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4368 posts

Uber Geek

  # 1013025 26-Mar-2014 11:09
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Dangermouse: I hadn't realised photovoltaic systems were so very feeble; I really thought this sort of thing would be pretty commonplace these days.

The greatest minds in the world haven't been able to figure it out yet. Solar systems (at least one at a lodge in remote bush I've had experience with) are possibly able to power a few lights at night time but not much more. 

This was huge news back in 2006 and my interest was piqued at the time....

But, nothing ever came out of it. Some germans were going to manufacture the panels but it never happened. I guess there were technical issues or the breakthrough was not really one. 

195 posts

Master Geek


  # 1013067 26-Mar-2014 12:03
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Photovoltaic systems aren't so feeble, otherwise I wouldn't be able to be sending this reply.

The problem in the situation we're discussing is that a robust, usable photovoltaic-based system is not very inexpensive.

The price of photovoltaic panels has plummeted, switchgear seems to be coming down but batteries are still dear.

If you want to operate 8 1-ampere (12 W) lamps for 3 hours an evening, you'll want a 48 Ah battery (24 Ah consumed and you only want to drop the battery to 50% of capacity).  And that's assuming that you get enough sunlight to recharge the battery fully each day.

It's more than likely that you'll have stretches of 3-4 days at a time with no significant sunshine, so you'll want a battery with 100-200 Ah of storage capacity.  That's going to blow your budget.

If you have to start hauling the battery up (down?) the hill to the house to recharge it, that's going to be a nuisance.


43 posts


  # 1013101 26-Mar-2014 12:32
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In truth, it’s very difficult to say what the exact usage would need to be; six hours/week is probably very much a worst case scenario. Most of the time it would probably be far less or not at all beyond a minute or two here and there, particularly in winter when it’s too cold to spend much time up there anyway.

I see a 115Ah battery on TMe now for $250 with free shipping (made in the USA but in my experience that’s usually on a par with Chinese rubbish); is this a good price or does anyone know of a better/best source for these in NZ?

How would you determine the most appropriate size of panel to feed this or is it just a case of the bigger the better?

If you have to start hauling the battery up (down?) the hill to the house to recharge it, that's going to be a nuisance

A large part of the time spent in the garage is working on my ’65 VW Beetle which is still 6V so I’m well accustomed to lugging batteries up and down ;-)

3267 posts

Uber Geek


  # 1013255 26-Mar-2014 15:44
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Lead-acid needs to be kept fully charger, they are not intended for cycle use (even deep cycle, they are just more tolerant).  Also not intended for discharge in 1h, but can give short bursts of very high current.  Also only 65% efficient in charging.  So the battery size gets pushed up.  From there you select panel size (or number of panels, it can be paralleled with diodes) based on sunlight and charge time requirement (which depends on battery size).  Then you pick a sufficiently rated solar charger that can handle the peak current.  Solar panels are about $3 per Watt retail, $1 per Watt trade.  You get different types of solar chargers, best if you can source one that does peak power point tracking (constantly adjusts the charge current to the most power out of the panel under different conditions).

You can never have enough Volvos!

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