# How many pcs of the 120W solar panel do I need?

1 Posts
Mar 19, 2011 09:41 am
How many pcs of the 120W solar panel do I need?

Hi,

I have a project that is:
System current: 14A
Operating voltage: 240V
Operating hr: 6hr
Panel operate voltage: 34V
Sunshine duration: 3.4hr

I would like to know the below calculation is correct or not:

Current capacity of the loading per day: 14A X 6Hr = 84AH
Current need to be created in the solar panel:
84AH/3.4hr = 24.7A
Power created in the solar panel = 24.7 X 34 = 840W
Assume 80% efficiency, power in the solar panel required = 840/0.8 = 1050W

No. of 120W solar panel required = 1050/120 = 5 pcs

Wrong or right?

462 Posts
Mar 19, 2011 10:54 am
Re: How many pcs of the 120W solar panel do I need?

wrong. What is the panel voltage? Try finding how many daily watts are needed first. A x V = W. Then how many panels are needed and then determine A output using sun hrs. More amps : more panels. Make sure you realize your output voltage is different than your operating voltage.

220 Posts
Mar 19, 2011 05:42 pm
Re: How many pcs of the 120W solar panel do I need?

I have a project that is:
System current: 14A
Operating voltage: 240V
Operating hr: 6hr
Panel operate voltage: 34V
Sunshine duration: 3.4hr

240v X 14a = 3,300 watts
3,300 X 6 = 20,160 watt hours
20,160 / 0.8 = 25,200
25,200 / 3.4 = 7,411
7,411 / 120 = 62 pcs of the 120w modules

99 Posts
Mar 19, 2011 07:00 pm
Re: How many pcs of the 120W solar panel do I need?

If I understand you correctly, you're saying that your LOAD is 240V x 14A and that it's running for 6 hours continuously, correct?  Is that 240V AC or DC?  If AC, you'll need to discount another 20% for inverter losses and power factor losses.  Even if DC, you should discount at least 5% for wiring and other losses.

240V * 14A * 6h = 20.16 kWh DC load (ideal)
240V * 14A * 6h / 0.95 = 21.22 kWh DC load (with losses)
240V * 14A * 6h / 0.8 = 25.20 kWh AC load (with losses)

If your panel produces 34V x 3.53A = 120W at full sun and you receive 3.4 hours of full sun, then to produce enough to sustain your load, you would need:

0.120kW * 3.4h * X panels = 25.20 kWh
X = 25.20 kWh / 0.408 kWh = 61.76 panels

And if you assume 80% efficiency in the panels themselves, then:

61.76 panels / 0.80 inefficiency = 77 120W panels for an AC load
21.22 kWh / 0.408 kWh / 0.80 = 65 120W panels for a DC load

That of course assums that 3.4h is the average and that you smooth out the fluctuations with batteries.

To me, this seems like quite a few panels.  You might want to try 210W panels instead and also look into making your load more efficient if possible.

If you could trim 20% off of your load and used 210W panels, you could do it with:

20.16 kWh / 0.714 kWh / 0.80 = 36 210W panels for AC load
16.98 kWh / 0.714 kWh / 0.80 = 30 210W panels for DC load

Remember that the more panels you need, the greater the mounting and wiring costs will be.  You should use the highest capacity panel you can, use DC loads where possible, and make conservation your highest priority.

I'd much rather be mounting 30 panels than 77.

7 Posts
Mar 22, 2011 10:23 am
Re: How many pcs of the 120W solar panel do I need?

And 34V is the solar panel opeating voltage.

But what's wrong in the logic of the calculation?

I used the Equation of Power = voltage X current:
Power need to be created in the solar panel = 34 X 24.7 = 840W

Is the figure 24.7 wrong? ("current need to be created in the solar panel")

I calculate the 24.7 based on the following logic:

Current capacity of the loading per day: 14A X 6Hr = 84AH
Current need to be created in the solar panel:
84AH/3.4hr = 24.7A

1) Could you pls kindly point out that which part in the logic have problem?

2) Would it be too long for the payback period either 77 pcs 120W or 36 pcs 210W for the 25.2KWH AC loading?

Thanks and Best Regards,
Eric Lee

462 Posts
Mar 22, 2011 11:18 am
Re: How many pcs of the 120W solar panel do I need?

Okay, you need 84 AH from a panel that needs to produces 24.7 amps. Unfortunately, one, 120 Watt panel only produces 10 amps operating at 12 Volts or 5 amps at 24 volts. (you say 34 volts, using that, 4 amps.)
So you would need 3, twelve volt panels to cover your needs. (or 5, 24 volt ) Now figure those numbers into your calculations.
Or just believe the other two post and learn from their examples. There is also many calculators out there to help you. One here at AltE.

26 Posts
Mar 22, 2011 01:49 pm
Re: How many pcs of the 120W solar panel do I need?

Is the figure 24.7 wrong?

Yes, it's wrong.  24.7A @ 12VDC is a lot different from 24.7A @ 240VAC.  Think of it as the difference between the power contained in water gently flowing through a babbling brook versus the same volume of water shooting out of a fire hose.  You can't use the brook as your power source in order to pressurize the fire hose even if the volume of water is the same.

It is best to convert it to kilowatt-hours first and then to amps at the lower voltage.  This is what I did in the math I showed you.

Would it be too long for the payback period either 77 pcs 120W or 36 pcs 210W for the 25.2KWH AC loading?

That depends on a lot of factors, such as if you consider other benefits besides monetary in terms of "payback".  E.g. independence, self-sufficiency, fault tolerance, saving the planet, sticking it to the man, not having to run grid power to a remote site, inflation/hyperinflation, ability to outlive catastrophic social collapse, etc.  If you're just trying to save a few bucks, then yeah, it's probably not worth it for that kind of load to replace perfectly good grid power with solar power unless you're getting every possible rebate and incentive.  Your money would probably be much better spent making your load much more efficient.  Then at that point, it might be worth thinking about going solar again.

If you describe your load, maybe we can offer suggestions on how to make it more efficient so that you could power it with fewer panels.

7 Posts
Mar 22, 2011 11:52 pm
Re: How many pcs of the 120W solar panel do I need?

Dear Mr. Tom Mayrand and Thomas Anderson,

Thanks for you pointing out that the current in the 120W 34V solar panel is  = 120/34 = 3.529A

And right now it need 24.7A, so the no. of solar panel needed should be = 24.7/3.529 = 7 pcs in Parallel, right?

So the total no. of the 120W solar panel to supply to this system (14A, 240V) for 6 hrs is 7 pcs?

This is of course not the answer as you have discussed so far, but pls kindly help me to figure out the key point or in which part there is error in the calculation......

I understand there is a difference in the 240V AC and 12V DC, but in which step I have lost in this calcuation can that can compensate this difference, pls show me....

thks.

99 Posts
Mar 23, 2011 01:33 am
Re: How many pcs of the 120W solar panel do I need?

Until you convert to kWh, you're not taking into account all of the factors relating to the power you need to produce.  You need the volume of electrons (amps), pressure of electrons (volts), and duration of flow (hours).  Without all three components, you will never arrive at a correct value.  You cannot compare amps to amps without accounting for volts and hours.  If you insist on handling these values independently without converting to kWh (the easy way), this is how the equation looks:

240V * 14A * 6h * 1 your load = 34V * 3.529A * 3.4h * x panels = (dividing both sides by common units)

Now if you want to figure out how many amps that is in your panels, it's 49.42 * 3.529 = 174.4A @ 34V.

To derate it for 80% panel efficiency: 49.42 / 0.8 = 61.775 panels
To derate it for 80% load efficiency: 61.775 / 0.8 = 77.22 panels

I hope that makes sense now.  You need to account for all components of the power.  You can't just drop the difference in voltage, amperage, or time.

7 Posts
Mar 23, 2011 08:26 am
Re: How many pcs of the 120W solar panel do I need?

Dear Mr.Thomas Anderson,

I understand now.  Thanks for your help.

Best Regards,
Eric Lee

462 Posts
Mar 23, 2011 11:48 am
Re: How many pcs of the 120W solar panel do I need?

Eric, it can get confusing. Especially when dealing with units associated with electricity. Just remember it is no different than any other calculations using ft^2 and such. Just know what the units are and what they become when combined, eg amp x volts = watts, and trust them. Follow math rules and the answer should come out the same no matter how you calculate it. The addition of hours can also make it confusing, which is why sometimes it can be easier to do your figuring based on an hours production, then do the time thing last. Glad you got it.
Hopefully you also learned why a lot of people do not go solar when it comes to PV. If someone personally cannot calculate a system size properly, then find out that the system they need is way to big and expensive, they often times opt out. As with most anything, dedicated PV systems can be beneficial in some cases and in others not.

99 Posts
Mar 23, 2011 03:54 pm
Re: How many pcs of the 120W solar panel do I need?

It can certainly be disappointing if you think you need 5 solar panels and it turns out you need 77.  The cost/benefit clearly becomes more onerous.  But this is why it's extremely important to make your loads as efficient as possible before considering powering them with solar.  Not only for the cost of the panels themselves, but also the charge controllers, inverters, batteries, and everything else.  The fact is, unfortunately, that 14A @ 240VAC for 6 hours is a LOT of power.  My entire off-grid house only uses that much power after 3 days!  I generate it with 6 210W PV panels, 4 75W PV panels, and a 1000W wind turbine.  I wouldn't want a much bigger system than I have and certainly tripling its size would be out of the question.  That's why I made sure my home only uses around 25%-30% of the power of a typical American household.

1 Posts
Mar 24, 2011 02:18 am
Re: How many pcs of the 120W solar panel do I need?

A.240V * 14A * 6h / 0.92 = 21.913kWh DC load (with losses)

21.913kWh/3.4=6.445KW

6445W/120W= 53.7(54 SOLAR PANELS)

B.240V * 14A * 6h / 0.80 = 25.2kWh AC load (with losses)

25.2kWh/3.4=7.4118KW (7412W)

7412W/120W=61.766(62 SOLAR PANELS)

Wish you success in solar power generator system.

Hi,it's David from china,we produce SOLAR PANEL,SOLAR POWER GENERATOR SYSTEM，as well as GEL AGM battery.welcome to our own factory in shenzhen china.

Bt rgds.

Shenzhen Sacred Industry Co .Ltd
David
Sales Manager

ADD：5C，Tianzhan Building，Tian’an Cyber Park Futian District，Shenzhen，China.  Zip:518040
Tel: 0086 755 82507899-Ext:8033
E-mail:    david @ sacredsolar.com
Skype:     solarpanelcn

« Last Edit: Mar 26, 2011 08:56 pm by Sascha Deri »

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