How do battery watts and solar panel watts relate to each other?

[LizardLady'sLittleLife]

Brand new to solar. Been using a little Rally power pack that I can charge via house current, 12 volt accessory plug-in, or even hand crank. 

Been looking at a solar kit that comes with a 400 watt battery pack and a 20 watt solar panel. Some of the reviews say you actually should have a 30 or 40 watt solar panel instead. I know the battery watts refer to how much power the battery can store, but what do the solar panel watts mean?

Thanks for helping me begin to understand this. ?

 

[Gypsy Jane]

Following

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[SternWake]

Watt hours or Amp hours is a measure of storage capacity, where watts or amps are the total energy flowing.

As battery voltage drops during discharge, watt hours is a more accurate measurement, but amp hours are a more common figure used
volts x amps = watts

20 watts at 14.4v is 1.39 amps
20 watts at 12.6v is 1.58 amps
20 watts at 11.8v is 1.69 amps

400 watt hours of capacity on a 12 volt nominal battery is ~ a 32 Ah battery.

for example/ comparison to whatever you've been looking at:

http://www.amazon.com/gp/product/B0..._m=ATVPDKIKX0DER&pf_rd_r=0C0RFF26B5MB1CB0E7T8

If this is a 32AH lead acid AGM battery cycled to 50%, the 'ideal' amperage for charging would be as high as 30%. 30% of 32 is about 9.6 amps which is in the 150+ solar watt range.

Minimum charge current would be about 10% which would be about 3.2 amps and about the 50 to 60 watt range.

That is 'ideal' when the battery is discharged to 50% or less range. When the battery is cycled shallower the charge current is less important and all sorts of lesser charge currents fall into the 'acceptable' range.

These solar battery kits are pretty much aimed at newb's who do not understand the details and do not necessarily want to understand them, and are willing to pay more for them.

If you are willing to learn more and can do some wire terminations, we can advise you on individual components that will work together and can meet your requirements with more reliably for possibly significantly less $$.

A hundred watt renogy suitcase solar system on the battery linked above would be a pretty capable system for less than 300$

 

[29chico]

Watts are watts.  However, a battery only stores electricity while a solar panel will produce electricity for as long as the sun shines.

Watts is defied as power(watts) equals volts times amps,.  I is the symbol for amps used in the actual equation.

To get a basic understanding of electricity think of a household water pipe that is connected to a faucet.  When the faucet is shut off you have water pressure but no flow.  Volts is like water pressure.  Then think of the faucet being turned on and the water is flowing out of it.  Amps is like the volume of water flowing.  You can have lots of pressure in the pipe or lots of volts in a wire with no flow in either one of them.  But without pressure in the pipe or volts in a wire you can have no water flow or electrical flow.

A bit of math:

Power= I(amps) times volts, or as usually expressed:   P=IV

with a bit of algebra you can have the following equations:

V=P/I  this can be used to determine the volts when watts and amps are known

I=P/V  this can be used to determine amps when watts and volts are known


A 12v 400w battery pack, at best, will deliver about 33.3 amp hours.  The usual minimum is one watt of solar for every amp hour of battery pack capacity.  So yes, a bigger solar panel would be better than the 20w panel in this case.  I prefer to see more like 2 watts of solar panels to 1 amp hour of battery bank capacity in a 12v system.

Consider that a true deep cycle battery can only be discharged to 50% without damaging it so a 400w battery pack should only be used to discharge 200w (or 16.6 amp hours at 12v) on a daily basis.  That is not a lot of power.

 

[29chico]

More info:

http://www.mrsolar.com/what-does-volts-amps-ohms-and-watts-mean/

Covers a bit more on basic electrical concepts than I did.

Mr Solar has very high prices on smaller panels. The large panels are about 20% higher than I can buy locally.

 

[akrvbob]

If you really want, I'll try to explain it simply. But I suspect you just want some practical advice what to do, I can give you that. Don't buy that solar kit, I'm almost certain it's way overpriced and won't do what you want. Instead do one of these:

1) if you think you can do it, or if you have friends family who will do it, or can pay someone to do it, then buy a Renogy 100 watt solar kit and mount it on your roof. Then buy a Walmart Marine battery. I'd guess this will cost less and work a hundred times better than that kit. Find it here: http://amzn.to/1pWoUBr

2) If you just can't mount the panel on the roof, then instead of buying that kit, bt a Reneogy 100 watt solar suitcase portable kit and a Walmart Marine battery. The panel folds up and for when you drive the van, when you get to camp it unfolds and its outside and you hook it to the battery with clips. It's super easy and works very well. Again, this will cost less and work a hundred times better than that kit. Find it here:
http://amzn.to/1Z1G5xp
Bob

 

[LeeRevell]

Batteries are rated by Amp-Hour (AH) amount, and solar by watts.  "Watts" is never used in regard to batteries.  The often quoted standard is you need one watt of solar to one AH of battery capacity.  So if your little jumper unit has a 35AH battery, a 35W solar panel would suffice.  This could be a simple folding panel unit, or a small roof mounted panel. 
You want to never discharge the battery below 50% of it's capacity, or for a 35AH battery, never below some 17AH.
The concept isn't really difficult. 
Just think of the battery as a storage jar you fill and draw from, and the solar is the thing filling it.  Only difference is, the solar panel won't put out the full amount of watts every hour - that changes with the strength of the sunlight.

 

[LizardLady'sLittleLife]

Just think of the battery as a storage jar you fill and draw from, and the solar is the thing filling it.  Only difference is, the solar panel won't put out the full amount of watts every hour - that changes with the strength of the sunlight.

That was very helpful. So, a 30 watt solar panel has the potential, in excellent conditions, of adding 30 watts of power to my battery per hour . . . but it might not do that well? Am I getting close? Hmm, I'm sure I read that my battery "holds" 140 watts . . .

I do actually want to understand how this works, as opposed to someone just telling me what I need. I do tech support for a living, and people are always wanting me to recommend a computer for them  :s when they would probably make a better choice if they understood what all those specs mean and chose for themselves. That's kind of how I feel about solar: I want to understand how it all fits together, so I'm in a position to evaluate options as my needs change. I have naturally had an aptitude for electronics since computers first started showing up in people's homes, but I'm just the opposite with electrical stuff (as opposed to electronic stuff). Can't seem to wrap my head around it.  :huh:

Found Bob's cute little elf analogy at the top.   That helped, too. I'm still confused, but seeing a light at the end of the tunnel. (solar powered)

 

[RV-Hopeful]

My eyes just crossed at the subject title ... 

Thank goodness there are so many knowledgeable people here!

 

[LeeRevell]

Ideally the 30W panel will supply 30 watts but nothing is ideal.  Weather, sun angle, clouds, etc. will affect the actual wattage.  And the panel is charging the battery over several hours.  Of course more solar wattage will "fill" your 35AH battery (get away from thinking wattage in batteries - it isn't done normally) in a shorter time.  Most folks use 200 to 400 watt systems to charge a 120 AH battery or a couple batteries.  More solar is always better but cost and roof acrage will limit you.

 

[Vivid-Dawn]

I suppose I need to learn this stuff too. I have an oxygen machine I need to use at night...so it has to be run from battery power. I can charge the battery during the day with solar.
My machine uses 150 watts while turned on. Now...I assume that's 150 watts per hour? And the calculation is watts divided by volts to get amps. So 150 watts divided by 120 volts (standard household outlet) is 1.25 amps.
So that would be 1.25 x 8 hours is 10 amp hours. Would one battery do that, without draining too much? Then as soon as the sun comes up it'll start charging again.

Or am I totally wrong?

 

[NeedAnEscape]

1.25a at 120v is 12.5a at 12v, and that is before taking inverter efficiency into account.

 

[akrvbob]

Part of the problems is it's a whole new language and it's a very difficult one.

Different things measure differently, some use watts, but most use amps. I know you don't want to, but you need to train yourself to think primarily in amps, not watts. Of course to do that you must be able to convert them easily.

Batteries are measured almost exclusively in amps and solar controllers are rated by amps, not by watts. Solar panels are the only thing commonly using watts so you need to learn to convert that to amps, and think in amps.

I'm so used to thinking in amps, I can't easily follow your posts because I would have to stop and convert the watts you're talking about back to amps.

Amps means the number of amps created or consumed per hour so you are really thinking in ah or amp-hours. But that is just more language, essentially we treat them as interchangeable.
Bob

 

[Vivid-Dawn]

Eh... I knew I didn't have it right. Here's the machines specifications, if that helps at all?



The manual says not to use DC (plug into cigarette lighter thing) unless the car is running. Obviously I don't want to run the car for 8 hours at night for both noise ordinance and wasting gas!

 

[Fivealive]

Brand new to solar. Been using a little Rally power pack that I can charge via house current, 12 volt accessory plug-in, or even hand crank. 

Been looking at a solar kit that comes with a 400 watt battery pack and a 20 watt solar panel. Some of the reviews say you actually should have a 30 or 40 watt solar panel instead. I know the battery watts refer to how much power the battery can store, but what do the solar panel watts mean?

Thanks for helping me begin to understand this. ?

As others have mentioned batteries are measured in AH, Amp Hours not Watts. There are two more pieces to the solar puzzel, a charge controller and a power inverter. The inverter would have a watt rating and 140, and 400 sound in line with that. I am assuming your looking at those all inclusive "solar generator" type kits where the battery, inverter, and charge controller are all-in-one. I have looked at these and always seem way over priced. Bob's advice was solid as for what to get instead.

 

[amwbox]

Eh... I knew I didn't have it right. Here's the machines specifications, if that helps at all?



The manual says not to use DC (plug into cigarette lighter thing) unless the car is running. Obviously I don't want to run the car for 8 hours at night for both noise ordinance and wasting gas!

Those specs say it draws a max of 8 amps on DC, which isn't an insignificant amount of power. At least 120 watts of power.

As others have mentioned, you should really stop thinking in terms of wattage. Try to think of it like this:

Electricity is water flowing through a pipe. The total AMOUNT of water is amperage. The pressure the water is flowing at is voltage. Wattage is just a way to express the work the water does as if flows over a water wheel. 

So, a skinny pipe at a really high pressure can move a water wheel at the same speed as a thick pipe at a really low pressure. The water wheel turning runs a millstone or pump or whatever. The WATTAGE is the amount of work the water wheel can do. If the wheel turns at the same speed, its the same amount of work, regardless of whether its high or low pressure water making it turn. 

So when it comes to your batteries, think of the amount of power stored there as amperage, and the voltage they run at as the amount of "pressure" they exert. 
12 volt batteries being drawn down at 10 amps is 120 watts of work.
120 volt wall socket having 1 amp drawn from it is ALSO 120 watts of work. 
See how wattage is simply a way to measure the relationship of amperage and voltage?

What really matters though, is AMPS. More specifically, amps per hour. or AH. 

According to those specs, your device will draw something like 85 amp hours from your batter over 8 hours. If you use an inverter, it would be closer to 100. So, you'd be drawing a couple of golf cart batteries down by about 50% each night, and would need to do quite a bit of charging during the day to recharge them. 

Basically, you really just need to sit down and play with Ohm's Law until you feel confident converting wattage into voltage and amperage.

 

[steamjam1]

Yep. Its all about the amps.

 

[SternWake]

Well, the amps are just the standard way of thinking that we've all gotten used to, and if voltage never changed would be just as accurate.

But since battery voltage drops during discharge, most devices draw the same amount of watts at a lower voltage, meaning the amps go up. Watt hours is a more accurate way of measuring electrical consumption over time, but then again the metric system also makes more sense, but most between these borders can't think distance in meters, or weight in kilograms, or temperature in celsius.

Sure I know a Kilo is 2.2 pounds, but I still have to do the conversion as I think in pounds. Had I learned to think in metric......
I know roughly that 75f is 25c but when someone asks me to judge the temperature of the ocean, I reply 64, not 19.

Oh well, too late for me, but someone new to the electrical world only finds resistance when confronting those of us who trained ourselves to think in amp hours.

KiloWattHours are a more accurate measurement of electrical consumption compared to amp hours. But we can't think in terms of them, nor do most devices oriented toward living on wheels, that can count electrical consumption, list a KWH spec.

Some do, I have one, and I never look at the KWH figure, as it is meaningless to my poorly trained brain, but 53.3 AH is chiseled in stone, but the less accurate measurement of electrical consumption.

I also wish i was exposed to more languages when A child. Too late for that boat too

 

[StitchExperiment626]

Brand new to solar. Been using a little Rally power pack that I can charge via house current, 12 volt accessory plug-in, or even hand crank. 

Been looking at a solar kit that comes with a 400 watt battery pack and a 20 watt solar panel. Some of the reviews say you actually should have a 30 or 40 watt solar panel instead. I know the battery watts refer to how much power the battery can store, but what do the solar panel watts mean?

Thanks for helping me begin to understand this. ?

 

[StitchExperiment626]

Solar panels have different voltages so not all solar panels have apple's to apple's comparison. A12 volt panel comes in different voltages and number of cells and construction material and conversion of percentage of light. I have seen YouTube comparison of numbers on a solar panel and tested output and the ratings lie. I have seen on YouTube smaller panels outperforming larger panels. Also solar panels degrade a % over 20 years. IMO hard panels last longer than flexible panels. A charge controller of mppt allows you to use higher voltage panels even a 24 volt panel which in the morning start producing current (basically sooner) than a lower volt panel till it gets more sunlight.
This is not comprehensive but gives you some insight into solar panels.