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noteven

Solar panel real world output

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I am an electric dummy. I have no skooling, credentials, degrees, or any quantity of the smarts for pretty much anything. I don't have a lot of interest is tech projects. 

If I take the advertised wattage of a good quality "100 watt" panel for example and cipher like this:

100/12v = 8.3amps/hr in the test lab x .5 = 4.1 amps/hr real world?

 

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You will actually see a little higher than 50% in the real world, but it will depend on many factors.  I have six panels mounted flat on my 5th wheel and have seen nearly 80% of the rated output from them at peak sun mid day.  Remember also that there are really only 5 solar hours in a day that you will see that percentage of peak output, so that also needs to be factored in.  You will get some output outside of the 5 solar hours, but it will be well below the percentage of actual that you see during peak hours.

If you are just looking for a number to plan out your system size, I would probably use 70% of rated output rather than 50%.

Edited by Chad Heiser

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Good question noteven, FWIW with my flat rooftop mounted panels (715 watts) on a sunny overhead day I can harvest with an MPPT Controller typically like 70% + of the rating, but once the sun dips and/or a cloud appears its, of course, down to maybe 50% subject to sun and angle. I agree with Chad as far as planning, 70% for 5 hours is a start, but if you have a full day of rain or no sun better factor that into your design parameters.........Generator time????????????

 

John T  Live from Austin Texas

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The realized output depends greatly on the time of year, your latitude, and whether you tilt your panels.  Of most concern is probably the total watts produced by the panel per day (i.e. amps x volts integrated over the period of a day).

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And with the price of Solar Panel's dropped to where they are now, adding more then you think you need is always nice. Better to have too much, then not enough, within reason. 

I spent ~$575 more when we did our project to have five 240W panels vs the three that would have probably covered us. (Of course now, GT 300W panels are available too.). 

Best of luck to you,

Smitty

 

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Supplemental question - what output advantage is given by tilting  panels in the SW US during winter? 

Latitudes S of Las Vegas for example.

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About 9 years ago I installed 1 single 120 W panel on the roof of our trailer.  The attached graph was recorded at the SKP park near Congress, AZ on two successive days and not nearly when the sun would have been at its lowest angel.  When tilted the panel was tilted directly to the south at a tilt angle that made it normal to the sun at noon.  Hope this give you some idea of what to expect.

 

panel output (2).jpg

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noteven, obviously you harvest more if the panels are kept better tilted towards the sun (all subject to location and time of year) BUT THE QUESTION BECOMES the time and effort and mounting and mechanics and structural strength IE is it worth it ????????? If so go for it, if not (what I chose) I just added more flat mounted panels. Now, if you're talking about small portable fold up suitcase panels (easy to keep pointed best) that's different then if you're talking about mounting panels to an RV.  

 

John T

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So, how do you tell when you have too much? The not enough is easy. Things stop working because there is not enough power. I'm assuming excess heat comes from too much output and where it will go is my question.  With an RV feeding excess power back to the "Grid" is not an option. (at least not yet) that I know of. 

Anxiously waiting for the day when I can fill my roof with panels and have everything I need and want powered where ever I may be. Then it will be how to collect enough water. That may be more difficult in certain areas. 

 

Rod

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6 minutes ago, lappir said:

So, how do you tell when you have too much? The not enough is easy. Things stop working because there is not enough power. I'm assuming excess heat comes from too much output and where it will go is my question.  With an RV feeding excess power back to the "Grid" is not an option. (at least not yet) that I know of. 

Anxiously waiting for the day when I can fill my roof with panels and have everything I need and want powered where ever I may be. Then it will be how to collect enough water. That may be more difficult in certain areas. 

 

Rod

If you have 400AH of battery, then the minimum solar should be 400 watts.  600 watts would be preferable.  If you have the space a pair of 300-350 watt solar panels work really well.  The dimensions are about 4.5 to 5 feet by about 6 feet.  That pretty much covers the roof of an RV from side to side, while still leaving a small amount of room to walk around.  You need to be able to access the solar panel, to clean them off from time to time.  Dirt on the panel reduces the output.

As far as excess power beyond what is needed to bring your batteries to 100%, that is why you have a solar controller.  The controller works just like the three stage charger you have to charge your batteries from shore power or generator.  That is, the controller senses the SOC (State of Charge) of the battery and stops charging it when it is full.

However this is getting the cart before the horse. 

You first need to determine you power needs and the space you have to put your batteries in.  

You don't need 400AH of battery (4 golf cart batteries) to run your lights, laptop computer, over-the-air TV on a LED flat panel TV (satellite TV receiver uses quite a bit of power) and a furnace fan running for up to 1-3 hours in a day. 

If you are going to run a toaster, microwave (to heat things up, or bake a potato), coffee maker, satellite TV receiver, multiple computers, etc, etc, you will need 400-800AH of battery or more. 

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3 hours ago, Al F said:

You don't need 400AH of battery (4 golf cart batteries) to run your lights, laptop computer, over-the-air TV on a LED flat panel TV (satellite TV receiver uses quite a bit of power) and a furnace fan running for up to 1-3 hours in a day. 

 

This is typical of my power needs minus the furnace run time. 

Some basics I am understanding are:

Connect loads direct to D.C. If possible

Size large wiring

Charge batteries at the correct voltage per their manufacturer. 14.8 would be ideal for my 2 6v's. My PD9200 converter puts out 14.4...

space components where they belong in relation to batteries etc

have a "Trimetric" - it's your fuel gauge. Maybe even install this first to understand how the rig uses power?

Thanks for the info so far. 

I'm listening 

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5 hours ago, lappir said:

So, how do you tell when you have too much? The not enough is easy. Things stop working because there is not enough power. I'm assuming excess heat comes from too much output and where it will go is my question.  With an RV feeding excess power back to the "Grid" is not an option. (at least not yet) that I know of. 

5 hours ago, Al F said:

 

As far as excess power beyond what is needed to bring your batteries to 100%, that is why you have a solar controller.  The controller works just like the three stage charger you have to charge your batteries from shore power or generator.  That is, the controller senses the SOC (State of Charge) of the battery and stops charging it when it is full.

 

So then the "too much" then becomes accessible & available power to use, just as a home is powered but needs a switch to be thrown to turn on the light.

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1 hour ago, rm.w/aview said:

So then the "too much" then becomes accessible & available power to use, just as a home is powered but needs a switch to be thrown to turn on the light.

Yes.  If the solar panels you have are large enough and there is enough sun to put out 20amps of 12V DC through the controller to the battery and the controller only needs 10amps for battery charging (because the battery is getting close to 100%), then the controller will only be using 10 of the 20amps available.  When you plug in something or turn on a fan, or light, then the controller will proved more amps up to the max the panels can put out. 

 

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3 hours ago, noteven said:

This is typical of my power needs minus the furnace run time. 

Some basics I am understanding are:

Connect loads direct to D.C. If possible

Size large wiring

Charge batteries at the correct voltage per their manufacturer. 14.8 would be ideal for my 2 6v's. My PD9200 converter puts out 14.4...

space components where they belong in relation to batteries etc

have a "Trimetric" - it's your fuel gauge. Maybe even install this first to understand how the rig uses power?

Thanks for the info so far. 

I'm listening 

You have pretty much nailed it. 

Yes, to install the Trimetric as a first step would be excellent.  Knowing your usage so you can size your batteries and solar panels is the way to go. 

Some additional reading for you about RV electrical, batteries, solar:

The 12volt Side of Life (Part 1) Batteries

The 12volt Side of Life Part Solar & Inverters

RV Dreams Electrical Systems

The three links above have lots of basic info.

For excellent advice about what NOT to do:

HandyBob's Blog Solar & Elect

Getting into more details, especially for designing solar systems.

Jack Mayer RV Electrical

 

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8 hours ago, lappir said:

So, how do you tell when you have too much?

《snip》

I'm assuming excess heat comes from too much output and where it will go is my question.

There is no excess heat.  A solar panel sitting in the sun but not connected to anything just sits there.  It doesn't overheat, self destruct or do any other nasty things.

On the most basic level, the solar controller simply connects the solar panel to the batteries when power is needed, then it disconnects the panel when the power isn't needed.  

There's nothing to worry about if you have more panels than you need, as long as the total wattage is within the power handling capacity of the controller.  It will simply turn the panels on or off as needed.

Edited by Lou Schneider

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Thanks everyone for their responses. I have almost 42 foot of unused roof space. I have three waste tank vents and that is all. I would like to be able to run a system similar to the system Steve Dixon has done with a "Volt" battery system , but I want it more plug and play than  what he has had to do.

 

Still in the information gathering so I apologize if the question was too basic. I know someone who explained it differently to me with some residential systems where Emergency Services personnel are trained the panels are always producing electrical current with any form of light and are only considered safe when covered or in complete darkness and there are no exposed wires. If power is being produced it has to go somewhere, I was just wondering where it goes when the batteries are at capacity and there is no power needed at that specific time. I guess that's where the power need vs power available calculations need to take into account the time of year, ambient temperature, if it has rained or is raining and a ton of other information. Am I making too big a deal of this? These are the questions in my mind that I cannot get past to actually start the project. 

Thanks again. 

I will start another post when I am ready to really start. Now back to the original topic. It seems that I may have hijacked. 

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1 hour ago, lappir said:

Thanks everyone for their responses. I have almost 42 foot of unused roof space. I have three waste tank vents and that is all. I would like to be able to run a system similar to the system Steve Dixon has done with a "Volt" battery system , but I want it more plug and play than  what he has had to do.

 

Still in the information gathering so I apologize if the question was too basic. I know someone who explained it differently to me with some residential systems where Emergency Services personnel are trained the panels are always producing electrical current with any form of light and are only considered safe when covered or in complete darkness and there are no exposed wires. If power is being produced it has to go somewhere, I was just wondering where it goes when the batteries are at capacity and there is no power needed at that specific time. I guess that's where the power need vs power available calculations need to take into account the time of year, ambient temperature, if it has rained or is raining and a ton of other information. Am I making too big a deal of this? These are the questions in my mind that I cannot get past to actually start the project. 

Thanks again. 

I will start another post when I am ready to really start. Now back to the original topic. It seems that I may have hijacked. 

I am not seeing a difference between what you are concerned about with solar panels on an RV and the the electrical service coming to a house.  The typical 240V AC power coming to a house is always there  at the main CB panel.  As you say "it has to go somewhere".  If you don't want it coming into the house you turn off the CB. 

Same with the solar controller.  Put a CB on the input to the controller and a CB on the output of the controller. 

Now as far as a house with solar panels.  I can see where emergency personnel would be concerned when the house  solar panels feed power back to the grid.  They would want to be sure the CB from the house to the grid is off before working on the grid.  

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9 hours ago, lappir said:

Still in the information gathering so I apologize if the question was too basic. I know someone who explained it differently to me with some residential systems where Emergency Services personnel are trained the panels are always producing electrical current with any form of light and are only considered safe when covered or in complete darkness and there are no exposed wires. If power is being produced it has to go somewhere, I was just wondering where it goes when the batteries are at capacity and there is no power needed at that specific time.

It doesn't go anywhere.  The point you're missing is electricity has to flow through a circuit to produce power.  Interrupt that circuit (i.e. open a switch) so no current can flow, and no power is being produced.  You can have voltage at the power source, but without current flow there's no power.

That's what the solar controller does - it closes the switch and lets power flow out of the solar panels when power is needed, then it opens the switch and disconnects the panels when the power isn't needed. When the panels are disconnected they just sit there until a load is placed across them again.   If the panels are in sunlight they will have voltage on their terminals but without a circuit path (i.e. the switch is open) no power flows out of them.

If you have more solar power than you need, the controller will simply disconnect the panels when the batteries are full and keeps them disconnected until more power is needed.  This happens all the time in some systems, if your design delivers 100% of your need on a winter day, you'll have excess capacity on longer summer days.

What the Emergency Services personnel are concerned about is once a traditional house is disconnected from the grid, all of the wiring in the house can be assumed to be dead.   Solar panels are a second source of power that also has to be disconnected before the house wiring becomes safe.

 

Edited by Lou Schneider

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On ‎11‎/‎11‎/‎2017 at 5:13 PM, lappir said:

If power is being produced it has to go somewhere, I was just wondering where it goes when the batteries are at capacity and there is no power needed at that specific time

lappir, The way my system works is once my batteries become fully charged to 100% SOC, my Smart MPPT 4 Stage  Solar Charge Controller lowers its output to maintain the 13.2 Float Level Voltage supplying just enough current to keep them topped off HOWEVER if I start using energy from my batteries the MPPT sense the load and starts pumping more energy back into them (IF the sun is providing such). A charged up battery has X amount of stored energy but it goes nowhere and isn't being depleted (not withstanding electrochemical action and self discharge) UNTIL a load is connected, current starts to flow, and the stored energy is put to use. If the sun shines on your solar panel a voltage appears across its output terminals but with no load attached and no current flow its similar to a charged up but disconnected battery other then a battery stores energy while a solar panel converts sunlight  into electrical energy.

Batteries STORE energy, Solar Panels are a SOURCE of energy, the Charge Controller regulates how much of the energy gets pumped into the batteries, as and when and how much needed ...

 

John T

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