hours) = Power * Hours = Volts * Amps * Hours

So, running your loads, your battery bank will be around 12.5 volts to 11.5 volts. Recharging will be around 13.2 to 14.5 (or even higher) volts. So, using the ~29 volts is just one more place to account for power losses in the system design.

• Efficiency = Volt out / Volt in = 12.0 volts / 14.5 volts = 0.83 = 83% cycling efficiency

Batteries tend to be around 80-98% efficient (old lead acid to new AGM). Also, it depends on how you use your bank... If you cycle deeper (say 50% to 90% state of charge), battery banks tend to be more efficient. If you cycle between 85% and 100%, they tend to be less efficient. And for sake of being conservative, I use 80% efficiency for flooded cell and 90% efficient for AGM.

-Bill

arkieoscar

Re: Excessive panels???

If my 550 AH battery(without a load) was drawing 71 amps at 28.8 volts, I would be trying to find out what was wrong with the battery or meters. Of course that current would be normal with a discharged battery but at a lower voltage.
I'm just trying to justify using his numbers in the real world.
So, wouldn't it more less confusing to use real numbers and add another derating factor instead of using improbable voltages to get it?

Cariboocoot

Re: Excessive panels???

arkieoscar wrote: »

If my 550 AH battery(without a load) was drawing 71 amps at 28.8 volts, I would be trying to find out what was wrong with the battery or meters. Of course that current would be normal with a discharged battery but at a lower voltage.
I'm just trying to justify using his numbers in the real world.

Maximum recommended charge rate for FLA's is 13% of the Amp hour rating (20 hour rate).
550 * 0.13 = 71.5 Amps
If you need to push the limits, that's what you get. Not all batteries can or should be recharged at such a rate. Some can take more, most prefer less.

Sometimes it's a long journey from basic calculations to precisely functioning system.:roll:

Cariboocoot

Re: Excessive panels???

BTW if you use Trojan batteries on a 24 Volt system their recommended charge Voltage would be 29.6.

Just sayin'.

BB.

Re: Excessive panels???

The "real" charging model is more complex... But drawing 13% rate of charge at 14.5 is real and very doable if you have enough solar panels or large enough battery charger (this is the transition point between Bulk and Absorb charging stages--going from current limit to voltage limit on the charge controller).

Normally, one would expect to hit 14.5 / 29 volts at 80-90% state of charge (depending on lots of variables, size of battery bank, rate of charge, temperature of bank, etc.).

So, if you discharge to 50% or less--you will be charging in the 13.X volt range for quite a while. If you discharge to 80% state of charge, then you will be much closer to the bulk/absorb transition voltage of 14.5/29 volts.

I try to avoid "double counting" losses... So, sometimes you will see "all in one" deratings (like 0.52 for solar panel + charge controller + flooded cell battery + inverter losses) for solar panel to AC outlet available power calculations.

Others, like sizing loads to battery bank, use individual deratings to make sure we don't double count (recharing battery bank does not need inverter losses).

If you want "more accurate" calculations--we can certainly attempt to make them... However there is limited return. For example, clean solar panels, brand new flooded cell batteries (less self discharge), cycling more deeply (typically in winter, not so much in summer), warmer batteries (more true in summer, less true in winter), and the 10-20% variation in solar radiation due to monthly/yearly weather variations, specific weahter (3+ days of bad weather exceeds battery storage capacity), etc.

So, trying to get better than 10% accuracy is not really worth the effort as there are so many (sometimes competing) variables that it is not worth it.

Also, I try to design the system around "end of life" (old/dirty panels, old about to fail batteries, etc.) so that they work well for the first year and then you have "power shortages" for years 2-8.

In the end, it is not a good idea to plan on using 100% of your available power every day unless you will be taking the time and energy to manage the loads (i.e., moving wash day and vacuuming day around) based on daily weather.

Some loads do tend to manage automatically (people probably pump more water during hot/sunny weather for irrigation and pump less during poor weather).

-Bill

Cariboocoot

Re: Excessive panels???

The missing word here is "tolerance".

You've got to build a certain amount of tolerance into the system. Tolerance for varying weather conditions. Tolerance for wire losses. Tolerance for aging components like batteries. Tolerance for unpredictable loads.

And especially tolerance for Moderators who work very hard to keep the forum free of spam and try to makes sure everyone gets an answer to the large number of very broadly ranged questions that will suffice in general terms. It's not possible to be 100% correct in every post with every answer. We go for RMS: Relative Mean Solutions.

ggunn

Re: Excessive panels???

Cariboocoot wrote: »

The missing word here is "tolerance".

You've got to build a certain amount of tolerance into the system. Tolerance for varying weather conditions. Tolerance for wire losses. Tolerance for aging components like batteries. Tolerance for unpredictable loads.

And especially tolerance for Moderators who work very hard to keep the forum free of spam and try to makes sure everyone gets an answer to the large number of very broadly ranged questions that will suffice in general terms. It's not possible to be 100% correct in every post with every answer. We go for RMS: Relative Mean Solutions.

Well, you don't HAVE to be mean about it, do ya'?

But yes, tolerances can make your calculated results meaningless. Each calculation adds its own uncertainty (tolerance), so in a long chain of calculations, your accumulated uncertainty can exceed the final result in magnitude. Welcome to the World Of Engineering.

Cariboocoot

Re: Excessive panels???

ggunn wrote: »

Well, you don't HAVE to be mean about it, do ya'?

But yes, tolerances can make your calculated results meaningless. Each calculation adds its own uncertainty (tolerance), so in a long chain of calculations, your uncertainty can exceed the final result in magnitude. Welcome to the World Of Engineering.

Fun, isn't it?

Do you know how many times I've been taken to task for using a standard panel derating? Always by people who live in quite other than average circumstances whose systems perform outside the norm one way or the other.

And if anybody ever bothered to do the calcs on my system they'd see I don't "play by the rules" either; high altitude and clever load shifting lets me get away with murder!

techntrek

Re: Excessive panels???

Simple calculation to make your future posting easier:

(Panel voltage - battery voltage) / Earth's circumference * Pi = Maximum wattage of the incandescent light bulb you can run with this system

There, Bill doesn't need to post 2-page responses anymore. :-)

Cariboocoot

Re: Excessive panels???

techntrek wrote: »

Simple calculation to make your future posting easier:

(Panel voltage - battery voltage) / Earth's circumference * Pi = Maximum wattage of the incandescent light bulb you can run with this system

There, Bill doesn't need to post 2-page responses anymore. :-)

But what about CFL's? Since they have a small transformer inside they're induction loads so the Power Factor comes in to play which means you get an extra player on the ice!

Er, seem to have wandered a bit there for some reason. :-)

arkieoscar

Re: Excessive panels???

I wasn't trying to attack any of the mods. You guys have way more patience than I have ever had. I can't imagine getting out of junior high without some understanding of Ohm's law but that's another story.
I would still be looking for a problem if the OP's stated battery was drawing that much amperage at that voltage.

Cariboocoot

Re: Excessive panels???

It isn't that it's going to draw that much current, it's that there is that much peak current potential. The more discharged a battery is, the more current the charge source will put to it due to the Voltage differential.

Batteries are peculiar things sometimes.