Confused about batteries

fishsticker

Hello, I am new to solar so please bare with me. I am trying to build a simple system to start with. I have 2 devices i want to power that run 24/7. The devices are a linksys router (120v @ 1.2A) and a cable modem (120v @ .8 A). I have found some load calculators that say i need 16 to 18 12v batteries with i think 100 or so AH. Does this sound right for these 2 devices? Seems like a ton of batteries for just these 2 devices. Please tell me i am figuring wrong??

Thanks in advance!!!

mike95490

A single deep cycle battery, 12V 90Ah, should last 2 days, add 20W of solar, and it should be all set.
 BUT, battery charging voltage of 14.5V may make the 12V gear upset

BB.

The "labels" on AC devices are usually "wrong" (at least do not provide accurate information for our use).

If you can, get a Kill-a-Watt type meter (some libraries have them to loan out) and measure your loads. Run them for 24 hours to get a Watt*Hour per day average.

You also did not tell us much about your needs... is this 5/10/24 hours per day?

Also--Why solar power? For emergency backup? Many times, cable internet will fail if AC mains fail, or a few hours after AC mains have failed. So--Long term solar power/emergency backup may not be real useful.

And what about what connects to the router/modem (laptop, security system, etc.)--Solar power / backup power need to be supplied too?

-Bill

fishsticker

This would be a 24/7 setup. I am in just now in the learning stage and i thought i would start out with a few small things in our basement linke the linksys and cable modem and maybe a few LED lights. Eventually would like to go bigger maybe whole house and our lake house. All things that connect to the linksys are wireless, phones, tv etc....no need for solar for them right now. Thanks again!

BB.

I still highly suggest a Kill-a-Watt type meter... Just about required to plan an AC off grid power system. And very handy to understand your present home loads and what would be a good candidate for conservation (turn off when not used, refrigerator becoming inefficient from dust/lost of Freon, etc.).

A 500 to 1,000 WH per day system would power quite a bit of daily "nice to have" loads. This stuff all scales, so if a 1,000 WH per day system is too big, just cut the stuff below by 1/2. Using our basic off grid home design rules of thumbs (just a starting point):

• 1,000 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery bank * 2 days of storage * 1/0.50 max discharge = 392 AH @ 12 volt battery bank (nominal design)

So, that works out to 2x 6 volt @ 200 AH batteries in series * 2x parallel string for 12 volt @ 400 AH battery bank--A very reasonable/capable battery system.

Next, sizing the solar array--We size both based on battery bank size (standard charging rates) and based on hours of sun by season. First based on charging rate of 5% to 13% rate of charge for solar. 5% is good for weekend/summer seasonal use. 10%+ is good for full time off grid system (good battery life, don't have to "manage power usage" every day, etc.):

• 400 Ah * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 377 Watt array minimum
  
• 400 Ah * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 753 Watt array nominal
  
• 400 Ah * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 979 Watt array "cost effective" maximum

And then there is based on hours of sun per day by season...
http://www.solarelectricityhandbook.com/solar-irradiance.html

New Orleans
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 60° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.46	3.94	4.73	5.33	5.42	5.05
Jul	Aug	Sep	Oct	Nov	Dec
5.07	4.99	5.02	4.90	4.05	3.50

So... use 3.46 hours of sun (January) as break even month (a goodly amount of sun, may need genset/AC mains to run through stormy weather):

• 1,000 WH per day * 1/0.52 off grid system eff * 1/3.46 hours of sun (Jan) = 555 Watt array minimum (break even for January)
  

Note--If you are planning on a good amount of day time "base loads", you should plan on 10%+ solar array... In general, off grid solar works "best" (most effectively) with day time charging and afternoon/over night loads. If you have a "too small array", your loads take charging current from the battery bank (deep cycle battery banks worst "best" with ~10% rate of charge when daily cycling).


Next, what size AC inverter... For a 400 AH @ 12 volt battery bank--Our rules of thumb would be a maximum of 1,000 Watt of AC inverter... If you do not have high loads (more hours of smaller AC loads)--A 300 Watt TSW 12 volt inverter from MorningStar is a really nice unit. Has low power sleep and remote on/off function.

https://www.solar-electric.com/mosu300wasiw.html

And there you go with my suggested design knowing very little about your personal set of choices/needs regarding AC loads. Your thoughts?

-Bill