Want to build a small offgrid system

7 Posts
Jan 15, 2009 11:31 am
Want to build a small offgrid system

Any tips on battery bank sizing and electric average electric consumption?

What could one 200 watt PV panel do for me in 1 solar day? What is the correct battery bank size for 1 200W module?

Help is always appreciated.

351 Posts
Jan 15, 2009 02:53 pm
Re: Want to build a small offgrid system

You are doing it backwards.
Identify the loads that you are going to try to support.
When you have a total watt hours per day, then you size the battery bank and panels.
Start here:

7 Posts
Jan 16, 2009 11:35 am
Re: Want to build a small offgrid system

Yeah, I actually was trying to do it backwards because I know I only have room for 1 or 2 modules. But thanks for the info.
578 Posts
Jan 16, 2009 12:47 pm
Re: Want to build a small offgrid system

there is no correct battery bank size, but you could figure a correct load size by working backwords.  if you had a 200w pv array, you could assume (4 sun hours) that you can't support a load larger than 640wh/day, day after day without another source (like a generator).  that means you would in theory need at least a 1280wh battery bank, at absolute minimum.  if you kept the system at 12v that would be a group 31 battery or higher, or 2 golf cart batteries in series.

hope that helps,

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Jan 16, 2009 01:03 pm
Re: Want to build a small offgrid system

In a perfect world I would agree with Mr. Hall's remark of "your doing it backwards." I would like to add that, when I started out with PV, I was reluctant to peel off several thousands of dollars for a minimal amount of wattage at a low nominal voltage. Budget can have an strong influence on the decision to go PV.
We, my family and I, had lived off grid without any electricity what so every for the better part of 25 years. So you can see that electricity of any kind wasn't the temptation for us that it is to most others. It was more of a luxury to us. Much in the same way an indoor flushing toilet is compared to an outhouse. They both serve the same purpose, only one is way more pleasent ot use.

We started out small. I learned as much as possible first, mainly from the pages of Homepower magazine which prior to them was the Mother Earth News. Two MSX 77's, a C-30-A charge controller (both were new to the market at the time) and, a couple of "18 wheeler" cranking batteries. Of which, the two MSX 77's are still in service. The C-30-A is collecting dust. The two batteries, traded in. We learned from these items just how much power could be collected from PV in the real world, as opposed to advertising. They are actually pretty darn close when real world output is averaged out. As you might guess our system has grown substantially since then.
Here is a site that can give you a fair approximation of how many - hours of equivalent full rated charge - per day in your region depending on PV module orientation. What this means is -�  from the time a module first starts making power in the morning until it stops in the evening divided by its rated output.

Lets say its a yearly average of 3 hours per day with the PV modules facing solar south and set at latitude all year. One - 200 watt module multiplied by 3 hours is 600 watts hours or .6 kilowatt hours. If your charging a battery with this at 12 volts nominal that would be 50 amphours. To give you any idea of what this could do. One 50 watt light bulb at 12 volts takes 4.2 amps. That means you could burn that light for about 12 hours, in a perfect world. There are losses due to naturally occurring inefficiencies in turning sunlight into electricity, conducting it through wire to a charge controller then into a battery and then to a switch and finally the bulb itself. If the battery and the wire is sized right, you should still be able to get close to 11 hours. There are just so many factors that could change the outcome of this, the greatest of all is weather, so its still a matter of averages.
As for the battery size, that to can be a matter of budget restraints. If you want to go ahead with this as a way of, "sticking your toes into the water before taking the big plunge," I would recommend
1 - 200 watt PV module or closest wattage equivalent.
2 - 220 amphour 6 volt flooded cell lead acid batteries wired in series for 12 volts nominal.
1 - MPPT type charge controller. I suggest an MPPT type mainly because when PV modules get upwards of a 200 watt output its voltage is typically higher than what is required for a 12 volt nominal system.
Again, budget restraints can come into play. Be sure to read and understand the ratings of any equipment you purchase for your PV system. Here are some links.
I hope something in what I wrote is helpful to you.

« Last Edit: Jan 16, 2009 01:10 pm by Thomas Allen Schmidt »

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