Solar Panel

[john_morris_uk]

Can any one tell me what Compass means when they say the out put of a solar panel is in Wh/d I can guess at watts/hour/day - but is that average daylight? Maximum sunshine? Whats the criteria.

I really want to know either the watts or current out put of the panel at maximum sunshine so that I can make a guess at what amp hours I will achieve in a normal day, or some other reasonable guide as o the power of the things! Wh/d is not a standard measure to my knowledge.

 

[Searush]

Why not ask them, as they made it up?

It sounds to me as tho they are trying to do exactly what you are asking by making up a typical daily output. But mid winter days are shorter than mid summer, rainy days are darker & the angle of the panel to the sun is important too, as is shadow across the panel from sails, mast or adjacent quayside buildings or walls.

Basically it is all a "how long is a piece of string" question. Max daylight output is probably the only measureable parameter that has any meaning.

 

[john_morris_uk]

Why not ask them, as they made it up?

It sounds to me as tho they are trying to do exactly what you are asking by making up a typical daily output. But mid winter days are shorter than mid summer, rainy days are darker & the angle of the panel to the sun is important too, as is shadow across the panel from sails, mast or adjacent quayside buildings or walls.

Basically it is all a "how long is a piece of string" question. Max daylight output is probably the only measureable parameter that has any meaning.

Fair enough. I was going to phone, but thought I might get a spotty trainee who I would have to be polite to over the phone.

An e-mail might help - and I will let people know their reply.

I wish people wouldn't make up units.

 

[VicS]

The figure in the column to the right (WP) is the max output in watts. If you go to the manufacturers website it should tell you what the panel terminal volts will be at that power output.

I dont know which panels you are looking at but if you go to the Solara.de website you will find that
Wh/d is the average daily output in summer in Germany !

 

[Mistroma]

VicS makes sense as the value will change from one place to another and summer is a reasonable time to be using solar power. Peak W is a more common figure to let you compare like with like. Of course you won't get peak output all day every day.

I had a look at this recently and panel outputs are usually derived from 1000W falling on 1m2 of surface. Average at any location will always be less than that. So you will never achieve the rated output over any length of time. i.e. It will be low in morning & evening but high near mid-day.

A bit of a conundrum and I initially used some guesswork to estimate likely output during the day. However, I did find useful historical data (1983-1993) for europe. So if anyone wants to check my rough calcs. then read on. I'd be happy to have big errors pointed out (figs. below are rounded up/down a bit).

http://lightbucket.wordpress.com/2008/02/24/insolation-and-a-solar-panels-true-power-output

Table 1 gives useful information
i.e. Insolation levels for European cities – monthly and annual averages (Units are watts per square metre)

The figures didn't make much sense initially but then I realised that they included night as well as day (monthly average). It sounds silly but I suppose quoting it that way makes it easy to derive a theoretical Ah output each day.

e.g. Lisbon in July reads 287 Watts/m2

So in any day in July that would be around 287 Watts falling on a 1m2 panel
As it's a monthly average it includes nights as well (sounds odd but makes sense in context of the table data).

So the daily Watt hours per day is ~6888 (287 x 24) Wh each day.
Assuming the battery system sits around 13.8V whilst charging then that gives ~500Ah/day.

Great, but that's assumes a 100% efficient system and it is more likely to be 10%-12% giving 50-60 Ah each day from a 1m2 panel.

I have a 65W panel that's around 12% efficient with an area of .56m2 and that would be 32 Watts produced all through that month.

So manufacturer quotes peak output of 65W but in Lisbon during July I'd be more likely to get 32W (i.e. Around half).
In practice there are other losses and shadows etc. so it might be more like 25W in practice (but much higher if panel is moved and angled to follow the sun).

 

[whiteoaks7]

PBO Summer edition 2011

... has a buyer's guide to solar panels that gives direction to all the relevant data with examples on how to calculate the amount of power and energy you can get anywhere in the world in any month.

 

[pappaecho]

Unfortunately all the published figure are based on "ideal" or lab conditions.

On a yacht a whole new set of parameters comes into play. Firstly shading.
A small amount of shading has a large effect on the output of the panel(s)

Angle of inclination. If the panels are struck by insolation 90 degrees in summer it will be less than 60 degrees in winter with concomitant loss of efficiency

The angle of horizontal inclination is similar to above so early and late sun will be so oblique as to be useless.

The boat will either be in fixed position on a pontoon or similar or swinging on a mooring. The swinging option means that you have really no idea ona daily basis where the boat will be heading.

So the solution is to find out other users experiences, approximately sizing panels and then taking the plunge.

I treated my new old boat to an 80 watt panel, which charges six batteries in 3 banks using an intelligent controller. In high summer the banks got to about 13.3 volts, and in midwinter they dropped to about 12.5 volts, except the (unused) bow thruster bank which stayed at 12.9 volts.

My boat is on a swinging mooring, unshaded, and can be realigned either 45 degrees pointing to the bow or the stern. (Whichever way I point it, will invariably be wrong!)

Good luck