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Solar->Batteries with grid backup

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Posted by jkoder59 on May 26, 2008, 4:20 pm

I'd like to stick my toe into solar stuff by adding a panel. In order
to avoid the expense of an inverter and tieing to the grid it would
seem that using batteries as an intermediary would be a good idea.

In principle I'm doing this now with my computers, they run off of
small UPS systems, they are charged by the grid and deliver
electricity when the grid is down.

It would seem that all that I need is a way to add a charging
component to these that comes from solar.

Is there a system out there that will let me charge a battery with
solar, and when that fails the grid will charge the battery?


john in TN

Posted by Richard P. on May 27, 2008, 2:56 am
Heh!  I had to re-read your message a few times before I understood what you
were getting at.  It's unconventional to think 'when solar fails, to switch
to the grid for backup'.  But I guess that is a possible situation..!

I am not aware of any device that will detect when current from a PV array
is insufficient and then switches to the grid to maintain charging current.
Maybe some of the older Trace SW series units do this...????

Posted by jkoder59 on May 27, 2008, 12:47 pm
Well, what I meant was that I have an application (several computers)
that are on 24/7 and they run on little battery backups that are
charged by the grid. They actually run off of batteries, but the
batteries are always being charged. What would be perfect is if I
could have a small solar set up charge the batteries in addition to
having the grid charge them. That way I could reduce the load on the
grid without having to sever my connection to it.


Posted by Roderick on May 27, 2008, 4:30 pm
My guess would be that your UPS doesn't spend much of its energy
charging the battery, except for after the grid goes down.  That is,
the battery is *kept* charged, but if grid power is available, that
will be passed straight through to your computer.  If this is so, then
if you added a solar panel, that panel would have very little to do,
except just after a power blackout.

You could force the UPS to drain its battery by unplugging it from the
wall for a period at night, but that would quickly ruin the battery.
UPS'es are not designed for repeated charge/discharge cycles, but for
infrequent emergency continuation of power.

Posted by wmbjkREMOVE on May 27, 2008, 1:59 pm
 On Mon, 26 May 2008 09:20:11 -0700 (PDT), jkoder59

You might start by reading these articles
www.homepower.com/files/webextras/loadcalc.pdf. They describe how to
determine energy required, or in your case, what percentage of your
consumption could be supplied by a solar module. One of these would be
http://www.pocketdeal.com/deal-Super-Media-Store-Kill-a-Watt-Electricity-Usage-Monitor%5E44888.aspx .
In easy numbers, let's say that one of your computers uses 100W 24/7,
or 2400 Whrs per day. Let's also say that you want to start with a
100W module, which might cost $00. If your average production is the
equivalent of 5 full sun hours per day, then the module would produce
500 Whrs per day. Accounting for various losses, it would actually be
more like one-sixth of a single computer's needs. Since that's such a
small fraction of the UPS's consumption, you might be able to get away
with simply tying the correct choice of module to the UPS battery. If
the economics haven't scared you off, then you'll find this article
http://www.solar-electric.com/charge_controls/mppt.htm  useful in
understanding one element of the losses that should be considered.


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