Solar Panel and Battery Bank Sizing

[Matternmatt2]

Hello, I built a road side sign for my Detail Shop. The lighting is a string of LEDs, I intend to use 2 small lead acid batteries (lawn mower type). The light packaging says the string draws 1.9A, I have an inexpensive 30A charge controler. A 12v photo switch will activate the lights.

I would like the sign to be lit through out the night. I know that being December I'm looking at 12 to 14 hours of illumination. The panel would be pole mounted in a location with no shade and in direct sun light throughout the day.

My questions, how much battery power do I need to illuminate my lights for the 14 hours? And how big of a panel do I need to recharge my batteries?

I have been trying to figure this out on my own but I SUCK at math and I'm at a point of frustration. Any assistance here would be greatly appreciated.

[SunEagle]

Hello Matternmatt2 and welcome to Solar Panel Talk.

If each string is rated 1.9amps at 12volts then they would be about 24watts each. With 2 of them that is 48watts x 14 hours or 672 watt hours. With losses lets say 700 wh each day.

A battery bank that can safely provide you with 700wh each day would be sized like the following calculation. 700wh / 12v / 25% = 233 Ah. That means you will need more than a couple of lawn mower batteries for your system.

Your 30amp charger should work but you will need about 5 panels that have an Imp rating = 5amps each. Or total panel wattage that can deliver about 25amps of charging.

Some of the quality CC have a "load terminal" which is designed for minimal amount for lighting. There would be some type of timer that can control when the lights come on and how long they can stay on. Although that photo switch gets you more control if it gets dark sooner then the timer thinks it should.

In short while solar may be fun it is not cheap to run your loads via a battery. You may want to look into getting grid power out to your sign and then use a converter to change the AC to 12V DC. It will be cheaper in the long run.

[Matternmatt2]

Grid power really isnt an option for me, unfortunately.

I will be using a single string of lights( plenty for my needs).
So that would be
24wx14hr=336wh or say 350wh
350 wh/12v/25%= 117ah

If I understand things correctly I would need twice the ah so I dont drain below 50% so I would need batteries totaling 234 ah, right?

My CC does have a load terminal. My plan was to use this terminal to connect the photo switch and lights.
when I purchased the CC I was under the impression that it would work up to 30 amps. Did I get that wrong? Would a lower amp CC be better suited to my needs?

I dont have anywhere close to enough room for 5 panels. I understand that reducing the burn time will reduce everything else accordingly. I may just have to get a timer limit the lighting to evening hours, rather than all night... "sigh"

[SunEagle]

Grid power really isnt an option for me, unfortunately.

I will be using a single string of lights( plenty for my needs).
So that would be
24wx14hr=336wh or say 350wh
350 wh/12v/25%= 117ah

If I understand things correctly I would need twice the ah so I dont drain below 50% so I would need batteries totaling 234 ah, right?

My CC does have a load terminal. My plan was to use this terminal to connect the photo switch and lights.
when I purchased the CC I was under the impression that it would work up to 30 amps. Did I get that wrong? Would a lower amp CC be better suited to my needs?

I dont have anywhere close to enough room for 5 panels. I understand that reducing the burn time will reduce everything else accordingly. I may just have to get a timer limit the lighting to evening hours, rather than all night... "sigh"

Well I used that 25% in the calculation which is basically a 25% DOD so you really don't need to increase the battery to 234Ah. You can go smaller than 117Ah but as you go smaller you run the risk of draining your battery too much. A 50% DOD is too much in my opinion.

I would not go to a lower amp rated CC. Usually 30A is a good starting spot for a small system. Since you have reduced your battery size you can also reduce the amount of needed charging amps. A 117Ah battery would only need about 10amps or about 200 watts of panel.

If you under-size your solar/battery system you run the risk of killing off the batteries much sooner then expected. Once you get some experience on how your load and battery system is working you can build a better one. Keep us informed on what happens.