Hi Pete;
Not trying to make to much of a 'project' out of this (posting this FYI), but you want to see what your total amp draw is before specifying your solar/charge power system.
Once you do that you then want to determine how many days of 'no sun' and night time combo backup you want the system to have in hours. Multiply this hour number by the amp draw and that will be the minimum battery size you want (amp-hr). If it were me, I would double it so I never would draw the battery down more than 50% of its rating (ideally).
You then need to size the solar panels so they can optimally replace the above maximum amp*hr draw without sun during (ideally) the following day. To do this you need to look at a solar panel's current output vs. exposed (sun output) available solar radiation (watts/m[sup]2[/sup]) graph and determine 1) the minimum solar radiation available in your area during the winter (where it is the least) and the minimum amount of daylight hours available (again in the winter where it is the least). This will give you the minimum performance output of the solar panel during the winter.
Here is an example:
A 12 volt security system has a maximum (DC) current draw of one amp continuous.
You want to insure you have three days of battery backup in case you have three day of no sun (72 hrs).
You also want to insure the battery will only draw down to 50% of its rating. Therefore the battery size needs to be 1amp * 72hrs = 72 amp*hrs, then double it for an approximate 150 amp*hr rating.
Now you need to size the solar panels so you can replace the battery draw of 72 amp*hrs during one full day's exposure to sunlight during the winter time (worst case scenario sun exposure).
Your area has solar radiation exposure of 600 watts/m[sup]2[/sup] during the winter (should be able to look up this data for your area). Looking at solar radiation vs current output graph of the solar panel shows that it will output 6 amps of current when exposed to that amount of solar radiation. The winter time has about 8 hours of daylight so the amount of charge that this solar panel can replace during the winter months is 6 amps * 8 hrs or 48 amp*hrs. Of course the system is drawing current while it is charging so you need to subtract the 1 amp * 8 hrs or 8 amp*hrs from this number so you only have 48 amp*hrs – 8 amp*hrs = 40 amp*hrs of battery replacement charge available.
Since optimally you want to have 72 amp*hrs available you would need to double the solar panel's capability or live with less battery backup hours available (in this case it would be about half the battery backup time if you used this solar panel), or hope there are more consecutive days of sunlight available for charging.
Of course cost is always a factor and you have to certainly weigh the above specs with the funds available. For instance you may want to draw the battery down more then 50%, or you live with less battery backup. I've run the
Concorde SunExtender AGM type batteries down to nothing and they charged back up without issues.
There are some other factors to consider as well. For instance some equipment may get damaged if their 12 volt DC operating voltage gets to low. Certain charge controllers will cut off the output if this happens.
There are also 'smart' charge controllers (I liked the
MorningStar SunSaver MPPT technology ones) that will maximize the charge to the battery in low sunlight conditions. Some controllers incorporate a (remote) temperature sensor and will take the temperature of the battery into consideration for charging scenarios as well. They will also insure the battery will never over charge by emitting just a trickle to it if needed.
Make sure you size the charge controller by looking at the solar panel's maximum current output.
You also want to adjust your solar panels' direction and angle to maximize exposure to the sun at various times of the year (should be able to look up this data for your area). Also consider the amount of wind these panels will be exposed to and purchase a strong frame to match those conditions.
You also want to incorporate appropriate fusing at the solar panel's output and the output of the charge controller (to the equipment).
Of course, the above scenario is for more of a mission critical application, but you might just want to run through the numbers with your proposed system so you don't have any surprises when it's installed (especially if its for a friend).
Also, the above calculations are just rough numbers for approximate sizing of your components and others may have differing (better) methods. I ran through the above numbers very quickly so hopefully there are no any glaring errors!
One vendor that I've dealt with is
Arizona Wind Sun (not affiliated with them at all) for components as they seemed to have decent pricing.
