[MLB-WIRELESS] Solar Power - How much battery?

[clae at tpg.com.au]

Wasn't there a wiki page about this somewhere?

24 watts is a lot to run just a simple AP.  Jon's figures below for the WRT indicate you can do a lot better.

One thing that came up in previous discussions is finding an APs that doesn't use 12 V, or that internally 
regulates that down to 5 V (which I understand is pretty common).

If the WRT does this, it's dumping over half of those 3 Watts as heat out of the V-reg, and only using 
around 1.3 watts internally.

Solar cells and batteries are available at 6 V nominal rating as well as 12 V, so for a 5 V AP you could 
run the whole setup at 6 V.  You would still need a 5 V regulator inline, because the "nominal" 6 V from a 
battery is really closer to 7 V (and 14 V from a "12 V" battery).

Anway, if that works, you can halve the figures I give below (including the ones with a $ at the 
beginning). 

Quoting Jon Dean <jon.dean at wormhole.co.nz>:

> I would agree. An WRT54G, uses about 3W on average - so 72 Watt- 
> hours. 

... per day.

You need a battery rated at 

72 Wh 
x 3 days
------------
= 216 Wh
------------

 x 2 (for battery safety margin)
------------
= 432 Wh
------------

And as batteries are rated in Amp-hours and not watt-hours you divide by V

/ 12
------------
= 18 Ah
------------

Which is a lot more reasonable. 

The panels need to be able to supply 216 Wh each day.   That's what you're designing in to cover 3 days 
of poor sun.  Assuming the batteries are fully charged when installed, that means they won't dip below 
the 50% safety margin over those three dark days.   Budget permitting, designing in for more dark days 
will protect your batteries longer, but what we're calculating here is a minimum

So:

216 Wh
/ 4 hours (equivalent full sun)
------------
= 54 Watts
------------

Figuring $10 a watt for panels, that's around 

$500 panels
+ $30 to $50  batteries
+ $40 charge controller
+ $50 - $100 connectors, cabling, mounting etc.

Yes, solar panels are expensive.  A small sun-tracker might let you get away with a smaller panel, but 
they're not exactly cheap themselves.

If your location is suitable, wind generators are much cheaper ($2 - $5 per watt, or next to nothing if you 
build it yourself), and have the added advantage of charging at night :-).  

The best option would be a combination of panels and a small wind gennie, each rated to supply around 
half your power (ie about 25 to 30 watts in this example).  You're much less likely to go three days with 
both no sun _and_ no wind.

The only problem would be finding someone selling a 25 - 30 watt wind gennie - that's tiny.  The smallest 
commercial ones I've seen are around 200 W and $500.  But for a handy person who does his research, 
a gennie that size is a good beginner's project, and it wouldn't even require much of a mast.

> Plus if your using Lead Acids for 
> 
> your batteries you don't want to go below 50% charge.

That's a point:  it might be worth comparing the cost of NiMH batteries (which can withstand a full 
discharge, but around 80% is probably better)

YMMV etc,

Clae.


> 
> >
> > So going by the example of 2A @ 12V = 24W,
> > hence power required over a day is then 24W x 24hours =
> 576W-hours.
> > That's not sounding too good. umm... I suggest getting ap's that
> don't
> > need 2A to run.
> >
> >