Calculating Battery Size for 2 Channel NVR Security Camera System
Battery Size
I would want a battery sized, as already suggested by others, roughly 2x your total expected daily output (based on summer days = more usage). That would keep you to 50% discharge on the shortest nights (but remember, you have a couple of months of those in a row) and less on the longest nights (all confusing because this is the exact reverse of the typical &quot;charge with solar in the day and discharge at night&quot;). In other words, calculate your total usage, multiply by the number of hours of the longest day (i.e., longest discharge time) and then double it. That is your battery size requirement. Let's say (for simplicity) it is 70W x 16 hours = 1,120 Wh x 2 = 2,240 Wh. 2,240/12 = 187 Amp-hours. So using your 100 Amp-hour 12V batteries, I would go with 2 of them.
Make sure the batteries can handle the cycles - you are talking about a significant (~ 50%) cycle EVERY DAY. Some battery chemistries can handle that a lot better than others.
Charger Size
Get a MUCH bigger battery charger. 12V 5A = 60W - that is roughly the same as your discharge rate, so at best on a 12/12 (spring/fall) day you would be breakeven. I'd want to charge at least 2x the expected discharge rate to have a fighting chance for long nights and other factors. Obviously the batteries have to be able to handle it, but if your batteries can support it then I would look for a charger that, at peak, charges on the order of 140W for your 70W usage.
AC/DC
Why convert from DC to AC if your equipment (a) can't ever use AC directly because the AC isn't available during the time you need power (which is also quite unusual) and (b) your equipment is all electronics that generally run on DC anyway. You may need some sort of regulator (perhaps as simple as a &quot;car cigarette lighter power adapter&quot;) to use 12V DC, but that will avoid the losses of DC-&gt;AC-&gt;DC conversion, which will further reduce power usage.