Total newb question about power supply size

[antoinette3173]

I have a piece of vacant land that I want to add some wireless Wifi security cameras to. I'm trying to figure out what my best option is for the power supply for the equipment. The Wifi receiver and router need about 6-8w, per the technician, and will need to be running 24 hours a day. The security camera base station spec sheet doesn't specify its power consumption, but I've seen guesses of up to 18w and another person's claim of 4w using the Kill-A-Watt (which will also need to be running 24 hours a day). Assuming my total power need is between 12w-26w running constantly, how can I best determine what size battery bank I need to last a week...a month...? I'd like to calculate this as if there is no solar input (I want to determine the worst-case scenario baseline first), although I will be wiring up some small panels while I'm away and 100w panels while I'm there.

 

[John Frum]

You mention a Kill-A-Watt are we to understand that these are 120 volt loads, confirm?

Determine your daily usage in watt hours then multiply by the number of days you wish.

If the loads are 120VAC then you can just plug everything into a Kill-A-Watt and get accurate numbers.

When you go to run your loads off of an inverter remember that it uses a little bit of power just to run itself and that dc to ac conversion is typically ~85% efficient so you will need to factor that overhead into your battery size.

 

[antoinette3173]

12v DC... although now that I think about it, I need to confirm that.

By my (possibly inaccurate) calculation: 26w * 24 hours = 624w per day * 30 days = 18,720 w/month
18,720 w / 12v = 1560 amp hours

Am I way off? That seems crazy!

 

[John Frum]

12v DC... although now that I think about it, I need to confirm that.

By my (possibly inaccurate) calculation: 26w * 24 hours = 624w per day * 30 days = 18,720 w/month
18,720 w / 12v = 1560 amp hours

Am I way off?

Should be good assuming your 26 watt estimate is accurate.
You don't indicate if an inverter is required, if it is you will need to factor that in.
26 watts * 24 hours * 30 days = 18720 watt hours
18720 / .8 bandwidth management overhead = 23400 watt hours
23400 watt hours / 12.8 lifepo4 nominal volts = ~1828 amp hours
That is a lot of battery!

 

[John Frum]

The most cost effective cells here are the big blue prismatics.
3.2 volts nominal * 280 amp hours * 28 cells = 25088 watt hours.

 

[John Frum]

Oops forgot the @snoobler factor(plan on the cell capacity being 90% of the advertised capacity).
26 watts * 24 hours * 30 days = 18720 watt hours
18720 watts hours / .9 snoobler factor / .8 bandwidth management overhead = 26000 watt hours
26000 watt hours / 12.8 lifepo4 nominal volts = ~2031.25 amp hours
26000 watt hours / (12.8 * 280 ) = ~7.25
That means 32 cells.

 

[antoinette3173]

That is CRAZY! 32 cells to run an internet router and camera base station?!

 

[John Frum]

That is CRAZY! 32 cells to run an internet router and camera base station?!

Yep, 400 pounds of lifepo4 battery.
You don't mention whether an inverter is required.

 

[John Frum]

26 watts * 24 hours * 30 days = 18720 watt hours
18720 watts hours / .85 dc2ac conversion factor = ~22023 watt hours
~22023 watt hours / .9 snoobler factor = ~24471 watt hours
~24471 watt hours / .8 bandwidth management overhead = ~30588 watt hours
~30588 watt hours / (12.8 * 310 ) = ~7.7
That means 32 of the new 310 amp hour cells.