Please check my math

woowoo2woowoo2 Registered Users Posts: 13 ✭✭

I put together a small system for a storm shelter.

2 ea group 29HM / 675 CCA / 205 min reserve capacity deep cycle batteries

A 100 watt Renogy panel on a tilt mount

A 10 amp Renogy wanderer PWM charge controller

This was originally put together as a power system in case the storm shelter was in use, day to day there was not a load.

Now I would like to add a 20 watt led light, on a timer for six hours a night.

Does the math below seem right?

Battery Capacity:

2 group 29 deep cycle batteries = 410 min of reserve capacity = 170amp hours

170 / 2 = 85 amp hours to 50% discharge.

85 / 2 ( days) =42.5 amp hours (cloudy days)


100 watts de-rated to 80 x 4 hours of sun = 320 watt hours / 12 volts = 26.6 amp hours


20 watts x 6 hours = 120 watt hours, add 10 more for the timer =130 watt hours.

130 / 12 volts = 10.8 amp hours


  • woowoo2woowoo2 Registered Users Posts: 13 ✭✭


  • woowoo2woowoo2 Registered Users Posts: 13 ✭✭
  • mike95490mike95490 Solar Expert Posts: 9,358 ✭✭✭✭✭

    For float use, that small panel is OK. When you go to cycle the battery in a storm, you are going to consume the battery quickly and require several days to recharge - if there is sun after the storm and the panels are intact

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • woowoo2woowoo2 Registered Users Posts: 13 ✭✭

    Yes, After any event, the batteries would need to be charged by grid power or generator.

    But, as configured would the system support a 20 watt led light on every night?

  • PhotowhitPhotowhit Solar Expert Posts: 5,650 ✭✭✭✭✭

    Missed several things on the math,

    with an PWM charge controller you have to divide the wattage by the VMP not the nominal battery voltage (12 volts) so 320 watts / 17.8 = 18 amps. This may be a bit more as much of the losses are in voltage.

    Charging flooded lead acid batteries is only 80-85% efficient, so multiply that by 18 amps by .85

    Is the 20 watt led light a DC light? If AC you will have additional losses.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin

    A 20 Watt LED lamp can be pretty bright... So, it is always a good idea to try a few different lamps first, and see what works well for you.

    There are also color temperature to think about too (>6,000K--Blue White; 4,500 to 5,500 "neutral white; less than 3,000K is warm white).

    Also, the way our eyes perceive light is, more or less, logarithmic... Very roughly, you would have trouble seeing the difference between a 10 watt and 20 watt lamp (factor of 2x difference). A factor of 10x difference (2 Watts vs 20 Watts), the smaller light is pretty much "lost" when compared to the larger light.

    So, you would want to have as small of lamp as possible for long(er) battery life. And you may want to look at several different lamps (or dimming lamps)--20 Watts is (usually) a pretty bright lamp and you probably want something much less for when you are getting ready to sleep, want just enough light to take care of business (rather than turning on "the sun" in the middle of the night).

    Also you need to figure out how many hours a night you want to light(s) to be "on". Say 5 hours per night at 20 Watts, and 12 hours a night at 0.5 Watts (night light).

    20 Watts * 5 hours = 100 WH per night (big light)

    0.5 Watts * 12 hours = 6 WH per night (night light)


    106 WH per night total

    106 Watt*Hours / 12 volt battery bank = 8.83 AH per night

    If you allow 50% discharge of your 170 AH battery bank:

    170 AH * 0.50 max discharge = 85 AH of "nominal" capacity

    85 AH / 8.83 AH per night = 9.6 Days of storage

    Generally, "car batteries" (batteries rated in CCA and Reserve Capacity), are not "deep cycle" storage batteries. "Car Batteries" (SLI--Starting Lighting Ignition) batteries tend to work best if not taken below ~85% state of charge. Marine batteries are somewhere between SLI and Deep Cycle batteries.

    In general, using solar+battery bank for simple emergency lighting--It is usually "overkill" for the amount of energy needed, and not great for standby (sitting, hopefully, for months or a few years of non-use).

    Unless you want to run some other devices (cell phone, tablet computer, radio, etc.), I would suggest you look at something else... Flashlights and Headlamps (hands free usage of lights when you are working/reading/etc. is pretty nice).

    For example, just to give you an idea of how a multi-function headlamp stacks-up (there are many choices out there--Just an example).

    5 lumen = 100 Hours (enough light to walk around and not trip)

    50 lumen = 29 Hours (enough to comfortably read)

    150 lumen = 10 hours

    400 lumen = 3 hours

    950 lumen = 48 minutes (see ~300 feet out).

    That is with a single 18650 2,500 mAH 3.7-4.2 volt Li Ion battery

    2.5 AH * ~3.7 amps = ~9.25 Watt*Hour battery (less than 1 AH @ 12 volt battery)

    Going back to Watts (estimated Wattage at LED):

    9.25 WH / 100 Hours = 0.09 Watts

    9.25 WH / 29 Hours = 0.31 Watts

    9.25 WH / 10 hours = 0.925 Watts

    9.25 WH / 3 hours = 3.1 Watts

    9.25 WH / 0.75 hours = 12 Watts

    A 1,000 Lumen (~10-12 Watt) LED flashlight is about the brightest you would need around the home (not doing search and rescue).

    A Li Ion battery would probably last >5 years in storage between charging (for rechargeable), and you can get 10 year Alkaline and Li Ion non-rechargeable batteries.

    There are lots of LED flashlights out there these days--And I would highly suggest them for emergency use and skip the solar (less than a week or two). Get common battery powered lights and (for example) an AM/FM Radio--And you are pretty much set (and don't have to worry about the solar panel(s) being blown off the roof or damaged by debris carried by the wind.

    A couple of "flashlight" forums:

    Not to say that solar power is not useful... But I would suggest that is be used for larger power needs (exterior lighting, laptop, tablet, cell phone, radios, etc.).

    Lead Acid batteries in a pure standby application (waiting for a storm), are not usually the best store and forget solution. There are AGM batteries (sealed lead acid) which are nicer, but more expensive (you don't have to check their water level monthly). For inexpensive lead acid ("golf cart" batteries), they can last 3-5 years (cycling or float charging). And better AGM can last 5-7 years.

    Li-Ion batteries are really nice, but not cheap, and have some other issues (balancing, over/under voltage issues, etc.). Most have very low self discharge and can go 5-10 years between charging (or replacement).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,358 ✭✭✭✭✭

    This application would be suited for EXIT light batteries, designed for long float service and occasional heavy use.

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • SolfyrSolfyr Registered Users Posts: 22 ✭✭

    Hmmm , since using wordings like " min capacity " and amp hours instead of Ah ,

    there seems to be some quirky mind around , that puts a tin shelter beneath huge trees

    and pv panels encircled by tall trees ....

    Assuming 12 Volt Batteries at 205 Ah , and in parallel as the

    large MB pictures suggest , that would give a 12 V * 205 Ah * 2 bank of ~ 4860 Watt hours (Wh) ,

    or 4.8 kiloWatthours .

    When discharging those to 50% State of Charge , so that remain 2.4 kWh inside

    the batteries , then at 30 Watt consumption over night would likely draw 16 hours * 30 Watt = 480 Watt

    during "stormy" winter days , and 30 W * 6 h = 180 Wh during summer nights .

    2400 Watthours ÷ 480 Watthours = 5 days , 2400 ÷ 180 = 13 days , without any charging from the PV .

    When the lonely panel is capable to supply 30 Watts on average during 4 hours ,

    since encircled by trees , then 120 Wh could be stored in the batteries during daytime .

    At a total draw of 60Wh each day ( -180 + 120 ) , then in theory , the batteries would

    reach 50% SoC after 2400 ÷ 60 = 40 days , until they would need to be recharged by grid power .

    Overall , it appears , that a reduction to 25 or 20 Watts continuous draw could at least

    keep the batteries somehow up , without discharging them after a while .

    And as said by a former poster , 20 Watt of some LED bulb is pretty bright , if not "blinding" .

  • woowoo2woowoo2 Registered Users Posts: 13 ✭✭

    Sorry I was not clear on the lights usage earlier.

    It is a 12 volt, 20 watt outdoor fixture, on a 12 volt timer.

    My goal is to always have light outside the shelter.

    With or without the light, the 100 watt panel should keep the batteries from self discharging over time.


    Thanks for everyone's input.

  • SolfyrSolfyr Registered Users Posts: 22 ✭✭

    Depends on how much Watthours the panel would provide into the battery .

    Overall , it appears to be possible .

  • EstragonEstragon Registered Users Posts: 4,495 ✭✭✭✭✭

    To reinforce @Solfyr comment about trees, any shade from trees will severely limit output. Even if the tree is leafless in winter, bare branches of a tree that close will be enough to pretty much kill production.

    Assuming the panel in the pic faces roughly south, and the area to the east is wide open at all times of year, it looks like output will likely cease around solar noon. In most climates (eg not prone to morning fog, etc), that means cutting daily average production expectation by about 1/2.

    Personally, I'd consider taking out a tree like that anyway, leaning toward the garage as it appears to.

    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • woowoo2woowoo2 Registered Users Posts: 13 ✭✭

    The panel is facing due south, it gets all the eastern sun, and goes into shade around 4pm.

    Thanks for the comments folks!

  • woowoo2woowoo2 Registered Users Posts: 13 ✭✭
    I got the lights installed  2 ea - 10 watts /12v
    They are on a timer for seven hours a night.
    It's only been a few days, but in the morning, when the lights go out and before the sun comes up the batteries are at 12.6 volts.

    I put switches on the lights so that I can turn one off on days of reduced production.

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