I bought a Watt meter to mesure the load I want to power with my future PV system. I want to power two box fans in a greenhouse. This is what the Watt meter said:

.75 amps
Fan low= 84 watts
Fan Mid = 92 watts
Fan High= 94 watts

I prefer to run my fans on mid to low, but never high.

Here are my battery stats (two 6 volt golf cart batteries wired in series, led acid cell):

GC2 product Blk/Blk:
Amp-Hour at the 20 hour rate: 225Ah
Minutes at 75 amps: 110 minutes
Minutes at 25 amps: 447 minute

And I wanted to run Solar Guppy's software on a old XP laptop and use the SunSaver MPPT charge controller. I can buy 2 more golf cart batteries if I have to, but would rather not. They cost almost $75 each at Costco.

I have this on the roof, but I can remove it:

http://www.sunforceproducts.com/prod...?PRODUCT_ID=29

or, I can use it with future PVs. Can I post photographs here? I guess I can put in a link to flicker....

So....

In a perfect greenhouse world, I would prefer my PV system power up 184 watts 24/7 with a amp rating of 1.5 amps. But I can drop back and do 184 watts and 1.5 amps for 12 hours a day. I would rather over build my PV array rather than under build it so the batteries stay charged.

Suggestions welcome as to how to size the PV, and whether to bundle them with the PVs I have or not.
suggestions.

Thank you solar guppy for the cool software and this site.

jack

Hi Jack,
just a basic calculation if the energy you need for a day can be harvested for a day:
you want 185W for 12h, the energy you'd need is 185*12 Wh = 2220Wh.
Your PV panel's rating is 60W (the link did not work, so I used the ID=29 to find it):
so you'd need 37h of full sunshine per day to get the energy you need if you have one set of those panels.
Now the batteries:
225Ah * 12V = 2700Wh.
Not considering any other factors the batteries would be almost fully discharged when you consume 185W for the 12h.

All these numbers are full theoretical but give you a ball-park figure

Thank you Fish Eggs for the good data. I decided to down size. I have two 12V fans that prerhaps I could manage. The two together consume the following:

44 watts
64 volt amps
.52 amps

If I ran them for 12 hours a day, would I need additional batteries? And I am sure I would need move PVs on the roof. I do not mind overbuilding a bit. Can you help me with some basic numbers I might need to power the above?

Thanks for your help.

jack

running the 44W for 12h require (44*12) 528Wh of energy.

Your batteries should last for about 3-4days without any charging or sun.

You have to figure your sunhour number, in SoCal it's about 7.
So your panels rated at 60W would produce about (60*7) 560Wh of energy.

With that configuration you are about equal. I would leave it at this config, but also leave room to expand later for both, fans, panels and batteries.

Thank you for your help. I do not know how to figure my solar hour number, but using a tool at The Weather Underground site, it showed me a graph of numbers for each month. The average for the year was 4. Another source on the web called it a 3.5. I have 60 watts on the roof, and I can add more.

As I wanted to track data, I thought about the MorningStar SunSaver MPPT unit (model SS-MPPT-15L). I know it is over kill for my small project, but if I do some expanding, it will still work. Also, I want to run Solar Guppy's software and start measuring data.

What is your opinion on the PV on my roof? Should I add more? My batteries are OK, right? I don't need to add any more batteries?

Thanks you for your good data.

Sincerely,

jack

Hi Jack,
I would double the panels if your sun hour is 3.5-4.
Mine is about 7 and the location is southern CA. I forgot the link for this number.
Or you can wait, it seems you plan on a lot of monitoring so you can always add the panels at a later time - my guess it would be sooner than later.