Need help choosing correct solar panel

hoghunter41 · May 2017

I have a 12v 18ah lead acid battery that I will be running 2 LED light pods using 16awg wire. The pods are 9 watts each and I want them to run anywhere from 8-10 hours a day off of my battery. I will have them run from dusk to dawn from a photocell. My question is what size Solar Panel do I need to replace the energy that the LED lights draw on a nightly basis, in order to make sure the battery is recharged every day to produce enough energy for the following night. I understand the weather plays a factor. You input would be very much appreciated.

mvas · May 2017

One LED Light is 0.75 amps = 9 Watts / 12 volts
Two LED Lights is 1.5 amps
Two LED Lights for 10 hours = 15AH (Amp-Hours)
Your battery is not big enough for two LED's.

You should drain only 9AH from your 18AH battery per night.
That is about 6 hours max for both LEDs or you could operate one LED for 10-12 hours.
90 Watt-Hours (consumed) = 12 Volts x 0.75 amps x 10 hours ( 1 LED )
A 50 Watt Solar Panel with 4 hours of sun = 200 Watt-Hours, would be more than enough to recharge

hoghunter41 · May 2017

ok thanks, but I still need to know what size solar panel I am going to need.

mvas · May 2017

Read the last line.

hoghunter41 · May 2017

Didn't see that.. sorry. Thank you for your input.

Estragon · May 2017

180wh/day or ~15 amp hours. Do you have a way of charging the battery other than solar? Running the battery down more than 50% regularly, or letting it sit in a discharged state for long will cause it to permanently lose capacity.

Assuming you do have alternate charging, you may want to plan for one cloudy day, so 30ah draw. That would mean ~70ah battery and 100w panel. If you want to charge every cloudy day, half that or 35ah battery and 50w panel. You may find 100w panels easier to find at a reasonable price. You will also need a charge controller.

hoghunter41 · May 2017

When you say a 50 watt is more than enough to charge, are you referring to using only 1 or 2 lights.? Thanks:)

Estragon · May 2017

2 lights for 1 night, or 1 light for 2 nights.

hoghunter41 · May 2017

My idea is to use these lights as hog feeder lights. The only way the battery would be able to be charged is through the solar panel.

Estragon · May 2017

Does it get cold at your location (much below freezing)?

hoghunter41 · May 2017

i'm in texas, so it does not stay cold here very long, but it gets in the 30's.

hoghunter41 · May 2017

Since I want to use 2 lights, would you possibly suggest putting a motion sensor on it so they only stay on for a selected period of time? If I did this, would a solar panel have a much better chance of replacing the power every day.

Estragon · May 2017

A discharged lead acid battery will freeze and this can destroy it. With no way to charge besides solar, you may be better considering a lithium based battery. They're ok with sitting in a discharged state. A charge controller with a load control function would help prevent the battery from getting too discharged.

littleharbor2 · May 2017

Image: https://us.v-cdn.net/6024911/uploads/editor/tl/9he7bhxk8qha.jpg

There's always something like this for a simple approach if intermittent motion activated light is all you need.

westbranch · May 2017

LH2, got a name for him to search for ? googling led lights might get a few thousand or more hits

Estragon · May 2017

I have similar motion detector in my carport. It seemed to trip on a lot for no reason in winter though. Maybe too cold?

littleharbor2 · May 2017

I pulled this image from eBay. Didn't want to post an ad. Amazon or eBay have hundreds of these. Just search solar led floodlight w/motion sensor.

hoghunter41 · May 2017

Ok gentleman.. can you please tell me what size battery I need to run these to lights every night and the correct solar panel to recharge the power that was used the night before. This setup is going to be left down at my ranch on my deer feeder if this helps you out any better.
2 - 9 watt green led light pods is what I want to run every night from dusk to dawn and recharge the battery every day. I hope this may help you out better.

Estragon · May 2017

The problem is you won't recharge on cloudy days. If you get a charge controller with load control you can protect the battery to some extent. In Texas I don't suppose you get runs of weeks of gloomy weather, and if the lights don't come on for a night or two the deer would get by.

At a minimum you need a 35ah battery, a 50w panel, and a charge controller with load control. As I mentioned earlier, you might be able to find a ~100w panel for about the price of a 50w. Set the charge controller so if voltage is < 12.2 the load is disconnected.

hoghunter41 · May 2017

ALRIGHT! Now we are getting somewhere. Thank you guys for your help.

hoghunter41 · May 2017

Would a 12v 35ah agm deep cycle battery be a better choice or a standard SLA

mcgivor · May 2017

hoghunter41 said:

Would a 12v 35ah agm deep cycle battery be a better choice or a standard SLA

AGM is what is needed, not a Gel, thought you mentioned Gel firs,t theen edited?

hoghunter41 · May 2017

Ok I have another question. If I ran just one LED pod.. that's .75 amps an hour from my 12v 18ah battery, which is 6 amps running at 8 hours a night. So a -10 watt solar panel would most likely generate .83 amps an hour. My one battery using .75 amps an hour would recharge fully enough with 6-8 hours of sunlight a day? If my calculations are correct, then one 10 watt solar panel would be enough to generate enough power for one LED running 8 or so hours a night? How am I doing? Thanks to all who participate, I really need the help. LOL!

Estragon · May 2017

I doubt a 10w panel will produce that much current, likely more like .5-.6a, and 8 hrs of direct sun hitting panels is optimistic. Morning and later afternoon sun is weaker and doesn't hit panel straight on (without a tracker). Also, charging the battery isn't 100% efficient.

Overall, as a rough guess you might replace around 1/2 the 6ah on average through the year, a bit more in summer, less in winter.

mcgivor · May 2017

hoghunter41 said:

Ok I have another question. If I ran just one LED pod.. that's .75 amps an hour from my 12v 18ah battery, which is 6 amps running at 8 hours a night. So a -10 watt solar panel would most likely generate .83 amps an hour. My one battery using .75 amps an hour would recharge fully enough with 6-8 hours of sunlight a day? If my calculations are correct, then one 10 watt solar panel would be enough to generate enough power for one LED running 8 or so hours a night? How am I doing? Thanks to all who participate, I really need the help. LOL!

Your math is correct but terminology is wrong, minor issue, 0.75A * 8 hours = 6Ah taken from the 18Ah battery. The 10 W panel has a rated capacity to supply 0.83A in laboratory conditions , but in reality you would get around 75% of that value, as far as hours of sun are concerned, the window of maximum output would more than likely be 4 to 5 hours, depending on location, perhaps less during winter months. There are other losses which must be included such as wiring, charge controller and battery, although small, they all contribute to the overall picture. A simple calculation would be what you have done so far, but double the panel capacity to 20W to compensate for losses etcetera. Using a 10W panel would in all likelihood end up under charging the battery over time without you noticing, until it's too late.

hoghunter41 · May 2017

So a 20w solar panel would put out around 1.25amp given the 75% figure you just gave me? That would be 1.66amps x 75%= 1.25 amps per hour from the 20w solar panel if i'm correct? Down here in Texas we get anywhere from 8-10hrs of sun on a good day, or normal day.

Estragon · May 2017

Using Dallas as an example, full sun equivalent hours for panel at 20° and no tracking ranges from 3.64 in Dec to 6.84 in July.

You can look up your specific location at pvwatts.nrel.gov

littleharbor2 · June 2017

Sun hours are based on what you would get at high noon with optimally aimed panels. Everything before and after high noon isn't as intense as peak mid day sun. So sun hours are a way of figuring how much charging to expect, based on full direct sun, in a full day.

hoghunter41 · June 2017

When you say if the load is <12.2 it is disconnected, what exactly are you referring to. I have decided that I am going to run just one light. Should the parameters you mentioned above still be the same or would you recommend a different setting?

Estragon said:

The problem is you won't recharge on cloudy days. If you get a charge controller with load control you can protect the battery to some extent. In Texas I don't suppose you get runs of weeks of gloomy weather, and if the lights don't come on for a night or two the deer would get by.

At a minimum you need a 35ah battery, a 50w panel, and a charge controller with load control. As I mentioned earlier, you might be able to find a ~100w panel for about the price of a 50w. Set the charge controller so if voltage is < 12.2 the load is disconnected.

Estragon · June 2017

The 12.2v number is somewhat arbitrary, but isn't really affected by how many lights you run. 12.2v is very roughly a 50% state of charge in the battery. Discharging a lead acid battery much lower than that, particularly if it will sit in that state for any length of time, can permanently damage it. If you set the load drop voltage higher, it protects the battery somewhat better, but is more likely to be hit and disconnect the load. Set it lower, it will be less likely to be hit, but more likely to damage battery.

If you have lots of pv, adequate battery, and a small load, you may never hit it. Small pv, it's more likely.

hoghunter41 · June 2017

Thanks so much for the answers Estragon .What exactly are the PV'S you are referring to? I have no knowledge of electricity so forgive me! LOL!

Need help choosing correct solar panel

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