Calculating Battery Size for 2 Channel NVR Security Camera System

Question

I need to install a security camera system on a United States 110v power circuit that is only on a night. The circuit is using an astronomical clock to turn on power and more accurately measure the amount of sunlight in a day before turning on the night power, compared to just having a timer on the circuit.

I need to install a PoE NVR with a 2 terabyte HDD, and two PoE cameras on this 110v circuit. The cameras will run during the day only, no night time operation. I want to charge the battery during the night while 110v power is on and have the cameras power off the battery during the day.

I have selected an AGM sealed 100ah 12v battery, a 12v 5amp battery charger (to ensure the battery gets fully charged during the night when there is power) and a 400w inverter. I do not need 400w at all, I only need roughly a 100w inverter but I do not believe a higher watt inverter consumes more power just because it is capable of a high watt output, it is just the devices plugged into the inverter that consume power, so the 400w inverter in fine and not very expensive. It has the battery connectors that I want is why I choose it.

An NVR with HDD can consume a range between 40watts and 200watts from what I have read online. Since I only have 2 cameras I believe I will be in the 40w to 60w range for my NVR. And remember, I will only be recording during the day, no nighttime recording, no night vision usage of the PoE cameras, the cameras will consume around 4w each.

So I am calculating a 70w consumption average of my PoE NVR and 2 PoE camera system.

By my calculations, a 12v 100ah battery will provide 1200 watt hours of power. To maintain good battery health of an AGM sealed deep cycle battery I want to keep the depth of discharge to 80%.

1200 watt hours * .8 = 960 watt hours.

If I am consuming 70w than that means I should get roughly 13 hours from the battery, 960/70 = 13.7 hours.

I am debating on whether I need a larger battery or not and also if my battery charger will be sufficient to recharge during the night, particularly in the peak of summer when you have the longest days and shortest nights.

For the battery size, I am concerned about the broad range between 40watts and 200watts NVR power consumption. With the battery charger I am concerned about the 80ah that will need charged back with a 5amp battery charger during the night. It will take 16 hours to recharge an 80% drained 12v 100ah battery with this 12v 5amp battery charger.

Any insight into where my calculations might be wrong would be greatly appreciated.

Answer 1

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 "charge with solar in the day and discharge at night"). 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 "car cigarette lighter power adapter") to use 12V DC, but that will avoid the losses of DC->AC->DC conversion, which will further reduce power usage.