CALC: Off Grid PV Sizing Tool

Started by MountainDon, January 08, 2010, 09:16:00 PM

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Assuming there is no tracking device it's the number of hours of "good" sun. In NM that is about 6 to 8 in summer, depending on panel tilt.    A tracker makes it a whole new game.

Just because something has been done and has not failed, doesn't mean it is good design.


I've been trying to rework my solar requirements but have to admit I'm not certain even with the spreadsheet, as to what I need.

Just did an offgrid calc on another site and was given 500watts of solar power as my needed bank using 15k watt hours per week in my zone.  However when I use the sheet and plug in Surrette S530's I end up with a lot more solar power required.

I'm wondering this:  is the AH rating asked for the 20 hour rate or perhaps a different rating?  After all, many batteries list up to 100 hours.  The difference?  The 100hr rate is 532AH where as the 20hr rate is 400ah.

Not sure that matters a whole lot but I'm wondering.

Here's my situation:  I'm running 8 GCB's (~220ah @20hrs) and can draw them to 50% in one evening of using my fridge and lights if it's a particularly dark day if I assume (I know I know  d* ) the lights on the Charge Controller and Inverter are correct.  The logs might show a discharge of more then that even but I've learned that under a load the batteries show a lower voltage then when the load stops (i.e. the fridge running can show the bank at 50% until it stops where it jumps back about 80%).

I'm certain that I DO NOT have more then 2 days to 80% of real discharge but with lots of sun most of the time it isn't an issue because the bank charges back up the next day so I don't usually see anything near that.

My system is 615watts of solar panels in series running to a Morningstar 60MPPT controller.  It's a 12vdc bank @ 880ah and my calculated weekly usage should be about 9000 watt hours.  Though the controller logs suggest that could be as high as 12000 watt hours.

I've drained the bank down badly last winter (voltage reported as low as 9.9vdc) and plan to replace the bank but also want to make sure I don't ruin the new one.

Thanks for the help


20 hour rate. Always. At least for most "normal" users. And you can see why as you have noticed the difference in voltage reading when a load is being applied to the batteries. Large loads = more drop. If the loads were al;ways "large" then maybe even the 20 hour rate would be too high.

Get/use a hydrometer; voltages are a ROUGH guide. Also use a log book; record each cell.

Just because something has been done and has not failed, doesn't mean it is good design.


The charge controller log is recording the amount of power being placed into the batteries; bulk, absorb and float, usually for a 24 hour period.  That may be different from a calculated figure for many reasons.   

Do you have an actual power used meter as well? If so, is that figure for the AC watts used or the DC watts removed from the batteries?

If you have a rated capacity of 880 amp-hours and if you are planning on a normal/usual use pattern of 50% discharge daily, and if you expect the batteries to be recharged completely by a normal single day dose of sunshine, my guess is you need more PV wattage. Maybe double.
Just because something has been done and has not failed, doesn't mean it is good design.


Thanks Don.

No use meter though I have a plug in watt meter to tell you how much energy a given appliance uses and could plug that in between the inverter and main panel to see what I'm using past the inverter I guess.

I'd like to have the bank sit no lower then 50% after 3 days -- and maybe it's doing that actually.  I guess the frustrating part to both of us is that I don't have or use a hydrometer to check the batteries!  So I plan to pick one up today or tomorrow and use on the bank this trip so I can get a good reading.  Then I'll take a reading in the evening and in the AM each day I'm there and report back.