Solar ciphering. Check my math.

[NM_Shooter]

Trying to SWAG my electrical system. 

I'd like a "hard" day of use to only use 1/4 of rated capacity of bank.  Planning on 4 GC2 batteries in 12V configuration.  Small cabin = small wiring runs.

GC2 batts rated at 220Ah, with recommended draw to 1/2 of that.  But having two sets in 12V configuration brings me back to that 220Ah rating to a safe discharge level.  So this bank at 1/4 draw would be about 100Ah per day.  That seems like a lot for a hunting cabin.

Looking at my power needs, lights will be the most.

Worst case, 2 units of 32W flourescent for 4 hours.  Figure 40W total for ballast operation overhead losses.  3.3 Ah per fixture.  So two fixtures running 4 hours (this would be a lot of light use) 6.6 x 4 = 26.4 Ah

Water pump, one hour at 5A = 5Ah

Furnace 2.8A at 4 hours = 11.2Ah

Total day use = 42.6 Ah    Way under budget.  Not a bad idea for rainy day situations. 

So... solar panel sizing.  This is where things get fuzzy quick.  I'm going to have a fixed roof panel, and it points SSW with a 6:12 pitch (26.6deg)

I'm going to use an MPPT charger.  Dunno which one yet, or if it will have data logging. 

Let's just figure on 6 hours total of full charge capacity out of the panel.  Obviously I'll get less than full capacity, but hopefully it will be for a longer period of time.  My cabin is in full sun.

If I need to replace 45 Ah, and have six hours to do it, that's 7.5 A per hour.  At 12V, seems like that puts me right at 90W.  However, there is a lot of heat that gets wasted charging batteries.  Any idea how much? 

So I'm thinking a minimum panel size of 100W. 

Comments? 

[NM_Shooter]

Hmmm... 75 to 85% efficiency in charge discharge for wet cells.

http://wiki.xtronics.com/index.php/Sealed_Lead_Acid_Battery_Applications

[MountainDon]

Mr Conservative here.    I'd go 120 -150 watts or more just to be sure to get it done on a half cloudy day.


Just a side note:  We just discovered we make enough power to run a slow cooker (crock pot) in the afternoon when the batteries have gone into absorb or float mode. Dinner the other night was free. Road kill.  Well, no it was chicken but the power to cook it was free.