Power for Security lights

I need some help/advise for my first off the grid project. I have to start somewhere, so I am starting with my outdoor security lights and when this is done I will get my well water off the grid and eventually the house. Starting out small, but will grow as time and money permits.

For now I want to power my security lights at night with batteries and recharge them during the day with grid power, eventually recharging with solar power.

Parts I have to start with:
1) A 70-watt high-pressure sodium security light/standard area light with dusk-to- dawn photocell for automatic on/off.
2) Two standard Everstart 27DC-6 Deep Cycle batteries connected in parallel with about 300 amps.
3) A battery charger/trickle charger.

I would like to power the lights with battery power at night, 10 –12 hrs or so, and in the day use a timer to turn the charger on to recharge the batteries.

Questions:
1) What size/watt inverter to get? (maybe big enough to also use on the well water, later on)
2) Will the batteries power the light for 10-12 hours?
3) How long to charge the batteries? I don’t know if the charger will automatically turn off when the batteries are fully charged or should I just set the timer for X amount of time and keep adjusting the start/stop settings until I get it right?

Thanks,

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Power for Security lights

    37SB,

    What is your intent? To save money? Not going to happen with Off-Grid power...

    Basically, you are paying $0.10 per kWhr... For an off grid system, you will be paying roughly $1.00 to $2.00+ per kWhr (assuming 20 year system life, and replacing 1-4 sets of batteries over 20 years)...

    0.07W * 10 hours * $0.10 per kWhr = $0.07 per night ($2.10 per month)
    0.07W * 10 hours * $1.00 per kWhr = $0.70+ per night ($21.00 per month)

    To have a $$$ efficient off-grid system, you should work on reducing your energy usage...

    Replace the constant on light and instead use a filament or LED lamp with motion detector... Now, running something like that from solar is worthwhile.

    More than happy to run the numbers for you--but it is almost never worth the conversion from Grid Power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Power for Security lights

    We live out in the country with weak/crazy power from the grid. My electric bill is cheap, so no complaints about that. Problem is we can lose power when it rains, snows or even on a windy day.
    What really got me thinking about this, is our water well being on grid power. My well is 220 ft deep with an submersible electric pump, and I want to be able to draw water out and have available power for other things regardless of grid conditions, like an emergency backup system.
    The security light is just a good place for me to start, it does need to be on all night. This allows me to experiment/learn without doing damage to our well or anything at the house.

    Thanks Bill
  • mikeomikeo Solar Expert Posts: 386 ✭✭✭
    Re: Power for Security lights
    The security light is just a good place for me to start, it does need to be on all night
    This is what I use for a solar secruity light

    http://tinyurl.com/ykad2k6

    They are on sale at my local HF for $19.95 . Hard to put a solar light together cheaper than this
  • System2System2 Posts: 6,290 admin
    Re: Power for Security lights
    This is what I use for a solar secruity light

    http://tinyurl.com/ykad2k6

    They are on sale at my local HF for $19.95 . Hard to put a solar light together cheaper than this

    I will be by a HF tomorrow, maybe they will still be on sale. Is it dependable? At that price I may need to get a couple of them. ;)
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Power for Security lights

    The issue with exterior lighting is that you need the most power when you have the least sun (long winter nights)... And one thing that kills batteries is lots of deep discharging. Not many off-the-shelf products really have cumulative Amp*Hour meters that can measure energy usage from the battery bank...

    So, at best, they may have a voltage level cut-off--which is not very accurate. There are programmable Battery Monitors that can turn off the inverter at 50% battery usage--but it is hard to justify the $200+ for such units.

    However--a good Battery Monitor will help you save doing dumb things to your battery bank (as we all have when we first started :roll:). And just about a requirement for AGM/Sealed battery banks.

    Anyway, a good way to estimate the size of your system:

    70 watts x 12 hours per day = 840 Watt*Hours pe day (0.84 kWH)
    0.84 kWhrs per day * 30 days = 25.2 kWhrs per month

    Using the PVWatts Website, for Fort Worth Texas...
    • Use 1 kWatt for solar panels (a round number)
    • Use 0.52 derating for off-grid (AC inverter + flooded cell batteries)
    • Use defaults for everything else
    [FONT=Fixedsys]Results
    
    Month     
    Solar Radiation (kWh/m2/day)     
    AC Energy (kWh)     
    Energy Value ($ at $0.097 per kWhr)
    
    1      4.19          64        6.21   
    2      5.13          70        6.79   
    3      5.35          79        7.66   
    4      5.26          75        7.28   
    5      5.62          80        7.76   
    6      6.06          82        7.95   
    7      6.43          88        8.54   
    8      6.12          83        8.05   
    9      5.79          79        7.66   
    10     5.78          84        8.15   
    11     4.88          70        6.79   
    12     4.22          64        6.21   
    =====================================
    Year      5.40          918        89.05  [/FONT]
    

    So, you need 25.2 kWhrs per month and December/January generate ~64 kWhrs per month per 1,000 watts of solar panels:

    25.2*1/64kWhrs per month per 1,000 watts of panels = 394 watts minimum of solar panels

    Battery size... You need 840 watts per day of power, assume the inverter is 85% efficient and you are starting with a 12 volt battery bank. The rule of thumb is 3 days of no-sun, and 50% maximum battery discharge:

    840 watts * 1/12 volt bank * 1/0.85 invrt eff * 3 days * 1/0.50 max discharge = 494 Amp*Hours of 12 volt battery bank.

    You can look through our host's website for recommended vendors.... Your system is right on the edge of a 12 volt system vs a higher voltage (charge controllers are rated on Amp output... A small charger can output 15 amps at 15 volts = 225 watts or at 30 volts = 450 watts -- basically the higher voltage battery bank has advantages of lower current for the same amount of power--smaller wires, less voltage drop issues, etc.).

    If you want a "nice" system... Look at the Xantrex XW MPPT 60 Amp Charge controller (12-48 volt battery bank), the MorningStar 300 Watt 12 volt True Sine Wave 115 VAC 60Hz Inverter (12 volt only), and pick your batteries from here or a local supplier (shipping can be a pain).

    If you want to keep the costs down, look for "12 volt" (really Vmp=~17 volts) solar panels (try 100+ watt panels, generally larger are cheaper). And get a nice Xanterx or MorningStar PWM charge controller (cheaper and will work fine for your application).

    By the way, I have a thread where I (and others) have posted a lot of information for new readers here... See if it gives you some ideas on how to proceed (and feel free to add information/questions to that thread too--we are trying to make it good starting point for new-to-solar folks).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sawmillsawmill Solar Expert Posts: 93 ✭✭✭
    Re: Power for Security lights

    If you want a security light that stays on during hours of darkness Lowes has a 19 a 26 watt model that will really save quite a lot on electricity consumption.
  • blackswan555blackswan555 Solar Expert Posts: 246 ✭✭
    Re: Power for Security lights

    You also need to check the power factor of the HPS,
    It may say 70w on the label, but in real terms it will use more like 80 > 90 w

    Have a good one
    Tim
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