I have worked many hours at building an Excel spreadsheet that helps me calculate my daily loads against some battery configurations and how certain solar panel configurations impact the whole system.
I am trying to keep all my numbers in watts and watts/hour since it is , in my humble opinion, the ONLY clean measuring stick for power consumption (my local utility also uses that same measuring stick - coincidence!).
Then I came across an "alternative energy" provider that offers a "solar module worksheet" to help determine how many panels are required for a given application. In their worksheet, one of the steps is to divide total amp/hours (!) by the peak amps of the desired panel. The panels I would like to purchase are rated as follows:
Watts Peak: 128.40
Volts Peak: 16.86
Current at Max. power: 7.61
(The math works: 128.40W / 16.68V = 7.61A
If the MPPT was converting this properly (without factoring any losses) then the system would then see 10.7A. That is 128.40W / 12V = 10.7A. In comparison that almost a 30% loss!!!
What am I missing here? Is it the MPPT controller eating the difference? Is the "worksheet" out to lunch?
Thanks,
Humbly Andre
P.S. I found the worksheet at the www.foster-wills.com site or more specifically:
www.foster-wills.com/Foster_Wills_Solar_Calculation_Worksheets.pdf
Andre,
Quite simple. The worksheet doesn't factor in MPPT, but normal chargers. A normal solar charger does normal DC regulation and can only shave off the extra voltage (16.86 V-> 13.2 V or whatever the charge voltage is).
This is exactly why a MPPT is so much better than a normal solar regulator. As you say, up to 30% more efficient.
I do think that these calculations are somewhat of a misleading trap anyway. They calculate what you would get for an "average day" of so many hours of sun, assuming no further losses. This is OK if you are in a place where the amount of sunshine is predictable. Most places on this earth, not so much. So you can almost always use more panels.
BTW -- please don't say amp/hour where you mean amp hour. You're dividing current by time, instead of multiplying them. The worksheet does this correctly. See http://yachtelectronics.blogspot.com/2010/05/basics-of-electrical-units-and.html for a rant of mine on this subject.
Thank you so much "Kees" for you comments and yes you are right about amp/hours vs amp hours. Thank you for setting me straight.
Conclusion: make sure you purchase a first class MPPT.
Thanks again.
Andre
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