Measuring solar panel output

[Hamma]

Hi

I was watching a demonstration on one of the stands at the LBS, in which a volt meter was connected to a solar panel and the effects of shadowing were then examined by covering various sections of the panel. I don't recall exactly how much the voltage drop was but it was down from about 13V to say 11V.

There was no measurement of current.

My O level physics lets me down here, because according to W/V=I, current will increase as the V reduces. This does not make sense in the scenario we are describing, so I assume that the Watts is not a static value. I'm guessing it's a theoretical maximum result of the combination of V x I.

At this point I get myself tied in knots.

What I am asking is - what will happen to the current output from an 80W panel if the voltage drops from 13V to 11V?

Is it a good demonstration to only look at the V?

 

[elton]

My O level physics lets me down here, because according to W/V=I, current will increase as the V reduces.

You're wrongly assuming that power output remains constant.

As voltage decreases with shading of the panel, current decreases proportionately.

So if voltage decreases by 10%, so does current.

 

[macd]

according to W/V=I, current will increase as the V reduces.

That's only true if W remains constant, which it doesn't. Why would it? Express it another way, W=VI: if the voltage reduces, so does the output in Watts. In reality both the voltage and amps output would reduce when shadowed.

The voltage figures you quote seem rather low. What was the light source? Where they regulated? Even 13V wouldnt put much into a 12V battery.

 

[rob2]

The voltage figures you quote seem rather low. What was the light source? Where they regulated? Even 13V wouldnt put much into a 12V battery.

True, the demonstration was not really in the normal effective working range of the panel - they probably didn't have a portable sun to drive it! But the proportional effect of the shadow would be the same.

Rob.

 

[Boo2]

What I am asking is - what will happen to the current output from an 80W panel if the voltage drops from 13V to 11V?

Good question.

Is it a good demonstration to only look at the V?

No, the open circuit voltage is not going to tell you much. However if the panel was connected to eg a battery charger and battery or other realistic load then the current drop might be approximately proportional to the voltage drop. Ie, Watts supplied goes like (Vlow/Vhigh)^2.

Boo2

 

[VicMallows]

I was watching a demonstration on one of the stands at the LBS, in which a volt meter was connected to a solar panel and the effects of shadowing were then examined by covering various sections of the panel.

What were they trying to sell? .... a different type of PV panel? ....or perhaps wind generators?

Vic

 

[Hamma]

Thanks,

How rare to find everyone in comprehensive agreement - clearly I am operating at a very basic level here!

The panel was under the lights in Excel, hence the lowish voltage. If it had been under the noonday equatorial sun and reading say 18V, would the drop have been more significant when he placed a brochure over a portion of it?

My point is that I was surprised that it didn't have a more pronounced effect, having read that PC panels are very sensitive to shading. I wondered if the demonstration gave any useful information.

Finally - this 70W panel was about £700. Ebay panels are about half that - is the gap all margin or do you in fact get what you pay for?

 

[jordanbasset]

£700 for 70w seems a lot of money, I bought two of these 100w panels -
http://www.ebay.co.uk/itm/100W-Mono...ervan_Caravan_Accessories&hash=item1e63fed57c
Only fitted last year but so far so good, at £160 each willing to take the risk

 

[TQA]

If the solar voltage drops below the voltage of the battery the 'output' will be effectively zero and no charging will occur.

I am always amazed to see some expensive solar panels fitted with something above them giving consistant shade.

However all is not lost. The modern approach is a solar panel with an output of around 28 volts and a really clever controller which converts this to a suitable voltage and a higher amperage.

Such systems will still produce a charge even when partially shaded.

 

[PITCAIRN]

Still a bit confused with all of this - I have a 24v battery house bank and am looking to install Solar panels for the first time. Bearing in mind W/V = I, then on a 24v system one will need a panel twice the Watts as one needs on a 12v system in order to produce the same Amps : A 200w panel/24v = 8.33 amps, A 100w panel/12v = 8.33 amps : GIVEN THE SAME SUNSHINE/TEMPERATURE etc.
Since the object of the whole excercise is to recharge the batteries, i.e. put back into them as much Amps as possible, does it not make sense to have a 12v panel connected to an MPPT charge controller which then 'steps up' the voltage to say 27v (which necessary to recharge the 24v house bank ?)
I have been told so many conflicting things e.g. to recharge a 24v system one must use 24v panels and to recharge a 12v system one must use 12v panels.
But others have said there is an MPPT controller which will do the 'step up'. Obviously 12v panel s are a lot cheaper than 24v panels ...
what about using two separate 12v , 100w panels connected sparately to two MPPT controllers which then all connected to the house bank.?
Thoughts ??

 

[ianj99]

Still a bit confused with all of this - I have a 24v battery house bank and am looking to install Solar panels for the first time. Bearing in mind W/V = I, then on a 24v system one will need a panel twice the Watts as one needs on a 12v system in order to produce the same Amps : A 200w panel/24v = 8.33 amps, A 100w panel/12v = 8.33 amps : GIVEN THE SAME SUNSHINE/TEMPERATURE etc.
Since the object of the whole excercise is to recharge the batteries, i.e. put back into them as much Amps as possible, does it not make sense to have a 12v panel connected to an MPPT charge controller which then 'steps up' the voltage to say 27v (which necessary to recharge the 24v house bank ?)
I have been told so many conflicting things e.g. to recharge a 24v system one must use 24v panels and to recharge a 12v system one must use 12v panels.
But others have said there is an MPPT controller which will do the 'step up'. Obviously 12v panel s are a lot cheaper than 24v panels ...
what about using two separate 12v , 100w panels connected sparately to two MPPT controllers which then all connected to the house bank.?
Thoughts ??

I can't see why you can't use two 12v panels in series. Same current, but twice the voltage. So if each was 8amps (~100watt) then its equivalent to one 200watt 24v panel. (ie the same 8 amps but at 24v)
You cannot step up (or down) voltages at 100% efficiency so its a lossy way of doing it.

 

[TQA]

Still a bit confused with all of this - I have a 24v battery house bank and am looking to install Solar panels for the first time. Bearing in mind W/V = I, then on a 24v system one will need a panel twice the Watts as one needs on a 12v system in order to produce the same Amps : A 200w panel/24v = 8.33 amps, A 100w panel/12v = 8.33 amps : GIVEN THE SAME SUNSHINE/TEMPERATURE etc.
Since the object of the whole excercise is to recharge the batteries, i.e. put back into them as much Amps as possible, does it not make sense to have a 12v panel connected to an MPPT charge controller which then 'steps up' the voltage to say 27v (which necessary to recharge the 24v house bank ?)
I have been told so many conflicting things e.g. to recharge a 24v system one must use 24v panels and to recharge a 12v system one must use 12v panels.
But others have said there is an MPPT controller which will do the 'step up'. Obviously 12v panel s are a lot cheaper than 24v panels ...
what about using two separate 12v , 100w panels connected sparately to two MPPT controllers which then all connected to the house bank.?
Thoughts ??

Notl twice the Watts but twice the volts. Watts stay the same.

In practice you want one of these http://www.ecodirect.com/Morningsta...2-24-Volt-p/morningstar-sunsaver-mppt-15l.htm OR something similar and 3 x 12 volt panels giving 36 volts.

ALSO some people giving advice here have forgotten that we are charging a battery with our output so say the battery is at 24 volts and we have an output from the solar panels of 26 volts we will get amps flowing into the battery [say 10 amps ], however if the output voltage of the panels drops to 24 volts or below then the amps flowing into the battery will be zero.

If the panel voltage drops to 25 volts in the above situation then the amps flowing into the battery will drop by about 50% eg from 10 amps to 5 amps.

 

[William_H]

Solar

For a 24v system you want 2 "12v" panels connected in series. The size of the panels is dictated by your electric needs. Bearing in mind that a 24v system uses only half the current of a 12v system. Hence individual batteries can be smaller (less AH).
A "12v " panel is so called because it is designed to charge a 12v battery. The actual output voltage with no load is around 19 to 20 v. So for a 24v system the 2 "12v" solar panels in series will give no load 38 to 40 volts. Don't let this voltage worry you as the voltage will fall to that of the battery with current flow from the panel.
The watt rating of the solar panels is a bit misleading. It is the product of the max voltage times max current. So 20 watt panel will give max 1 amp which into a battery at 14v on charge will be only 14watts.
True an MPPT controller will convert some of that excess voltage into more current and run the voltage/current regime at max power point. However consider that for a small panel system you could buy extra solar panels rather than the MPPT controller. Assuming you have room for the panels.
So to Pitcairn I would suggest something like 2 X20 watt panels wired in series with no regulator straight to the battery 24v +ve and -ve. This should keep your reasonable sized batteries charged against natural discharge and give you some recovery from use. (not much). You will get 1 amp into the batteries for about 8 hours maybe per day. This depends on weather, location and the panels being aimed at the sun.
One amp will probably easily provide a family with (LED) lighting while camping on the boat. It won't drive a VHF turned on continuously or a heater or fridge.
You would need much more solar power for running fridge heater etc.
Tip... You may get a good deal on a household solar PV panel which often run at 40 volts in one panel so good for your 24v battery typically at 5 amps max. But pretty big to mount and it will need a regulator (cheap sort).
good luck olewill