There have been times when I've set out to learn something new. I would read the books and look over the charts and pictures and all of that but then one phrase or sentence would clear it all up and give me the understanding of it all, kind of like, the last piece in a puzzle. Where batteries are concerned it was this: Batteries do not store electricity, the convert electric energy into chemical energy and store that, until such time as electric energy is needed and then it converts it back.
A simple thing that one may know or at least speculate, but until I read that, everything I had read about; lead, spongy lead, sulphuric acid, amp hours, C rates, and the like still had an ora of mystery.
First: know what your electric needs are and translate them into watt hours. Most electric devices that have a UL listing have there wattage labeled on them but some list only volts and amps.
To do this one will need to know the volts and amps or watts that an electric load demands as well as the amount of time the load is on demand.
Example: a 12 volt nominal motor draws 3 amps for 6 hours, mathmatically this is simply 3 x 12 x 6 = 216, thats 216 watt hours. Also a 50 watt light at 12 volts for 4 hours - 4 x 50 = 200 plus the 216 for the motor is a total of 416 watt hours. (It makes no difference when caculating watt hours if its volts ac or volts dc.)
Second: Once one has the total watt hours both for ac and dc loads, translate this into amp hours by divding the watt hours by a nominal battery voltage. Continuing the example from above: 416 divided be 12 equals 35 amp hours.
This would absolute minimum battery storage capacity. Now figure what number 35 is 20% of, because staying in the top 20% of battery capacity increases its overall life and will give 2 or 3 days of reserve power. This would be a 180 amp hour battery at the 20 hour rate. If anything go larger when pricing a battery bank.
Third: sizing the PV array, at minimum I like to size the PV array so that it can replace that top 20% in one sunny day. To do this one needs to know the number of "hours of equivalent full rated charge." Use worst case which would be winter time. If it were 3 hours then, 35 amp hours divided by 3 hours would be 12 multiplied by the nominal voltage which is 12 which gives us a total pv array size of 144 watts.
This is all over simplified of course and bare minimum in example. Allways round up and when actually making a purchase go to the next size up. In the example above where a battery capacity of 180 amp hours at 12 volts is needed "bump" it up to 220 when making a purchase, this is true the PV array as well, where 144 watts is called for in the example "bump" it up to 200.
If you managed to get this far without me putting you to sleep, goto
www.homepower.com and read what they all have to say. Out of all the monies I've spent on my of grid system, the best value over all was a subscription to HomePower magazine.
