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Thread: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

  1. #1
    pmaru77 Guest

    Default newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

    Hi all,

    Been reading up on solar sytems etc. I'm going to put in a small system in my Baja house, but I'm not sure of my needs yet.
    From what I've read a 120w 120vac bulb would use 1amp on a 120vac system and would use 10 amp in a 12vdc system. A 12v battery with 35ahr would run this bulb for only 3.5 hours ...and less because of the efficeincy of the inverter. Is this correct?

  2. #2

    Default Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

    You're on the right track. However, fully discharging a battery will lead it to an early death, so it's good practice to not discharge the battery below 50% state of charge, and typically not below 80% SOC (= using just 20% of capacity) on a regular basis. Using the 50% rule and assuming a 90% inverter efficiency, you could run a 120 W 120 VAC bulb for 35 Ah x 50% / (10 A / 90%) = 17.5 Ah / 11.1 A = 1.58 hours.

    This is a good example of why standard incandescent lamps should be replaced with compact fluorescent lamps (CFLs). A 14 W CFL puts out as much light as a 60 W incandescent. So, the same battery could power two 14 W CFLs instaed on one 120 W incandescent bulb for ~6.77 hours, and require a smaller (cheaper?) inverter.

    Six-packs of mini-spiral CFLs are available at Home Depot for ~$10. DC CFLs are also available, but they're typically fairly expensive.

    HTH,
    Jim / crewzer
    120618: System off-line for a while...

  3. #3
    Join Date
    Sep 2006
    Location
    Willits, CA
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    5,714

    Default Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

    Quote Originally Posted by pmaru77
    Hi all,

    Been reading up on solar sytems etc. I'm going to put in a small system in my Baja house, but I'm not sure of my needs yet.
    From what I've read a 120w 120vac bulb would use 1amp on a 120vac system and would use 10 amp in a 12vdc system. A 12v battery with 35ahr would run this bulb for only 3.5 hours ...and less because of the efficeincy of the inverter. Is this correct?
    Just about. Assume 30 useable amp-hours (50% discharge). That's 360watt hours, spend it anyway you like.
    You could get a 12v LED bulb and run direct. and skip inverter losses.
    (some expensive ones here, but good descriptions http://www.ccrane.com/lights/led-light-bulbs/index.aspx )

    So for 120V via an 80% efficient inverter 360WH gets 2.4 hours
    360WH : 80% = 288WH remaining / 120V = 2.4hrs



    Camping, I take a small inverter, and hook many strings of LED xmas lights on it, runs all night on a single COSCO deep cycle battery.
    http://www.sbainc.com/PStuff/060828-...s/img_4153.jpg panel is on right edge of light strings
    the photo is of a more elebrate setup, with lots of EL wire, strung up on a 45' tall tower with sign, 2 batteries, and 2 panels ran it all night for a week.
    http://tinyurl.com/LMR-Solar
    http://tinyurl.com/LMR-BigLug
    http://tinyurl.com/LMR-NiFe

    Grundfos 10 SO5-9 with 3 wire Franklin motor (1/2hp 240V 1ph # 214505 ) on a timer for 3 hr noontime run - Runs off PV
    Powerfab poletop PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe battery | 15, Evergreen 205w "12V" PV | Midnight ePanel || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT

  4. #4
    Join Date
    Jun 2003
    Location
    pittsburgh, pa
    Posts
    10,282

    Default Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

    you are correct as far as the inverter efficiency goes. if an inverter would have a 90% efficiency then it needs 10% more power to produce what you are using. now this efficiency will vary with different makes and models and will also vary with the inputted power to the same inverter. in other words the efficiency changes throughout the inputted power range and usually the peak efficiency is what a company will list.
    as to the battery, that would be wrong. you would need roughly 10-15% more for a standard lead acid battery due to charging efficiencies and you cannot excede the 50% dod point. around the 15% point would mean another 5ah of current for a 35ah battery, but because of the 50% dod i'd say a good 85-90ah battery would be needed to do what you describe.
    NIEL

  5. #5
    barshnik Guest

    Default Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

    OK, another newbie question - you talk about a 35 or 65 or 90 AH battery - what exactly does this mean? I see many ratings for my Trojans: Capacity in minutes at 25 amps, capacity in minutes at 75 amps, cranking performance (I guess not a factor), 5 Hr AH rate, 20 hr AH rate. So, what do these really mean, and which do I use (if I know approx. how many amps per hour my trailer accessories consume)? The batteries in question are Trojan (2 ea, in parallel) SCS225. Thanks,

    John F
    LV, NV

  6. #6
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    Default Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

    Quote Originally Posted by barshnik
    OK, another newbie question - you talk about a 35 or 65 or 90 AH battery - what exactly does this mean?
    The amount of energy stored in a battery.
    [edit - hypothetical values, for educational use, your mileage may vary ]
    A motorcycle battery has 30AH
    A car battery has 500AH
    A large deep cycle battery has 1,000AH

    AH = Amp Hours It can put out (car battery)
    500Amps for 1 hour,
    1 amp for 500hours
    It's a standardized rating system, so that you know what size battery you are buying.
    In actuality, you only get about 80% of that rating, and it varies at the speed you withdraw it.
    http://tinyurl.com/LMR-Solar
    http://tinyurl.com/LMR-BigLug
    http://tinyurl.com/LMR-NiFe

    Grundfos 10 SO5-9 with 3 wire Franklin motor (1/2hp 240V 1ph # 214505 ) on a timer for 3 hr noontime run - Runs off PV
    Powerfab poletop PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe battery | 15, Evergreen 205w "12V" PV | Midnight ePanel || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT

  7. #7
    Join Date
    Mar 2006
    Location
    SF Bay Area (California)
    Posts
    21,374

    Default Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v

    I think the typical 12 volt car battery is only rated for around 75-120 amp*hours (weight of ~50-70 lbs). And because a car battery is designed for starting, it should only be discharged by about 20% (not the 50% or more for batteries designed for storage)...

    500 AH car battery would be pretty rare I would think. 1,000 amp*hours is a very big storage battery (for 12 volts worth of cells--that would be around 600 lbs).

    Here is a pretty good link that explains 12 volt power systems in terms of RV's (recreational vehicles)... Might answer many of your questions.

    http://www.ccis.com/home/mnemeth/12volt/12volt.htm page 1

    Page 2 includes solar chargers and inverters...

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  8. #8

    Default Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

    you talk about a 35 or 65 or 90 AH battery - what exactly does this mean?* I see many ratings for my Trojans:* Capacity in minutes at 25 amps, capacity in minutes at 75 amps, cranking performance (I guess not a factor), 5 Hr AH rate, 20 hr AH rate.* So, what do these really mean, and which do I use (if I know approx. how many amps per hour my trailer accessories consume)?* The batteries in question are Trojan (2 ea, in parallel) SCS225.
    John,

    GREAT question! Unfortunately, the answer is fairly complicated. Let’s use your Trojan SCS225 batteries as an example.

    http://www.trojanbattery.com/Product...px?Name=SCS225

    For deep-cycle applications, the important specs are voltage, reserve capacity (RC) and 20 hour AH capacity. Your batteries are nominal 12 V models. The RC spec is 225 and the 20 hr rate capacity is 130 Ah.

    RC is a specification for how long (in minutes) a healthy and fully charged battery will sustain a 25 A discharge at 77 degrees F until the battery cell voltage drops to 1.75 V. For a 12 V battery with six cells, that’s 10.5 V, at which point the battery is considered fully discharged. Continued discharge may permanently damage the battery. So, under the conditions described, each of your batteries can sustain a 25 amp load for 225 minutes, or 3-3/4 hours.

    To me this spec is useful because it is load based (25 A) and it allows direct comparison of batteries’ RC specs.

    The 20 hour rate spec defines how many Amp-hours (Ah) the battery can deliver at 77 degrees F when under a constant-current load that will cause the cell voltage to drop to 1.75 V (10.5 V for a six-cell 12 V battery) in 20 hours, at which point the battery is considered fully discharged. Continued discharge may permanently damage the battery. So, under the conditions described, each of your batteries can sustain a 130 Ah / 20 hours = 6.5 amp load for 20 hours.

    I don’t care for this spec because it is time based. I’d rather see this spec based on a constant load of 5 A, as this would allow direct comparison between different batteries for the same anticipated load.

    You may see other battery specifications such as CA, CCA, and MCA. These generally apply to starting batteries (“SLI” batteries) and to hybrids (Marine/RV) used for both starting and deep-cycle applications, but they are of little practical use in just deep-cycle applications.

    It’s not a good idea to fully discharge a battery, as that treatment will substantially shorten its useful life. Although Trojan allows for a maximum 80% discharge, 50% is a more common rule-of-thumb. For daily discharges, a 15% - 20% discharge limit is typical. No matter what, it’s good practice to recharge your batteries as soon as possible after each discharge.

    Battery specs and performance can get complicated when temperature or different loads get thrown into the equation. Figure on a 1% loss of capacity for every degree C drop below 25 C (77 F). In other words, a fully charged battery may be able to deliver only ~75% of its rated capacity when ambient temps are at 32 F (0 C).

    Varying loads throw the capacity numbers for a real loop. For example, the SCS225 RC spec of 225 suggests that the battery’s capacity is 25 A x 3-3/4 hours = 93.75 Ah. But, the 20 Hr spec says its capacity is 130 Ah.

    ***?

    Welcome to Peukert’s Law, which basically states that a battery’s capacity is temporarily increased as discharge current is decreased, and that capacity is temporarily decreased as discharge current is increased. Related topics are Peukert’s Equation and a battery’s Peukert Number. You can research these topics in my previous posts and/or on the Internet. Once you’ve mastered the concept, you’ll no doubt be the center of attraction at just about any bar that I can think of.* :evil:

    What it really means is that a battery’s capacity is not a constant. Instead, battery with a 20 hour rating of 130 Ah will deliver 130 A for far less than one hour, and it can deliver 1 A for a bit longer than 130 hours.

    As a practical matter, the Peukert Number for your batteries, based on their RC and 20 Hr specs, is 1.24. This suggests that your two well-maintained and healthy batteries, which together appear to be 20 hr rated for 260 Ah, could sustain a 5 A load for ~65.57 hours (100% discharge), indicating a load-normalized capacity of ~328 Ah at 77F (25 C).

    In other words, that’s a fine pair of batteries you have. Aren’t you glad you asked?* :wink:

    HTH,
    Jim / crewzer
    120618: System off-line for a while...

  9. #9
    barshnik Guest

    Default Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

    Thanks for taking the time for the explainations, crewzer/Jim, and the link provided by BB.* Between them, I'm just a little less confused than before.* I'll have to re-read everything a few more times (my mind is in 'bulk' mode right now, not even approaching regulation) before I can absolutely fascinate everyone at the bar.

    I got a cheap digital 20A current meter off eBay, will install it when it arrives, then will be able to accurately measure each of my loads, estimate time usage, and arrive at a run-time based at (for winter camping) 32 degrees and a absolute minimum 50% SOC.* I'm anticipating my max drain at any one time to be around 16 amps for short periods of time (furnace fan + water pump + lights + LP leak detector) and max charge current of 8.6 A (the highest I saw reported by the TriStar).

    BTW, new TriStar is installed, custom program created for the Trojans (including their .028 temperature compensation) according to charge specs on their website, and is working.* It did take 1.5 days for them to finally reach 'float' mode, at which time I considered them fully charged.* Measured SG of each cell, and they are close to spec (although more variation than they recommend, but I'll give them a few more discharge/charge cycles before I consider having the distributer check them under warranty.)

    Armed with all my new knowledge, I'll be able to explain in detail to my wife why we must freeze our a**es off sleeping without the furnace after a couple of overcast days and batteries at 50% SOC <g>.

    Sorry, didn't mean to hijack the thread, but I've really been wondering how all these battery ratings work...

    John F
    LV, NV

  10. #10
    pmaru77 Guest

    Default Re: newbie question......How long will a 120 watt 120vac bulb run on a 60Ahr 12v bat

    Thanks all .... for the feedback. Hijack thread? This thread could only go so far anyways.

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