Configuring my first pv system

sprintman · May 2011

Greetings from Iowa, I was all set to do a wind gen. set up and at the last moment thought I'd check on need for a permit...was told it's a no go in my county--so am switching to PV. My intent is just to supplement the grid and have some back up available. I was/am going to run a few circuits in my house off of a battery bank, hopefully on a daily basis. Mostly lighting, which is all cfl and maybe the outlets in the family room, ie. tv and stereo. Here's what I'm thinking to start with. 2- 24v panels 150-200 watts each, a four battery 6v golf cart battery bank, a xantrex mppt controller. About 30 feet max from panels to batteries. Batteries set to 12v configuration as I have a 12v dc to ac pure sine inverter. From what I understand the controller will use panels in any configuration (either 24 or 48 depending on how I wire them) and then charge a 12 volt system. My questions are: Is what I'm proposing make sense? Will this be a balanced system....enough charge capacity to keep the batteries up. Is this size battery bank large enough to handle my proposed usage. Few hours of tv and even less in llighting.
What size wire do I run from PV's to controller...Is 10awg ok? I'm assuming braided wire is better than solid? Should I just keep the panels at 24 volts or bump them to 48. Thanks indvance...This is a great forum.

Cariboocoot · May 2011

Re: Configuring my first pv system

Welcome to the forum.

It's probably just as well about the wind turbine; they tend not to perform as well as advertised.

I'll not dwell on your reasons for going this way instead of GT and/or gen back up. I'm guessing it's more of a learning experience than a practical economic project.

So let's analyze the plan.
Two 200 Watt panels perhaps? 400 Watts might supply:
20 Amps of charge current and 1200 Watt hours on the DC side. Possibly 800 Watt hours AC.
Four T105's configured for 12 Volts @ 450 Amp hours?
Immediately that's low on panel for recharging. One set would work with 400 Watts of panel.
That's a potential of 125 Amp hours @ 12 Volts = 1500 Watt hours DC - very close to the panel potential.
The Xantrex MPPT controller is overkill for this configuration, unless you're planning on expansion. The Morningstar 45 Amp MPPT might be a better choice. As a general rule array Voltage shouldn't be more than 2X system Voltage.

Will it meet your needs? That depends on what those Watt hours really are. You could get a Kill-A-Watt meter and measure the actual loads you want to use. That will give you the info you need to size the battery bank, which in turn determines how much solar you need to recharge it. Although recharging from the grid is an option (with a proper battery charger).

Does this help any?

sprintman · May 2011

Re: Configuring my first pv system

Yes, this is helpful and Fast! Do you mean 400 watts of solar at 24v would be better balanced with a pair of T-105 batteries. The reason for the xantrex is that the only way they would take the wind gen back was with a restock fee and exchange and that was the controller closest in value. Also this is for a vacation home that I could see becoming more permanent at retirement and would expand system then. As an after thought then would I need an 800 watt 24v panel system to charge four of those batteries. Thanks

Cariboocoot · May 2011

Re: Configuring my first pv system

Please keep in mind that is all done with basic calculations and typical deratings, plus one or two rules-of-thumb. To get a precise design you must have information about how much power you want to supply and for how long. Then you can determine the size of inverter, battery bank, panels, et cetera. When you work from already purchased equipment you may find you have to make more compromises.

icarus · May 2011

Re: Configuring my first pv system

As we all too often say, all off grid ( battery based) system calculations begin with the loading. First, determine the loads, then work backwards. If you are building a "hobby" harem, blanche the PV and the battery,and don't worry about the inverter until you need to.

Tony

jcgee88 · May 2011

Re: Configuring my first pv system

sprintman wrote: »

My intent is just to supplement the grid and have some back up available.

Unless your need for backup is critical, it is usually more cost
effective to deploy a generator for that purpose. You can then
convert the money saved from not buying and maintaining
battery infrastructure into additional PV capacity. Assuming
your utility offers net metering, this then turns your utility into
a free, zero-maintenance battery. Plus, grid-tied power
production efficiency is double that of a battery-backed system,
which means every month your electricity bill is less.

Putting some numbers to this... Say you spec out a
400 watt, battery-backed system. If you delete the battery
back-up part, let's conservatively estimate that you can buy
one extra panel, now you are up to 600 watts. That 600
watts is then doubled because you're not doing the
battery-charging process, so now you are at 1200 watts.
Would having 3X more capacity on a daily basis be worth
more than have an hour or two of power during an outage?

Note also that in a small battery-backed up system as you
describe, a relatively larger proportion of your funds goes to
infrastructure, instead of production capacity. Thus, you end
up with both less capacity than you could have had for the
money you have AND the battery capacity you have is not
sufficient to completely tide you over even a minor outage.

Of course, before you even do any PV, you should do whatever
conservation measures (measuring loads, insulating, replacing
energy hogs, etc) that have the most bang for the buck.
Consider solar when you have reached the point of diminishing
returns for further conservation.

John

Configuring my first pv system

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