Solar panel advice please

I would have thought 5 to 10 Watt should be enough to keep the systems happy. 

Cheers HB,

That's what I have read, but I have also read that 60 watt is the lowest to consider! Taking into account lack of sun in the winter.

So you see my dilemma!

GC there is no perfect answer. If all you want is to keep a battery charged, then a smaller one will do it. I see you want to run an alarm and tracker. I would say that a 60 w is minimum. What you have to do is consider the shortest darkest days- like right now.  I have a 135w roof mounted SP and that feeds 2 x 110ah batteries. I have an alarm and tracker. The batteries are always fully charged in the winter and in the summer produces enough electricity for me to go off grid and have everything inc my microwave, toaster etc. I would suggest that a 100 w is the minimum! It is far better to have too much electricity being generated rather than too little.

Thanks Merve, 

So 60watt minimum?

Anyone else?

I suggest that you compare costs, which is what I did and I found that a 100w unit was very little greater cost than a 60w, so I have brought a 100w unit. 

Today I think the rational is basically they are so cheap that physical limitations aside you might as well buy a unit that is for most of the year an overkill.

If you want to winter camp off an EHU then here in the UK you are going to be severely challenged with anything of a viable physical size and weight; considerably more so if mounted horizontally on a roof out of the way.

A great deal contrives to work against you;

• The demand for power is high.
• The hours of effective light are low
• The sky can be more overcast.
• The sun angle is low. This means the incidence angle on the horizontal panel is way off optimum. Tree etc shadows are much more likely to hit the panel.[these yield a much bigger loss than a prorata effect]

Winter because it is colder improves the panels efficiency and we can have some crystal clear days but on balance you need to fall very lucky to do even adequately in winter [say Nov through till end of Feb] 

Historically I have thought 40 Watts was just about okay for Mr & Mrs average summer campers; going bigger being solely restrained by the then cost.

Now, for a fixed panel I see little point in buying less than 100 Watts, its price size and weight all stack up as viable and it would give adequate main season yield for all but the power profligate campers.

 
Buying a decent controller is though very important. I have tested some that’s performance is awful. Both in losses and high parasitic night time battery drain. Reportedly there are a lot of cheapo fakes out there and particularly mis described MPPT technology units as these are the expensive high performance type. To gain the better yield of a MPPT controller you need a panel outputting a lot higher voltages than the 20 odd volts that are designed for use with PWM technology controllers.

I buy and recommend Morningstar PWM controllers as they work very well for me and where I have helped others and interface correctly with the 20 odd volt standard, our sized solar panels aimed at the camping market. Things totally change for fixed domestic system using far greater areas of panels. If spending less than 35% of the pa\nels cost on the controller, then know what you are doing or invite issues.

Thank you oscid, very good post,

However, I will stress again, that I have no plans, ever, to fix a solar panel to the van, it will never be transported in the van, as we always use EHU, My plan is to mount the panel on our utility roof, next to where the caravan is parked, and just hook it up when we return.

But I totally agree with you the comments on size and price, I have noticed that the price jump from say 40watts to 100watts is considerable. And I have taken note on your comment on which controller to use.

Many thanks.

Dave.

Interesting discussion. I didn't want a fixed panel on the roof either so this is what I did to keep my battery charged in storage:

• I bought a 30W panel. I had tried a 10 watt Maplins panel in the past but it wasn't up to the job in keeping the battery topped up and alarm functional.  30W in my view is enough even in the winter.
• I put a 30W panel on the caravan roof placing/removing it by hand through the skylight. To keep it stable I put two Velcro strips on the roof and on the panel to prevent any movement in high winds.
• I ran a 5A cable just past the edge of the skylight to a plug.
• The other end of the plug cable was connected to a discretely placed solar controller and then  (fused) to the caravan battery.
• I just remove the panel, roll up the cable and put in a bed box when travelling or on site.

It works perfectly. I hope this helps.

Bob D.

The only sure fire way of getting a good idea of what size panel you need for a winter layup is to measure your current drain from your battery and use this figure to come  up with what is required. The risk you run of fitting too large a panel without a regulator is you will overcharge (cook) your battery. If getting a good idea of what is required is too difficult I would use an oversized panel fitted with a regulator. It is almost certainly your safest option. I would consider anything greater than 10 to 20 watts oversized

peedee

Even knowing the drain, then having any idea of a panel's yield during the winter is a huge gamble and few winters will be the same. I know its crude but you only need a bird to dump onto the unattended panel and its yield will be but a fraction of what it could have been.Wind blown leaves in locations with trees do just the same if even one gets stuck to the panel.

Whilst with things stacked in your favour a 10 Watt could be adequate the more reserves the better in winter if you want to walk away and forget about it. However,  you could write off a battery if things were not going your way for a month or so, the more so with a high drain controller.

I would work on what is practical and affordable if that is 40 Watts then given a decent controller then that is a reasonable solution. Given a decent controller even 100 Watts is not over the top, just probably more costly than needed. So much depends on a winter locations site and how shadows throw, plus the frequency you can check the panel is clean.

If you can prop the panel near vertical facing due south, off the ground vegetation and other shadows then it will be more self-cleaning and potentially increasing the real world winter yield. Summer presents nothing like the challenges or size of panel needed.