## How Long Can an Average Car Battery Power 30 Watts (12v DC)?

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Topic Author
JSparks
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### How Long Can an Average Car Battery Power 30 Watts (12v DC)?

I've researched this online and I have no idea how this works. I'm trying to determine how long the average car battery (left out of the car, but fully charged) can power a 30 watt 12v DC light bulb. Any ideas?
dukeblue219
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

Car batteries are around 50 amp hours.

30 watts at 12v is about 2.5 amps. That would last roughly 20 hours then.

Assuming the 30 W bulb is rated for 12v DC, that is.
Jack FFR1846
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

dukeblue219 wrote: Sat Nov 18, 2023 2:54 pm Car batteries are around 50 amp hours.

30 watts at 12v is about 2.5 amps. That would last roughly 20 hours then.

Assuming the 30 W bulb is rated for 12v DC, that is.
Close. Incandescent light bulbs are pretty much resistive. So as the voltage of the battery drops, the current through the bulb will drop and light output reduce. So the bulb will continue to light all the way to zero from the battery, but of course it will get dimmer over time.
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Turbo29
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

A quick search shows a group 24 battery with a reserve capacity of 115min. That means it can supply 25Amps for 115 min before the voltage drops below 10.5V. A 12V 30W bulb requires 2.5A so a quick calculation gives 1150min. It may be longer as batteries can usually provide more total power at lower discharge rates.

But, things are usually not that simple so I await the replies of those who really know lead-acid battery chemistry. (Also a group 24 battery is one of the smaller ones.)
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Topic Author
JSparks
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

Thanks everyone. I should clarify that the bulb I mentioned is actually many LED bulbs totaling 30 watts.
quantAndHold
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

You might want to rephrase the question and tell us what problem you’re trying to solve. My first though is that a car battery would be a poor choice for this kind of task, and we might be able to guide you towards something better, if we knew what you’re trying to do.
Topic Author
JSparks
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

We have a large tree in our front yard and my lovely wife wants to put LED Christmas bulbs on it. I think it is 2,000 bulbs total and that the total draw is 30 watts. The tree is way too far from the house to run extension cords to it. But I have a large new deep cycle battery in the garage for which I have no use.

Since LED Christmas lights are 12v DC, I thought of running them off the battery. So I wanted to know how often this holiday season I would have to bring the battery back to the house and recharge it. I supposed a solar charger could help, but I'd like to avoid that.
sailaway
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

JSparks wrote: Sat Nov 18, 2023 3:12 pm We have a large tree in our front yard and my lovely wife wants to put LED Christmas bulbs on it. I think it is 2,000 bulbs total and that the total draw is 30 watts. The tree is way too far from the house to run extension cords to it. But I have a large new deep cycle battery in the garage for which I have no use.

Since LED Christmas lights are 12v DC, I thought of running them off the battery. So I wanted to know how often this holiday season I would have to bring the battery back to the house and recharge it. I supposed a solar charger could help, but I'd like to avoid that.
Kind of curious where you got the 12v lights from!

What Ah are listed on the battery? Is it FLA, AGM (or Li+)?
Topic Author
JSparks
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

sailaway wrote: Sat Nov 18, 2023 3:18 pm
JSparks wrote: Sat Nov 18, 2023 3:12 pm We have a large tree in our front yard and my lovely wife wants to put LED Christmas bulbs on it. I think it is 2,000 bulbs total and that the total draw is 30 watts. The tree is way too far from the house to run extension cords to it. But I have a large new deep cycle battery in the garage for which I have no use.

Since LED Christmas lights are 12v DC, I thought of running them off the battery. So I wanted to know how often this holiday season I would have to bring the battery back to the house and recharge it. I supposed a solar charger could help, but I'd like to avoid that.
Kind of curious where you got the 12v lights from!

What Ah are listed on the battery? Is it FLA, AGM (or Li+)?
I guess she got them on Amazon when we used them for the bushes at the front porch last year.The little AC adapter at the start of the strand says the output is 12v DC. We have ten of these with 200 bulbs each.

The battery says Ever Start Deep Cycle - 29DC, 845 Marine Cranking Amps, 122 Amp Hour @ 1A. It says it contains sulfuric acid so I guess it is an FLA.
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

Note that to preserve the life of a lead acid battery you should never discharge it more than 50%. So a 50 amp hour battery should only have 25 amp hours drawn from it before recharging, unless you are fine with damaging the battery and shortening its useful life.
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dukeblue219
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

The LED string may behave very different from an incandescent lightbulb. It may not tolerate a 12v battery that starts at more like 13v fully charged, and it may not gracefully degrade as voltage falls below 12v. The cheapness of the lights will play a part in how well it behaves on battery rather than its own regulated supply.

It's worth a shot.
notmyname
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

I would connect the lights to the battery in the garage and run a test. That would give you a solid answer. Could also put the lights on a timer so that they don't stay on all night.
quantAndHold
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

That should be plenty of power to run the lights at night and recharge the battery every day or two. I suspect you’ll get tired of humping a heavy battery back and forth to the house to charge it, though.
JoMoney
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

Unless you already had a "car battery" you don't have other use for and don't care about ruining, you might want to consider the difference between a "car battery" that's designed for short bursts of cranking power (and then gets recharged), and an "RV battery" or "Deep-cycle battery" that's designed to be deeply run-down and recharged.
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dukeblue219
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

JoMoney wrote: Sat Nov 18, 2023 9:01 pm Unless you already had a "car battery" you don't have other use for and don't care about ruining, you might want to consider the difference between a "car battery" that's designed for short bursts of cranking power (and then gets recharged), and an "RV battery" or "Deep-cycle battery" that's designed to be deeply run-down and recharged.
OP indicated it is a marine deep cycle battery.

But I wouldn't want to drag that thing around for charging every day or two.
Topic Author
JSparks
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

dukeblue219 wrote: Sat Nov 18, 2023 9:08 pm
JoMoney wrote: Sat Nov 18, 2023 9:01 pm Unless you already had a "car battery" you don't have other use for and don't care about ruining, you might want to consider the difference between a "car battery" that's designed for short bursts of cranking power (and then gets recharged), and an "RV battery" or "Deep-cycle battery" that's designed to be deeply run-down and recharged.
OP indicated it is a marine deep cycle battery.

But I wouldn't want to drag that thing around for charging every day or two.
Yeah. Two days is too frequent. I was planning to put a 12v DC timer on the lights which might extend it, but it's still probably going to need frequent recharging anyhow.

I thought about getting a solar charger but it's too complicated to figure out how large of a panel I'd need for this.
MathWizard
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

JSparks wrote: Sat Nov 18, 2023 3:12 pm We have a large tree in our front yard and my lovely wife wants to put LED Christmas bulbs on it. I think it is 2,000 bulbs total and that the total draw is 30 watts. The tree is way too far from the house to run extension cords to it. But I have a large new deep cycle battery in the garage for which I have no use.

Since LED Christmas lights are 12v DC, I thought of running them off the battery. So I wanted to know how often this holiday season I would have to bring the battery back to the house and recharge it. I supposed a solar charger could help, but I'd like to avoid that.
For a one-time use, the battery would work, but that's too much lugging. If you need to find a use for the deep cycle battery, I'd get a pure sine wave inverter, and it can be a backup during a power outage. I ran my 1/3 hp sump pump that way once, with an 800w modified sine wave inverter, when pure sine wave inverters of that size were quite expensive.

I'm curious how far from the house your front yard extends.
For 30 watts you don't need a heavy cord. I have a 100 ft outdoor extension cord like this which is rated for 1250 watts (about \$25)

https://www.lowes.com/pd/Project-Source ... lsrc=aw.ds

I used it for a string trimmer , and using outside outlets on the front and back of my house, I could reach the entire property,now I have a battery powered trimmer, so it doesn't get much use anymore.

I have heavier 50 ft extension cords for my circular saws and a 25 ft. 20 amp extension for my electric chain saw, but you don't need anything that hefty.
LotsaGray
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

JoMoney wrote: Sat Nov 18, 2023 9:01 pm Unless you already had a "car battery" you don't have other use for and don't care about ruining, you might want to consider the difference between a "car battery" that's designed for short bursts of cranking power (and then gets recharged), and an "RV battery" or "Deep-cycle battery" that's designed to be deeply run-down and recharged.
OP posted two times this is a “spare” deep cycle marine battery
Topic Author
JSparks
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

MathWizard wrote: Sat Nov 18, 2023 10:50 pm
JSparks wrote: Sat Nov 18, 2023 3:12 pm We have a large tree in our front yard and my lovely wife wants to put LED Christmas bulbs on it. I think it is 2,000 bulbs total and that the total draw is 30 watts. The tree is way too far from the house to run extension cords to it. But I have a large new deep cycle battery in the garage for which I have no use.

Since LED Christmas lights are 12v DC, I thought of running them off the battery. So I wanted to know how often this holiday season I would have to bring the battery back to the house and recharge it. I supposed a solar charger could help, but I'd like to avoid that.
For a one-time use, the battery would work, but that's too much lugging. If you need to find a use for the deep cycle battery, I'd get a pure sine wave inverter, and it can be a backup during a power outage. I ran my 1/3 hp sump pump that way once, with an 800w modified sine wave inverter, when pure sine wave inverters of that size were quite expensive.

I'm curious how far from the house your front yard extends.
For 30 watts you don't need a heavy cord. I have a 100 ft outdoor extension cord like this which is rated for 1250 watts (about \$25)

https://www.lowes.com/pd/Project-Source ... lsrc=aw.ds

I used it for a string trimmer , and using outside outlets on the front and back of my house, I could reach the entire property,now I have a battery powered trimmer, so it doesn't get much use anymore.

I have heavier 50 ft extension cords for my circular saws and a 25 ft. 20 amp extension for my electric chain saw, but you don't need anything that hefty.
Google Earth shows I would need 503 ft.
cmr79
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

OP, how cold do you expect it to get overnight where you live? A significantly discharged lead acid battery can freeze at temperatures above 0F, which would ruin the battery.
Topic Author
JSparks
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

cmr79 wrote: Sun Nov 19, 2023 6:19 am OP, how cold do you expect it to get overnight where you live? A significantly discharged lead acid battery can freeze at temperatures above 0F, which would ruin the battery.
Possibly into the 20's. But I'm close to giving up on this whole flight-of-fancy since the re-charging rate is too frequent and I can't figure out a solar charging solution.
MathWizard
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

JSparks wrote: Sun Nov 19, 2023 6:24 am
cmr79 wrote: Sun Nov 19, 2023 6:19 am OP, how cold do you expect it to get overnight where you live? A significantly discharged lead acid battery can freeze at temperatures above 0F, which would ruin the battery.
Possibly into the 20's. But I'm close to giving up on this whole flight-of-fancy since the re-charging rate is too frequent and I can't figure out a solar charging solution.
In the past, I have wired lamps with zipcord.

For 503 ft, you could make your own extension cord with ends from Lowes , and green zipcord made for Christmas lights.
Not cheap , but lugging a battery 500 ft. is not what I'd do.

For example see:
https://www.1000bulbs.com/product/221905/CMS-10130.html
Kenkat
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

Just as an alternate idea - I have a number of Ryobi 18v tools, one of which is an 18v to 120v inverter. With a 3.5 amp Ryobi Lithium Ion battery, it will power a 10w led bulb in a traditional indoor lamp for around 24 hours. So I would guess it would power 30w led lights around 8 hours. The nice thing is the Lithium Ion batteries are light and easily chargeable. You could probably make one battery work if you wanted or swap two to minimize the walking.
Ependytis
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

From Chat GPT:

To calculate how long an average car battery can power a 30-watt device, you can use the formula:

Time=Battery CapacityPowerTime=PowerBattery Capacity
However, car batteries vary in capacity, usually measured in ampere-hours (Ah). An average car battery might have a capacity of around 45-75 Ah.

Let's assume a car battery has a capacity of 60 Ah (which is fairly common). To find the time it can power a 30-watt device:
Battery Capacity=60 AhBattery Capacity=60Ah Power=30 WPower=30W
First, convert the battery capacity from ampere-hours to watt-hours:
Battery Capacity (Wh)=Battery Capacity (Ah)×VoltageBattery Capacity (Wh)=Battery Capacity (Ah)×Voltage

For a 12V battery:
Battery Capacity (Wh)=60 Ah×12 V=720 WhBattery Capacity (Wh)=60Ah×12V=720Wh

Now, use the formula:
Time=Battery Capacity (Wh)PowerTime=PowerBattery Capacity (Wh)
Time=720 Wh30 W≈24 hoursTime=30W720Wh≈24hours

So, on average, a 60Ah car battery could power a 30-watt device for approximately 24 hours.
telemark
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

Depending on how much you're willing to spend, you could also consider a portable power station. Easier to carry, faster recharging, and shouldn't mind the cold. For example

https://www.amazon.com/EF-ECOFLOW-Porta ... B0B8MXPRDB
flarf
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

Maybe you could build a little track to shuttle the battery back and forth between the garage and tree. Just watch out for the third rail.
bertilak
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

dukeblue219 wrote: Sat Nov 18, 2023 9:08 pm
JoMoney wrote: Sat Nov 18, 2023 9:01 pm Unless you already had a "car battery" you don't have other use for and don't care about ruining, you might want to consider the difference between a "car battery" that's designed for short bursts of cranking power (and then gets recharged), and an "RV battery" or "Deep-cycle battery" that's designed to be deeply run-down and recharged.
OP indicated it is a marine deep cycle battery.

But I wouldn't want to drag that thing around for charging every day or two.
Maybe OP has a garden cart.
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andypanda
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

Harbor Freight, Amazon, Northern Tool, etc. all sell a hand truck for \$59.

I have 3 x 67# trolling motor batteries and it is very handy. Good for other things too and easy to pull. Putting a battery on the mesh garden wagon requires more lifting than the 1/2" lift it takes to get a battery on the hand truck.
dukeblue219
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

To be clear, there are solar solutions to this on Amazon for under \$100. They aren't usually as bright as something powered from mains AC (or a marine battery substituting for such) but they do work.
andypanda
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

I hope they last longer than all of the solar sidewalk-lights-on-a-stick my wife has ever purchased.
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

This is an interesting topic. At least I know now that I'm not alone in my project ideas.
hudson
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

JSparks wrote: Sat Nov 18, 2023 3:12 pm We have a large tree in our front yard and my lovely wife wants to put LED Christmas bulbs on it. I think it is 2,000 bulbs total and that the total draw is 30 watts. The tree is way too far from the house to run extension cords to it. But I have a large new deep cycle battery in the garage for which I have no use.

Since LED Christmas lights are 12v DC, I thought of running them off the battery. So I wanted to know how often this holiday season I would have to bring the battery back to the house and recharge it. I supposed a solar charger could help, but I'd like to avoid that.
Please hook it all up, run it, and post the results.

I'm sure that you've already eliminated putting in underground wire. With that distance, you might have to go 220V?
For a light load, probably not. The EEs or electricians would know what to do. You could also add a yard light.
n00b
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

JSparks wrote: Sat Nov 18, 2023 3:36 pm The battery says Ever Start Deep Cycle - 29DC, 845 Marine Cranking Amps, 122 Amp Hour @ 1A. It says it contains sulfuric acid so I guess it is an FLA.
UPDATE: Sorry, you said 30-watts not 30-amps. Adjusted below.

30-watts at 12-volts is a 2.5-amp draw.

Off-grid solar here, so my whole house runs on batteries. The poster who noted the 50% depth-of-discharge before serious damage to lead-acid batteries is correct so the calculation is: Amp-hour Rating * 50% / Load Amps. In your case 122 * 50% / 2.5 = 24.4-hours.

* Actually there are different amp-hour ratings for different time periods but close enough for a shot from the hip.
Topic Author
JSparks
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

n00b wrote: Tue Nov 21, 2023 8:29 am
JSparks wrote: Sat Nov 18, 2023 3:36 pm The battery says Ever Start Deep Cycle - 29DC, 845 Marine Cranking Amps, 122 Amp Hour @ 1A. It says it contains sulfuric acid so I guess it is an FLA.
UPDATE: Sorry, you said 30-watts not 30-amps. Adjusted below.

30-watts at 12-volts is a 2.5-amp draw.

Off-grid solar here, so my whole house runs on batteries. The poster who noted the 50% depth-of-discharge before serious damage to lead-acid batteries is correct so the calculation is: Amp-hour Rating * 50% / Load Amps. In your case 122 * 50% / 2.5 = 24.4-hours.

* Actually there are different amp-hour ratings for different time periods but close enough for a shot from the hip.
Thanks. So with a timer set for 6 hours, we're looking at charging the battery every 4 days. This is a bit too much considering the DW wants these lights to be out there for about a month. I wish I could understand what size solar panel I'd need (given this load) to recharge the battery during the day.
n00b
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

JSparks wrote: Tue Nov 21, 2023 8:41 am
n00b wrote: Tue Nov 21, 2023 8:29 am
JSparks wrote: Sat Nov 18, 2023 3:36 pm The battery says Ever Start Deep Cycle - 29DC, 845 Marine Cranking Amps, 122 Amp Hour @ 1A. It says it contains sulfuric acid so I guess it is an FLA.
UPDATE: Sorry, you said 30-watts not 30-amps. Adjusted below.

30-watts at 12-volts is a 2.5-amp draw.

Off-grid solar here, so my whole house runs on batteries. The poster who noted the 50% depth-of-discharge before serious damage to lead-acid batteries is correct so the calculation is: Amp-hour Rating * 50% / Load Amps. In your case 122 * 50% / 2.5 = 24.4-hours.

* Actually there are different amp-hour ratings for different time periods but close enough for a shot from the hip.
Thanks. So with a timer set for 6 hours, we're looking at charging the battery every 4 days. This is a bit too much considering the DW wants these lights to be out there for about a month. I wish I could understand what size solar panel I'd need (given this load) to recharge the battery during the day.
Your latitude and weather are part of that calculation. I suggest your reach out to someone like Northern Arizona Wind and Sun who are very experienced in this. I am not affiliated with them except as a happy customer. (https://www.solar-electric.com)

You will also likely want to add a small charge controller (many of which include a load timer) to adjust for weather etc. as batteries dislike both under and over-charging.
eigenperson
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

The required solar panel depends a lot on your location. Assuming this is to be used in winter, in the average US location with average weather, I think about a 60 W panel would do the job. But you might not get average weather. If you want to power through cloudy stretches without a recharge, you need more. In the far north, you also need more.

To get the best performance, you need to mount the panel tilted and facing south. There are a bunch of tilt angle calculators online.
quantAndHold
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### Re: How Long Can an Average Car Battery Power 30 Watts (12v DC)?

If you have a nearby spot with full sun, this should do the job.

Note that this works better in a sunny climate. Your production will drop a lot on a cloudy day. And it needs to be adjusted periodically to face the sun. But unlike your battery, it’s 20 lbs and has a handle.