Solar....how much wattage is enough

[WotEver]

It’s actually far simpler than that. Charging voltage is 14.4V, not 12V. There will also be some degree of loss through inefficiencies in the controller. 

 

Edited May 29, 2018 by WotEver

[Murflynn]

solar panels are tested under ideal laboratory conditions and the result is a NOMINAL rating;  this will never be achieved in practice - in the tropics the surface temperature will be too high and the efficiency of the panel reduces accordingly, whilst in temperate conditions the intensity of the sunlight is lower than in the tropics so that the amount of energy falling on the panel is less than the theoretical maximum, so the output is less than the nominal.   

 

.......  plus, of course, the actual charging voltage is greater than the nominal 12v stated by the battery industry, as Wotever has said, and there are losses in the controller.

Edited May 29, 2018 by Murflynn

[WotEver]

15 minutes ago, Murflynn said:

solar panels are tested under ideal laboratory conditions and the result is a NOMINAL rating;  this will never be achieved in practice - in the tropics the surface temperature will be too high and the efficiency of the panel reduces accordingly, whilst in temperate conditions the intensity of the sunlight is lower than in the tropics so that the amount of energy falling on the panel is less than the theoretical maximum, so the output is less than the nominal.   

However, on a cool but bright day, with the panels tilted towards the sun, it is possible to achieve max output for a short while in the UK, so the rated output isn’t purely theoretical. 

[Alan de Enfield]

As I have said (many times) previously I find my own 'fudge factor' seems to work in practice and takes real world situations into account (clouds, rain etc.)

 

Average Output :

 

May, June, July, August output based on 50% of rating for 14 hours per day

April & September anywhere between 25% and 50% depending on if it is an early/late Summer for 12 hours per day

March & October 25% for 8 hours per day

November, December, January, February a maximum of 10% of output, more often 5%. for 6 hours per day

 

Taking the above and comparing  to 'actuals' recorded the 'estimates' are slightly pessimistic (but not by much) and I would rather underestimate than overestimate. Getting more than you expected is always nice.

[Theo]

10 hours ago, Horace42 said:

I understand enough for my own satisfaction - and likewise I could say the same about you.

 

You might have got there - The battery size is to do with the demand - and solar panel is to do with charging it. They compliment each other.

but not 40A - do you mean I should have said 40.83333.....  or?

So what current will a 490W panel give

I love that last question (the one without the question mark)!

 

The answer is complex and I, for the sake of those interested, will try to answer some of it.  The answer to the question "How many panels should I fit?" is really simple though and you can skip the guff and go straight to the bit in bold at the foot of this posting.

 

It depends.

Upon what does it depend?

 

1.  The brightness of the sun.

2.  The angle of the plane of the panels to the line from the sun to the panel surface.  The power output (not necessarily the current) varies as the sine of the angle.

3.  How clean the panels are.

4.  The nature of the contaminants on the surface.

5.  The state of charge of the batteries.

6.  The instantaneous power consumed in the boat by the various appliances.

7.  The diffuseness of the incident radiation.

8.  The age of the panels.

9.  The predominant wavelength of the incident radiation.  This in turn depends largely on the time of day and local atmospherice conditions

 

There might be more but those are the ones that spring to mind as definites plus a few that I have dreamt up just now as possibles but wholly unresearched.

 

It's not very helpful to put numbers on it at the but let's assume that you have perfect panels brand new and shiny but working at hlf their rated output because of the conditions prevaling then with stupidly discharge batteries being charged at 12V you would get 490W/2 divided by 12V or a bit more than 20A.  If they are charging the batteries at a more realistic 14.5V then the answer comes to nearly 17A.

 

I am convinced that the answer to designing solar pv systems for boats is not a technical one and runs like this:

 

I will never be able to get enough solar panels on my boat to supply all my needs summer and winter, therefore I will add as many panels as I can with regard to aesthetics, getting under bridges, cost, boat handling, walking on the roof and the need to clean them.  I know that I must supplement that with extra charge sources whatever they may be.

 

There is one further thing as a hint for the non liveaboard.  As I see it as an ex liveaboard, now that I am leaving the boat on its tod for lengthy periods the solar panels are superb for keeping the batteries at 100% and for lifting them the last few percent when you get back from your cruise.  For this you don't need much.

 

Enjoy the thread but try not to be confused by Horace.

 

Nick

 

11 hours ago, Mike the Boilerman said:

 

I've remembered!

 

I think it was graham-m (or something similar). 

You are right!

Edited May 29, 2018 by Theo
To correct typos.

[Robbo]

5 minutes ago, Alan de Enfield said:

As I have said (many times) previously I find my own 'fudge factor' seems to work in practice and takes real world situations into account (clouds, rain etc.)

 

Average Output :

 

May, June, July, August output based on 50% of rating for 14 hours per day

April & September anywhere between 25% and 50% depending on if it is an early/late Summer for 12 hours per day

March & October 25% for 8 hours per day

November, December, January, February a maximum of 10% of output, more often 5%. for 6 hours per day

 

Taking the above and comparing  to 'actuals' recorded the 'estimates' are slightly pessimistic (but not by much) and I would rather underestimate than overestimate. Getting more than you expected is always nice.

Was that tilted or flat, do you follow the sun's path as well or is fixed position?

[reg]

58 minutes ago, WotEver said:

However, on a cool but bright day, with the panels tilted towards the sun, it is possible to achieve max output for a short while in the UK, so the rated output isn’t purely theoretical. 

Which I know as an absolute fact to equal 3.275 minutes per annum ?

In practical terms I have just added panels as and when I could afford them, laid them flat (because it makes sense on my boat), knew enough to go for a half decent mppt controller and upgraded batteries as and when needed and pennies allowed it. 

Didn't bother with much pre calculation just installed them and then left it to magic to give me electrons. 

Seems to of worked out OK. 

 

Edited May 29, 2018 by reg

[Theo]

Don't these solar threads run?!

 

N

[cereal tiller]

1 minute ago, Theo said:

Don't these solar threads run?!

 

N

Depends how Sunny it is ...

[Theo]

9 minutes ago, reg said:

Which I know as an absolute fact to equal 3.275 minutes per annum ?

 

I absolutely agree.  I guessed much the same figure the day before yesterday.  Can we start an opinion poll to gather more data?

[Alan de Enfield]

3 minutes ago, Robbo said:

Was that tilted or flat, do you follow the sun's path as well or is fixed position?

On my Cruiser  - Fixed with the possibility of a slight 'tilt' (on triangular brackets) but I rarely bother and just leave 'flat'

 

The figures also work out reasonably accurately for my sailing Cat but the boom does occasionally obscure part of the panels (3x 100w mounted above the tender) but when sailing the boom is normally out to one side or the other.

 

 

[MtB]

11 minutes ago, Robbo said:

Was that tilted or flat, do you follow the sun's path as well or is fixed position?

 

I think this must be with panels tilted and rotated to point at the where the sun is, because it Alan's figures bear no relation to the performance of any of my solar installations with panels flat on the roofs. 

[Robbo]

1 minute ago, Alan de Enfield said:

On my Cruiser  - Fixed with the possibility of a slight 'tilt' (on triangular brackets) but I rarely bother and just leave 'flat'

Thank you for confirming, my panels when I install will be flat as well so handy to know what they may provide.

[Alan de Enfield]

1 minute ago, Mike the Boilerman said:

 

I think this must be with panels tilted and rotated to point at the where the sun is, because it Alan's figures bear no relation to the performance of any of my solar installations with panels flat on the roofs. 

But you are further South and with more boats sucking up the sunshine each slice of the cake is smaller.

 

(I'm sure that you have previously agreed with the 50% Summer output and less than 10% Winter).

[Theo]

Woudl there be any mileage on combining solar pv and solar water heating? Pipes monted on the underside of the panels to keep them cool and prewarm water for domestic use?

 

[WotEver]

9 minutes ago, Theo said:

Don't these solar threads run?!

What else are we supposed to do in the monsoon season?

[MtB]

2 minutes ago, Alan de Enfield said:

(I'm sure that you have previously agreed with the 50% Summer output and less than 10% Winter).

 

Strictly speaking yes.

 

Less than 1% winter is typical in my experience which I agree, is less than 10%.

 

Peak summer charge current from my biggest installation (560W) is hard to establish as my batts are always full long before midday, but I have seen 15A into 24V.

 

Peak output in mid winter is easier to comment on. I don't think I've ever seen more than about 2A in full sun at 1pm (very rare in Dec). On a typical dull December day I get 0.1A into 24V at 1pm. 

[WotEver]

5 minutes ago, Theo said:

Woudl there be any mileage on combining solar pv and solar water heating? Pipes monted on the underside of the panels to keep them cool and prewarm water for domestic use?

Let us know, Nick.  We look forward to your results ;)

[MtB]

9 minutes ago, Theo said:

Woudl there be any mileage on combining solar pv and solar water heating? Pipes monted on the underside of the panels to keep them cool and prewarm water for domestic use?

 

 

No!

[reg]

20 minutes ago, Theo said:

Woudl there be any mileage on combining solar pv and solar water heating? Pipes monted on the underside of the panels to keep them cool and prewarm water for domestic use?

 

Let's not go there please. 

 

On a similar theme I am intrigued by these BiFacial panels currently available a t Bimbles. 

https://www.bimblesolar.com/solar/individual/LG-390W-bifacial-solar

 

Add this for when above link eventually disappears

Bifacial Energy Yield
LG NeON® 2 BiFacial modules use highly efficient bifacial solar cell with Cello technology..Through the Cello technology, LG NeON® 2 BiFacial can achieve up to 30% more energy than monofacial modules (under optimal conditions)

 

These are approx 6ft 9inch by 3ft 4inch 

Edited May 29, 2018 by reg

[Horace42]

11 hours ago, reg said:

A fair question. As far a I understand it it goes as follows 

490w rating is given as the STC rating, which is an industry wide rating which allows one manufacturers panel to be fairly measured against anothers and is arrived at by measuring under a controlled set of conditions and parameters. 

In practice though you would do your calculations based on normal operating conditions, as a rule of thumb you would multiple your 40a by 0.75 to give a real world figure, which in your example would arrive at 30A.

Basically the published figures are a device for panel output comparison between manufacturers. To calculate for normal operating conditions use the 0.75 multiplier to give a more accurate output figure when trying to establish an array Size.

 

Google Solar Panel STC and NOTC for more info(which is what I did) 

 

I'm happy to be corrected on the above if I am wrong. 

Thank you Reg. What you say makes perfect sense and useful to know. I appreciate you taking the trouble to come back with technical answers.

When I have got a moment I will look at the links above.

Pity the standards were not applied at the outset 30 odd years ago when I had my bad experience.

 

 

[Horace42]

3 hours ago, Murflynn said:

solar panels are tested under ideal laboratory conditions and the result is a NOMINAL rating;  this will never be achieved in practice - in the tropics the surface temperature will be too high and the efficiency of the panel reduces accordingly, whilst in temperate conditions the intensity of the sunlight is lower than in the tropics so that the amount of energy falling on the panel is less than the theoretical maximum, so the output is less than the nominal.   

 

.......  plus, of course, the actual charging voltage is greater than the nominal 12v stated by the battery industry, as Wotever has said, and there are losses in the controller.

Thanks Murflynn. Use and helpful information. Thanks for taking the trouble to explain.

3 hours ago, WotEver said:

However, on a cool but bright day, with the panels tilted towards the sun, it is possible to achieve max output for a short while in the UK, so the rated output isn’t purely theoretical. 

Surprise, surprise. Some useful information - instead of insults - keep it up.

[Nightwatch]

Those that have read my posts before will realise that I'm not up to full speed with electrics. 

Finally got round to installing my two 100w solar panels yesterday. Wired up to a Victro MPPT controller. This had been wired by an electrician three or so years ago. Inline fuse etc. The lights on the controller making the correct flashing.

 

i also have a Victron 702 battery monitor, fitted at the same time as the controller. This has done me proud since. Anyway, in sunshine, there was nothing showing on the battery monitor to indicate that there was anything going into the batteries. Just the expected output for the fridge etc. So I left it alone for a rethink. I thought for ages, as the light was fading we found that our lights weren't working. Checked the fuse board and the lighting circuit had tripped. Switched in on and all okay. 

Whilst up and about I tugged on the solar panel connections at the MPPT controller and due to my incompetence one of the connects came adrift. I disconnected the other terminal. That's where I'm at presently.

 

A number of questions.

1. Is the Victron 702 monitor capable of showing state of solar input.

2. If so, do I have to rest the settings. (I have read the booklet, no reference to this)

3. If the 702 doesn't detect solar will I need a separate monitor? Bimble informed me that the 702 should show solar input etc.

4. Finally, what have I done wrong?

Theres possibly a million more questions but I'll save them until later.

 

HELP please.

Matty, if you are reading this and are aware that I've sent a PM to you, I have gathered you are busy as you hadn't got back to me. 

Edited May 29, 2018 by Nightwatch

[Horace42]

2 hours ago, Alan de Enfield said:

As I have said (many times) previously I find my own 'fudge factor' seems to work in practice and takes real world situations into account (clouds, rain etc.)

 

Average Output :

 

May, June, July, August output based on 50% of rating for 14 hours per day

April & September anywhere between 25% and 50% depending on if it is an early/late Summer for 12 hours per day

March & October 25% for 8 hours per day

November, December, January, February a maximum of 10% of output, more often 5%. for 6 hours per day

 

Taking the above and comparing  to 'actuals' recorded the 'estimates' are slightly pessimistic (but not by much) and I would rather underestimate than overestimate. Getting more than you expected is always nice.

Thanks Alan - again - some useful information of practical benefit.

[Robbo]

14 minutes ago, Nightwatch said:

A number of questions.

1. Is the Victron 702 monitor capable of showing state of solar input.

2. If so, do I have to rest the settings. (I have read the booklet, no reference to this)

3. If the 702 doesn't detect solar will I need a separate monitor? Bimble informed me that the 702 should show solar input etc.

4. Finally, what have I done wrong?

 

1 - The V702 is for monitoring in/out of the battery bank, it will include all charging sources including solar, but it's not really for monitoring solar input.

2 - The Battery capacity will always need re adjusting to get the percentage right. - it's easier just to ignore this view.

3 - Depending on the Victron MTTP model (let us know) you can have another display similar to the V702 for showing pure solar stats (ie panel voltage). - https://www.victronenergy.com/panel-systems-remote-monitoring/mppt-control  -  It will give you additional information over the V702 on what the solar is doing, but you don't really need it - just a nice to have.

4 - Dodgy connection is the plausible answer.

Edited May 29, 2018 by Robbo
Rekindled my answers...