Multiple solar panels to a common bus bar?

[Hoohaa]

Originally Posted by Wotname

OK, those panel figures are at the best, aspirational but perhaps better put, bull-dust.

Let’s go back to basics: the total power you will get is dependant on the area of the panel; it’s efficiency and the amount of solar radiation you are receiving on the day. The area of the panel is fixed; the efficiency varies with temperature (and to some extent, the amount of radiation); the solar radiation is dependant on location, time of day, and atmospheric conditions (cloud cover, dust etc).

From Wikipedia - https://en.wikipedia.org/wiki/Solar_irradiance Average annual solar radiation arriving at the top of the Earth's atmosphere is roughly 1361*W/m2. The Sun's rays are attenuated as they pass through the atmosphere, leaving maximum normal surface irradiance at approximately 1000*W/m2 at sea level on a clear day.

Usually panel data is calculated on this figure of 1000 W/m2 with a temperature of 20C. While we don’t know how the data of these dodgy panels are specified, we can still run some numbers. Panel area is claimed to be 0.931 m2 but I think they are including the frame and the gaps between the cells, still let’s give them the benefit of the doubt and call it .93 m2. Claimed efficiency is 24% (this is very high and also suspect) but let’s use it. So max power will be 1000W x .931 x 24% which equals 223.2W. The only way to get 350W from this panel is if the solar radiation is 1,568 W/m2 and that is never going to happen until the earth moves much closer to the sun.

However I think the real figure for this panel is more likely to around 170 W. I am using an area of .9 m2 and an efficiency of 17%. Remember these numbers are predicated on 1000 W/m2 and 20C. What is the actual numbers for solar radiation in Sydney? These guys https://en.tutiempo.net/solar-radiation/sydney.html suggest it peaked today (at 1300) with 889 W/m2 so I think the very best you could have expected would have been 150 W at 1300 and 120 W two hours either side of that - assuming it was sunny and the panel was pointing directly towards the sun.

EDIT - the temperatures mentioned above should have been 25C.

Thanks Wotname. My thoughts exactly! Lol
I really appreciate you taking the time to do all that for me. I've checked the Victron app and the watts topped out today at 296 which is pretty much spot on for the Sydney calc.
So if we assume 170 watts per panel then there not even half what's being advertised.
I take your point about the higher voltage panels. My main problem is the width I have available. Which incedently was the main reason I went for these particular panels.
I'll make contact with the dealer tomorrow and see what they have to Say.

[Wotname]

FWIW, I see how they determined the panels are 350W. From the specs on the eBay site, they have used the used the "max power voltage and the max power current". Without knowing more about the testing regime used or seeing proper output graphs, I am not sure this is a legitimate method of calculating output power power although a first glance, it seems appropriate. Unfortunately the output power determined in this manner doesn't tally with other ways of determine the output power and it also doesn't tally with what you are seeing.

Others more versed in solar panel technology might know more; I'm pushing at the limits of my understanding given the scant (and conflicting) data available.

I can't think of a simple DIY way of testing the panel to replicate the stated output V and A. Ideally a 1 ohm, 400W resistor placed directly on the panel would test the stated specs. Point the panel (clean and cool) directly at the sun at midday (on a clear day) and measure the V and A. Look up the solar radiation for your location (using previous website) and adjust for the difference between the actual radiation and the ideal radiation (i.e. 1000W). Getting a DIY 1 ohm 400W resistor is the sticking point, maybe Raymond's bicycle spoke could be cobbled into some useful.

My gut feeling is the numbers on the eBay site need to be taken with a good of salt but ATM, I can't think of a sure way method of disproving them.

@RaymondR, you are a wealth of DIY left field knowledge, what are your thoughts for finding a load that replicate 18V and 19.4A?

[RaymondR]

Hi Wotname,

I have a SS bike spoke habit of many years and they're generally my first port of call whenever I need a very high wattage variable resister (although I have been known to use a 5/64" SS metal thread in a permanent installation such as shunting a wind generator)

In a previous post I suggested alligator clips to connect the flying leads to the bike spoke because the spoke can get very hot however fir temporary connections I generally just bare a couple of inches of wire and wrap it around the spoke and use a screw driver to slide it along to vary the resistance.

By using a variable resistance to short the output of a panel whilst measuring both the voltage across the resistance and an in series current meter as panel voltage concurrent voltage and current readings are available to calculate the power outputs.

If one starts the series of readings with the alligator clips widely separated a high voltage low current circumstance should occur. Then as the clips are moved towards each other and the resistance decreased the voltage reduce and current increase. This circumstance should prevail until the maximum power point is achieved (MPP) and then the power decrease as the voltage further decreases and current continues to increase.

The MPP defined by the experiment should then be that extant for that particular panel in the prevailing sun shine and operating temperature circumstances.

[Hoohaa]

Just a quick update.
I've been in contact with the supplier.
To-date they have been helpful and responsive.
They are waiting for me to supply them some photographic evidence.
I should be back on board tomorrow so will let you know how it goes.
Jeff