Thank you Brian. You have made a good point. Often times I have simply wrote that one should "take the electric power needs over a 24 hour period" when determining a battery bank size. That is some what vague.
(Please keep in mind that I have kept such things as budget restraints out of the equations. I feel that to be a personal matter for the individual to compute.)
More precisely, one should sum up the electric power needs over a 24 hour period, worst case scenario. If it is known that there will be a day when electric power usage is going to be higher than average over the winter months, then use that days electric power needs. (Unless one lives on the equator of course.) I use the winter months because these days are shorter and the occurrence of inclement wheather is more predominate.
Of course this is mainly all about sizing an "offgrid" battery bank with a stand alone PV system, where a minimum of backup generator usage is desired.
Even with this, there could be weeks during the winter months that a stand alone PV battery charging system just can't keep a battery bank fully charged no matter how big the PV array is.
The multiplying factor of 5 that I use is synonymous with 20%. 20% of 100 is 5.
For Example: if after summing up the electric power needs over a 24 hour period, worse case scenario, the total is 1.5 kW h's (kiloWatthour's) then the math is as follows -
1.5 kW h's X 1000 = 1500 watt hours
1500 watt hours / 12 (nominal battery voltage) = 125 amp hours, bare minimum.
125 amp hours X 5 = 625 amp hours at 12 volts nominal.
It always best to round up any fractional numbers to the next highest whole number. Even in the case of 1.5 kW h's. Of course this translates into higher cost $ as well (sorry, couldn't help myself.)
What that factor of 5, that I use, does is help to keep the battery within the top 20% of full charge. This is what is known as shallow cycling, which helps with having an overall longer battery life. It also will help give several days of power with little to no sunshine (in a stand alone PV system.)
Any type of re-chargable battery is made with a predetermined number of cycles starting with a full 100% charge, then down to a 100% discharge, then back to 100% re-charged, that is one full cycle. Treating a battery this way though will mean having to replace it, a lot, sooner than if one were to increase the size of the battery bank and shallow cycle it.
All of this is concerning discharging a battery bank but just as important if not more so is recharging it. If it is at all possible, what is taken from a battery bank should be replaced ASAP. This too helps the over all life span of the battery. Nothing will ruin a battery faster than sitting there totally discharged. With the exception maybe of a Ni Fe.
http://www.beutilityfree.com/content/index.php?option=com_content&view=article&id=44&Itemid=129After all is said and done, when sizing a battery bank, its better to error too big! Than to error too small.
