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Q & A: battery power storage for solar cells

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Most recent answer: 04/25/2013
I'm looking to replace 25 AC light fixtures with DC fixtures to run 12v 13w LED's off solar charged 12v batteries that are 550amp. Can you maybe tell me how many watts I would have to generate per day from the solar panels to keep the batteries charged and how many batteries it would take to run this many lights and not have the batteries drain more then 70% per 14hr no sun period??
- Aaron (age 23)
Muskegon Mi USA
With 12Volt 13W LEDs, you'll draw I = Watts/Volts = 1.08 Amperes per lamp from the battery.  One LED light switched on for 14 hours will consume 15 Ampere-Hours of the total  number of amp-hours available from each battery.   Multiply that by the number of LED's (25) to find the total capacity needed: 375 Amp-hours.

Perhaps the '550amp' you mention is actually 550 amp-hours, the battery's total storage capacity. If so, you're already set. If instead it's the 'cold start cranking current' that is sometimes used in battery specifications, you'd need to look around for what the storage capacity specification is. Make sure it is of the so-called 'Deep Cycle' variety, designed to provide a medium-sized current for a long time rather than a lot of current for a short time.


p.s. You also asked about how many watts of solar generation capacity you'd need. Your 25 LEDs will use 325 W. It's unclear if you'll want them on all day or part of the day. Let's say you don't need them all the time and just need 200 W on the average. The battery storage-release is only around 90% efficient, or worse, so maybe you'll need around 250 W average power generation over the day. Since there's no sunshine at night and you will also want to handle some cloudy days, it would be a good idea to have about 1000W peak solar capacity. BTW, the need to handle cloudy days suggests that you might want some excess battery capacity too. Mike W.

(published on 08/22/2012)

Follow-Up #1: solar powered lights

How do I figure the size of a solar battery needed for a solar panel. I am an artist creating a major sculpture and need to light the objects with halogen bulbs which will be electrified as well as run on solar power. Remember I am an artist and not a "Mechanically adroit" person!!!
- rick (age 69)
We've answered something like this before, so I've marked it as a follow-up.You should also consider LEDs, which come in a variety of color temperatures. They're far more practical than halogens for this sort of use.They cost more, but you'll only need about 1/4 or less of the batteries and solar cells that you'd need for halogens.

Mike W.

(published on 02/19/2013)

Follow-Up #2: battery rating systems

Batteries are normally expressed on volts and AH (Ampere-hour), so I go to a store looking for a 12V 4AH battery and he gives me this 12v, 6 cell, 21W @ 15 minute-rate to 1.67V per cell @ 25°C (77°F) battery and says it's a 12v 5AH which is better (or more). Can you tell me if that is true and how do you get from that long thing to 12v 5AH?
- Leonardo (age 34)
This is tricky. I looked around on the web and found a battery with those specs, an HR1221WF2. Some sites say its capacity is indeed 5.1AH. The interesting thing about the other specs is that they say it runs down significantly (to only 6x1.67V=10V total) in 15 min when supplying 21W. Since 21W at 12V requires 1.75 A, and 15 min is only 1/4 hr, that sounds like only 0.44 AH. That's a lot less than 5.1 AH!

 If I understand correctly, what this means is that it can supply a lot more energy when it's supplying a low current over a long time than when it's supplying a fairly large current over a short time.

So, if the specs are honest, you should be ok if you're planning a low-current application.

Mike W.

(published on 04/25/2013)

Follow-up on this answer.