2.12.2012 – Sunday
I recently took a few minutes to plan a few posts for the next month or two; stuff that I’ve been working on for a while and feel should be on this blog because as anyone who has ventured into alternative energy will tell you, there is a lot of head scratching. So if you ended up at this blog because the way you understand reality doesn’t quite mesh with what your off-grid system is telling you, I might be able to help. So along those lines, there is something that probably gets overlooked quite often when designing an appropriate system for the electrical loads planned: efficiency.
“Derate to 70%” is the most common number/statement you’ll hear. If only it was that simple. While that might be the case for the mechanical bits in a system, nothing is ever simple when it comes to batteries. Battery charge efficiency is not a stable number. It changes depending on how charged up the battery is because of something known as internal resistance. Basically internal resistance increases as a battery nears a fully charged state, making the battery more resistant to accepting a charge. Here is an article from Sandia National Laboratory addressing this very issue: FLA study – charging efficiency
About a year ago I calculated the efficiency of our off-grid system… and a few days ago I adjusted those calculations to take the batteries into account. When taking a look a my numbers you might notice I am a bit optimistic about the efficiency of the panels. Most solar panels will be labeled “X” number of watts +/- “X”% so it’s not unlikely that in a PV array some panels will be capable of producing above the rated output and some will produce below. The panels at the cabin are rated 135W ± 5%.
If you would like to play with my numbers or use the spreadsheet as a template, here are a few more file formats. Since I have a Mac and prefer to use the iWork suite of apps, the Excel document was converted from a Numbers document so it will look different than the PDF.
PDF – System Efficiency
XLS – System Efficiency