How small awg can I get away with?

[cottonpickers]

Hi,

I am trying to set up a 'solar laptop' -ie set up enough solar to power a laptop, so its really a 'hobby' than a serious solar implementation.

I'd like to put the solar panel in the garden and run the wires the long way around to keep the lawn looking nice :-)

I know that I should be looking for no more than 3% drop from panel to battery, but looking at the sizing charts that means I need some pretty serious cables which is what I'm trying to avoid. Even 5% drop means large cables. I don't need to rely on the solar, and I've hopefully overspecced the panel so some losses wouldn't worry me if it means that the garden can stay looking neat and I can still get enough juice for the laptop.

Anyway, heres the specifics:

20. watt polycrystalline panel (I also have 55w available)
    
120. foot from panel to battery (long way around)
     
17. ah battery (also have 100ah available)
    
    

dc-dc laptop converter (kensington)

Dell d420 latop with 68wh battery (& 5 hour runtime)

Latop usage 2-3 hours a day

Based in the UK

The plan would be to leave the laptop always plugged in to the dc-dc converter.  During the day the sun would charge the 17ah and laptop battery up (hopefully)on cloudy days I can always plug back in to the mains or use it less

What gauge wire could I get away with with just the 20w panel? any suggestions? Is there any danger to using thinner cable? (eg awg 14 or 2.5mm in the UK?) would it cause any risks? (I don't understand why the websites I see seem to say NEVER more than 3% drop from solar to battery.  Would changing increasing the solar and battery size to the others I have available make a big difference to your advice?

Hoping for some real world experience and advice :-)

Thanks for your time!

[ghurd]

It could be OK with the 20W PV and #12. Not good.

With proper wire that is still marginal at best.

The 17Ah is an SLA?  It should have a charge controller.

"available"?

You will be a LOT happier with the 55W, 100Ah battery, and #8. #10 maybe.  It would be a functional system.

Even the 20W and #12, with the 100Ah battery will be a big improvement.  It will be able to take more in on sunny days, and have more reserve for cloudy days.

If you get 'the solar bug' and decide to keep at it, buying #14 or #12 is money pretty much wasted.

A couple Pounds extra now will not be such a waste.

The biggest danger with small wire is it won't work worth a hoot, and you will believe solar is no good / not worth the money / only for tree-huggers.

G-

[Flux]

With free energy you can make many compromises. As it is not likely to save you money, just ease your conscience,then you don't have to do it very efficiently.

Thin wire will get you less power but will cause no other problem. I doubt that it is ever cost effective to choose wire to give a 3% loss. You have to balance the cost of wire against all the other factors.

In your case 120ft run will be 240ft of say 2.5mm wire. That works out to about 1/2 ohm. Most of the time a 20W panel is not going to give more than about an amp or just over with no cable loss. You will loose about .5V. That will bring your fully charged battery to just over 14.5v. The peak power point of the panel will be nearer 17v when reasonably cool so at this point you are running within the constant current part of the panel curve and you are loosing nothing. On a very hot day when the panel surface temperature is high the peak power point will fall. Even at this point you are only just likely to be nearing the corner of the curve and even now the loss will be tiny.

As long as your total voltage of battery and cable drop is below 16v you will probably see no gain with thicker wire. Even if you do reach the point where you see more loss then you are only loosing some free power and you have to balance the cost of cable against the battery replacement cost that is the killer financially.

At 55W you will be seeing some loss except on cold bright days with that cable but you will still be ahead of your 20W panel and infinitely thick wire.

These cable loss calculator things are fine for working from battery to load but for wind turbines and solar panels which are more nearly current sources they don't help much.

flux

[SteveCH]

I think, using the laptop as little as you expect, you can get away with this. I'd much prefer to see you use a larger ga. Cu wire [10 ga.]. Or use the larger pv panel. And if you have some other use for that wire, should you go with the 12 ga., for some other home project or improvement, you won't lose except your labor.

Another thing to consider is that getting started on this project, even if you have to improve on it later when it doesn't quite do the job, you'll at least be learning about how to collect, store, and use the pv-sourced electricity. We've all spent some money [in some cases, a lot] getting our feet wet. I, for one, am glad I did as our cottage here has been off-grid since we finished building over 20 yr. ago [we're all PV].

Let us know what you decide and how it works out.

[scottsAI]

Hello cottonpickers,

How about going the short way thought the garden and bury the wire?

At first just laid the wire out, later buried it when happy with it.

The plants covered the wires looked just fine.

I have several wires/tubes in my gardens, use a marker, only hit then now and again:-)

Have fun,

Scott.

[cottonpickers]

Thanks for ALL the comments!

I'll let you know how it finally ends up, but I've taken on board the suggestions and have changed my plans a little already -which was the purpose of my posting.  I'm trying to do this on the cheap so will need to wait for the right price on ebay for cabling then I'm all set :-)  So exciting!!  and the best news is that I'm in time for summer :-)

[cottonpickers]

Here's an update on how it all went! (next time I'll try to use the diary)

CABLE

I was originally hoping to use 2.5mm cable over a distance of 120m but ended up buying some 4 core 10mm armoured cable costing £130 OUCH off ebay -as its 4 core, I assume it means its the equivalent of 2 core 20mm?. But I took the advice to think of it as an investment.  This came in a large drum of 150m. And being 4 core means I'm now well inside the voltage drop limit. I decided not to cut the cable, so I can maybe use it again in another project later.  I then went around the garden rather than under it.  I think I'm glad I did it this way.

BATTERIES

I was originally planning on using one 17ah battery that I had as part of 'Jump Start' and just use it as a voltage regulator and not worry that it couldn't really store the energy, having the laptop do the storing.  In the end I bought 4 27ah 'New' AGM batteries off ebay with the expectation that these would store much more.  These should have a 10 year life according to the spec, but when they arrived they were not new :-( but 7 years old and just never used.  Consequently they all read 6v or theabouts.  After contacting the seller he refunded my money which was nice, and I spent 2 weeks trying to coax them back to life.  They now work, but I'm really dubious that they hold 100ah between them. More like about 20 I should think. My learning is that I'll ask more questions next time! Anyway, I guess it worked out for my learning phase.

PANEL

I was going to go with a 20w panel, but this really failed to charge the batteries enough each day, so I was seeing them drop to 40% too quickly.  I used the laptop a lot less to compensate, but in the end that was too much hassle, so I upgraded to a 60watt panel. This works much better

CHARGE CONTROLLER

Always planned to have one :-) but my 5amp cheap Maplin one broke after 2 weeks. I upgraded to another cheap one, but 10amp.  It has some nice features though for the £20 I spent on it, so I'm happy here.

<BR<p>
NEXT STEPS:

I think I need more battery or more panel for winter (or both)  I have some panels I can use, but would need to buy batteries, so I plan to plug in my other 55w panel in winter, or the additional 20w panel.  I'm also upgrading the socket in the kitchen from a standard 12v car socket to one with a USB port as well to charge up the mobile phones from.

Thanks for the advice so far.  More pics and a bit more information is on my website if you're interested: www.suburbangoodlife.com/energy

[cottonpickers]

should have been www.suburbangoodlife.com/energy.html

[AbyssUnderground]

None of the images on your energy page seem to work.

[fungus]

Seems there's a %5C in place of the / to get to the images directory .. looked that up and it's a backslash ..

[cottonpickers]

Thanks. Funnily enough IE doesn't complain about that, but thats Microsoft for you. I appreciate you letting me know, as I'd have likely never have found out otherwise.  I'll have to check a few of my pages now :-)

[ghurd]

Geeze!  Get that panel up out of the weeds!

Even the amount of shade that is on it in the photo will reduce the output a lot.

Looks like it will be more shaded earlier and later.

I don't see any reason not to add the 20W or 55W panel.  Or both if the total Isc is below 10A (controller limit).

The wire losses may be high, but the amps into the battery will be more.

If the panels are sitting in the shed doing nothing, might as well get them connected.

I got the photos here,

http://www.suburbangoodlife.com/photo.html

G-