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Solar Project to Power Computers during daytime

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Posted by Scott Kraemer on August 21, 2008, 8:07 am
Greetings Everyone!

I am finally taking the step to lower my utility bills!  In the Summer
Months in Arizona From Jun - Sep I average 2900 kWh
Mostly is due to AC unit running (15 year old model! )  - I am upgrading
that in a few months.  I run gas for heat and water/oven/stove.

Its the on peak kWh that is killing me (im on the 9am - 9pm plan)  Typicaly
this month on-peak is 1137 kWh=  $43.61 and off peak is 1770 kWh =
$4.18... (montly bill avg $00 in summer) so you see its basically just the
daytime I need this free solar power.  During the non Summer times my bill
usually averages $5.00

However.  I run some online servers 24x7.  Two Computers one with a 450w
power supply (dont know actual load) and a 350w.  and a few misc devices
(low wattage)

I have been doing alot of reading lately on this subject and getting ready
to order.  I know that there are many panels to choose from however the
inverter/charger needs to fit my needs and I wont skimp on that.

I plan to start off with two or three 100w panels to a  Xantrex DR2412
Inverter/Charger 12v 2400w.  I am wondering if I can configure this to use
the panels durning the day, and when voltage drops switch to grid power
without the need for batteries? or will I need the batteries to offset the
wattage drops? If so what battery model would you suggest and quantity?
If I had more battery storage I have the grid charge them on off-peak hours
and use them for on peak.

Is the DR2412 the right inverter/charger package for what I want?

I appreciate your imput.  As I learn I hope to contribute as I learn.  Are
there any online web forums for this? I couldnt find much that wernt pushing
stuff on you.


Posted by bealiba on August 21, 2008, 10:03 am

Well, first off, let's say 3 - 100W panels feeding a twelve volt

300W / 12V = 25A

Now lets say you have 1000W of load

1000W / 12 = 83A

You see the problem, you need to produce more than you use. And you
haven't even looked at the inverters efficiency yet. Your free solar
power is going to be quite a long ways from being free.

Posted by Ron Rosenfeld on August 21, 2008, 12:04 pm

Your goal of trying to reduce your electric bill is a good one.  But
reducing consumption, as you are already planning to do, is really going to
be the best method.  Your refrigerator, if electric and old, will also be
an electricity hog, and could be upgraded to a newer model.

If you haven't, you should also look into compact flourescent lights
(CFL's) or LED lights if they have colors and prices you can live with. Not
only will those reduce your electric consumption, but by replacing
incandescents, also reduce the a/c load.

Looking at the economics, while solar panels will reduce your "electric"
bill, they will probably cost you more.  For example, you write that you
are paying $43.61 for 11.7 kWh during peak times -- that's only

According the the calculator at

using the data for Phoenix, and even assuming you pay $.126/kWh *all* the
time, 300 watts of panels might save you about $1/year.

Perhaps you are looking only at the cost of electricity, and not the cost
of delivery?  If so, you might be able to save a bit more.

300 watts of panels will probably cost about $,500; and then there are
mounts, cost of installation, cost of the inverter, batteries, etc, which
may double that price.

Arizona does have some decent credits available, depending on where you
live, that might reduce your total costs by $,000 or more.  Check here:

So while this plan will lower your electric bill, you need to look closely
at the numbers to see if it is "worth it".  You should compare "investing"
in a solar PV system to "investing" in a safe CD, in order to make that
comparison.  For most places in the US that are grid-connected, even with
incentives, it is more expensive to generate electricity with solar PV
panels, than to buy it from the grid.

If your goal is also to "go green", then you might choose to install panels
even if there is some net cost to you.  Compared with other things we spend
discretionary money on, this is not bad.

So far as batteries are concerned, there are grid-tied inverters that do
not require batteries.  I don't believe the DR2412 is one of them, though.
With a battery-less inverter, you won't have any power during a grid-power
outage. I don't know if that is a consideration for you.

So, you have some work to do.  

As a starter, determine exactly how much energy your system requires. There
are inexpensive meters (Kill-A-Watt; Watt's-Up) available that you can use
to do that.  The power supply rating doesn't tell you much about what the
computer actually draws -- that's a function of the components inside and
is likely to be less than 1/2 the PS rating.  Measure your usage for a day
or three.

You may need to determine maximum power draw, if you are going to be
starting any heavy motors, but that's probably not an issue at this time.

Figure out if you want to have "battery-backup" provided by this system. If
not, and if you have other UPS's for your computers, I have read that some
UPS's will have problems with the "square" AC wave form put out by the DR
series of inverters.

Once you have the above information, you will be in a position to determine
your "optimum" system, and also how much of it you can afford to install at
this time.  Or you might decide that reducing your electric bill is not
worth the cost of solar PV :-(

Good luck!

Posted by Ron Rosenfeld on August 21, 2008, 2:40 pm

Typo:  That should be $43.61 for 1137 kWh  (my "." is just below the 3 on
my keypad).

Posted by Cydrome Leader on August 21, 2008, 6:07 pm
There are 48 volt drop-in AT and ATX power supplies out there. I haven't
measured their efficiency, but it appeared they ran far cooler than the AC
equivalents. This was judged standing behind full racks of the exacty same
machines, some on 120, the rest on -48.

I suspect the telco power supplies were must made far better than generic
chinese garbage power supplies where higher efficiency = costs too much to

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