Amps from 120w PV panel??

Submitted: Wednesday, Feb 27, 2013 at 15:54
ThreadID: 100792 Views:2236 Replies:7 FollowUps:11
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To all the PhotoVoltaic experts here:

What amperage would I be optimistically expecting from a 120w (actually 2x60w folding) panels on a high sunshine days, good conditions and the panel aimed?????

Cheers
Wombat
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Reply By: Member - Rosco from way back - Wednesday, Feb 27, 2013 at 16:03

Wednesday, Feb 27, 2013 at 16:03
G'day Wombat

Theoretically 120/12=10 A.
Realistically consider about 80% efficiency i.e. 8 amps

May be more, may be less but I would work along those lines as a best case scenario.
More competent persons may offer a more accurate answer
AnswerID: 505694

Follow Up By: garrycol - Wednesday, Feb 27, 2013 at 16:31

Wednesday, Feb 27, 2013 at 16:31
Well not quite - these type of solar panels actually make 17v at the panel on a bright sunny day so if they are are true 120w the amps will be 120/17= 7amps and a little bit. The regulator will then reduce that 17v to 12v with appropriate losses as well.
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Follow Up By: Ross M - Wednesday, Feb 27, 2013 at 17:08

Wednesday, Feb 27, 2013 at 17:08
garrycol is on the money, ideal conditions, square on sun and the panels not too hot with a light cooling breeze maybe 7 amps.
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FollowupID: 782634

Follow Up By: Member - Rosco from way back - Wednesday, Feb 27, 2013 at 18:44

Wednesday, Feb 27, 2013 at 18:44
No doubt correct chaps. But more or less different road to the same address .. ;o))
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Follow Up By: Member - mechpete - Thursday, Feb 28, 2013 at 20:17

Thursday, Feb 28, 2013 at 20:17
I,m with you rosco ,
who cares about realisticlly or optimistically deal with the facts ,
I have a 64w panel it gives 3.8amps give or take a bit . how simple
can it be to work out
mechpete
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FollowupID: 782703

Reply By: chisel - Wednesday, Feb 27, 2013 at 16:04

Wednesday, Feb 27, 2013 at 16:04
What controller do you have?
In theory you might see up to nearly 9A (at, say, 13.5V) but that's assuming a very good mppt controller and ideal conditions for the battery you're charging.

Poor/cheap mppt controller or PWM and you're looking at whatever the max is for the panel(s) so probably in the 6.5A range but check your panel specs.
AnswerID: 505695

Follow Up By: Racey - Thursday, Feb 28, 2013 at 09:28

Thursday, Feb 28, 2013 at 09:28
The maximum I have experienced, and I guess will ever see from 2 x 120 panels connected via a Plastronics controller is 17.5amps which close to chisel's comment.

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FollowupID: 782671

Reply By: Member - John and Val - Wednesday, Feb 27, 2013 at 17:50

Wednesday, Feb 27, 2013 at 17:50
What starts out as a simple question so often has some complexities hidden in it!

As Garrycol and Ross M have said, panels are rated at their maximum power point, which occurs at about 17-18 volts. So 120 watts divided by 17 volts tells us the panels will deliver about 7 amps under ideal conditions.

But what is delivered to the battery? Connected directly to the battery, the battery will initially load the panels so the panel voltage drops and the panels will deliver their 7 amps until the battery is fully charged and can't accept any more current. When that point is reached, the current drops and the panel voltage rises and can damage the battery. A controller is used to prevent this occurring. The simplest controllers recognise that the battery is fully charged and disconnect it from the panels. MPPT controllers allow the panels to operate at their Maximum Power Point, 17V and deliver their maximum current, 7A. The battery needs about 14-14.5V to charge it; the controller effectively swaps the excess panel voltage for some extra charging current. The better MPPT controllers are pretty efficient and provide a significant increase (around 15+%) in charging current. The not-so-good ones don't do so well.

So - the straight answer is that the panels can provide about 7A. How much reaches the battery? Depends on the controller, but 7 to 8.5A.

You might find Electricity for Camping a useful read.

Cheers

John
J and V
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AnswerID: 505697

Follow Up By: oldtrack123 - Saturday, Mar 02, 2013 at 23:01

Saturday, Mar 02, 2013 at 23:01
HI
Re [quote]" MPPT controllers allow the panels to operate at their Maximum Power Point, 17V and deliver their maximum current, 7A"[end quote}


.Mppt Reg can, under good conditions, produce in excess of the panel's MPP into the battery
If the battery SOC is 13V the 7A MPP [120Wpanel@17VMPP] of the panel will CONVERT to around 9Amps
The formula then is: panel Watts divided by battery voltage= Amps into battery ,MINUS some small conversion losses

A PWM does not /cannot convert the panel available Watts @ one VOLT AMP combination to ANOTHER combination
It simply switches /controls the panels available current whatever it may be @THAT point in time


PeterQ
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Reply By: Member - Peter H36 - Wednesday, Feb 27, 2013 at 18:06

Wednesday, Feb 27, 2013 at 18:06
Wombat,

I purchased two "cheeeeep" units on ebay. They are rated at 120 watts. This is at the maximum voltage of 18.6 volts. I have checked all the specs of lots of panels and the same applies if you spend $300 or $800 per panel.

P=EI so P (power) 120w = E (volts) x I (current or amps)

My system runs up to 120w at the maximum voltage of 18.6 volts so.... 120=18.6 x 6.45 amps
In fact it runs at 12 to 14 volts depending on the amount of sun and gives me between 4 and 6 amps per panel. (each panel is giving me realistically between 60 and 80 watts at 12 volts)

As my two fridges draw 3.5 amps each when running, one did not give me enough, so I now have two panels and everything is great for 3 weeks or more camped in the one spot

Hope this helps
Peter
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AnswerID: 505699

Follow Up By: olcoolone - Thursday, Feb 28, 2013 at 18:06

Thursday, Feb 28, 2013 at 18:06
Peter I take it you have 2 x 120w panels...... if that's the case you pretty spot on with your calculations.
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Reply By: Dennis Ellery - Wednesday, Feb 27, 2013 at 20:22

Wednesday, Feb 27, 2013 at 20:22
Depends on the quality of the panel. Some panels are rated more conservatively than others and can produce more than their rated output. I have a couple of Sharp 130 watt 22volt panels on a cheap PWM controller – these (2 in parallel) blow 15 amp fuses in ideal conditions (up to approx. 8.5 amps each). Had to change the fuses to 20 amp fuses.
AnswerID: 505704

Follow Up By: Peter_n_Margaret - Wednesday, Feb 27, 2013 at 21:11

Wednesday, Feb 27, 2013 at 21:11
I suggest that there is no value in a fuse between the panels and the controller.
It would simply add resistance.
Panels can not be harmed by short circuit, the cables should be large to minimise voltage drop losses, so the fuse protects nothing.

Cheers,
Peter
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Follow Up By: Dennis Ellery - Thursday, Feb 28, 2013 at 12:49

Thursday, Feb 28, 2013 at 12:49
I put them in a fair time ago – didn't give it much thought - possibly following the manufacturer's manual.
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Reply By: olcoolone - Thursday, Feb 28, 2013 at 18:02

Thursday, Feb 28, 2013 at 18:02
Good to see so many mathematical equations.... and most are sort of there.

One important thing people are forgetting is on most panels the rated output is with a panel temperature of 25 Deg C at maximum sunlight..... on an average day this temperature can increase to 45+ Deg C...... as the panel temperature increases the voltage decreases as does the amps.

The output of the panels can be up to 20% less at 45 Deg C compared to 25 Deg C.

So take into account the average 10-20% lost in general efficiency, wiring and sunlight input plus another 20% due to temperature you can be looking at an overall efficiency of between 60-70% of the advertised output.

On a 120w panel I would expect an output of around 80w.
AnswerID: 505759

Follow Up By: oldtrack123 - Saturday, Mar 02, 2013 at 23:17

Saturday, Mar 02, 2013 at 23:17
Hi
With a PWM reg ,the actual Watts into the battery can be quite low Even leaving out temp losses etc.
The formula becomes
Available PANEL current x battery actual terminal voltage AT THAT TIME
eg
[1]i
f battery voltage is 11.5 V & panel is putting out its PMPP current of say 7amps the actual energy into the battery will be 80.5 watts[neglecting losses]
[2]
if battery voltage is 12V under same conditions, 12vx 7A=84W
[3]with battery voltage of 14v & still in bulk charge mode ,14V x 7A =98W
Near Full available Watts output as energy into the battery under all SOCs can only be achieved with a good MPPT REG
there will always be some losses, but far less with a MPPT
reg

PeterQ
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FollowupID: 782833

Follow Up By: oldtrack123 - Saturday, Mar 02, 2013 at 23:33

Saturday, Mar 02, 2013 at 23:33
Hi
One other point
the MPP is constantly changing depending on energy into the panel [light intensity ]& panel temp
The MPP stated on the panel is it's rated MPPTunder specified conditions.
The panel will put out a higher than rated MPP under high light inputs [reflected sunlight from cloud being one case]

PeterQ
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FollowupID: 782834

Reply By: alhow - Sunday, Mar 03, 2013 at 15:20

Sunday, Mar 03, 2013 at 15:20
The cheap solar kits are a a great buy, but you do need to upgrade the wiring and connectors to Anderson plugs ( If you use connectors) as they are way to restrictive. Remove the controller from the back of the panels and shift to the battery/ies.

http://www.ecoonline.com.au/product-information/solar-pv-panels/solar-wire-sizing-calculator
AnswerID: 505968

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