12 volt and 24 volt conversions etc for solar

[Oneleggedcowboy]

Confused I am but I feel the need to understand this. First off I read somewhere that one 60 cell panel cannot charge a 24 volt battery bank???  But that a 72 cell could?
Next for the conversion confusion...
Let's use round hypothetical numbers, a 100 watt panel with 5 amps charging thru a Mppt controller to a 12 volt battery would at best put in a little over 5 amps right?
A 24 volt 300 watt panel with 10 amps would thru a mppt controller put in about 20 amps on a good day correct???
A 24 volt 300 watt panel charging thru a 24 volt mppt to a 24 volt battery bank would yield 10 amps???
Now for the inverter...
A 12 volt 225 amp battery bank into a inverter would have available about a 100 amps usage b4 causing damage?
A 24 volt battery bank would have 225 amp hours and the same 100 amps available?
A 24 volt bank into a 24 volt inverter that powers 110 volts uses how many amp hours compared to a 12 volt inverter???   

Remember small words, simple sentences for my 2 volt brain please. LOL

THANKS FOLKS!

 

[jimindenver]

A 72 cell panel has a higher voltage than a 60 cell panel. Then again are you talking a 230w 60 cell panel or a 275 watt panel? Also Mono's have a higher voltage than Poly's and MPPT uses more of the panel than PWM. In the end it is the ability of the system to bring the voltage up to charging levels and keep it there long enough to do the job.

24 volt and 12 volt may look like different animals but really they are just two different formulas that reach the same answer. Two 12 volt batteries in parallel hold the same amount of usable power as they would in series for 24 volts. A 24 volt inverter uses the same amount of power to produce 120Vac for a given load as a 12 volt inverter except it looks differently because at twice the voltage it uses half the amps. This is true across the board.

So if it all is equal in the end, why chose 24 volt over 12 volt? At half the amperage 24 volts uses thinner wiring to carry the current. So if you are moving large amounts of current it is more efficient to use 24 volts. That is the reasoning behind recommendations of inverter sizing as in 12 volt inverters should be limited to 1500 watts or so, then 24 volt for the next level and 48 volts for the really big continuous jobs. The stress on all of the wiring including that wiring inside of the inverter is reduced at higher voltage. There are 10,000 watt 12 volt inverters but the wiring to make them safe to use is massive. It is a case of just because they sell it does not make it a good decision to use.

Other things to consider is that 12 volt inverters and appliances are far more numerous than 24 volt, meaning less expensive too. Should you also need to run 12 volt items you will need a separate device to produce 12 volts from a 24 volt bank. That in itself is another expense and loss of power in the conversion.

The one place a 24 volt system can save money is in a MPPT controller if you want a larger system. A 20 amp controller that can take 300 watts at 12 volt can take 600 watts at 24 volts.

 

[John61CT]

First off I read somewhere that one 60 cell panel cannot charge a 24 volt battery bank???  But that a 72 cell could?

All depends on the controller.

MPPT units like Victron make things very flexible. Higher voltages make MPPT even more efficient.

Roughly, watts is your best unit for measuring panel output since voltage can vary a lot.

Amps is OK for charging output, since either 14V or 28V is close enough.

W ÷ V = A

> A 12 volt 225 amp battery bank into a inverter would have available about a 100 amps usage b4 causing damage?

Not amps but amp hours for capacity or usage over time.

So a 225 AH bank should not be often discharged below say 100AH remaining. Not damage as such, but shorter lifespan.

A 24V bank's AH are worth twice that of 12V, watts (watt-hours) is actual power.

You should only use 24V if you have a strong reason to. 12V being much more standard makes everything cheaper.

You should also only use an inverter where you really have to, they are expensive and waste power.

Most things you want can run off DC directly.

Many mains consumer devices are too inefficient to use off solar.

If you're off grid, spending money on using less power is much better value than spending it on generating more power.

 

[tx2sturgis]

Remember small words, simple sentences for my 2 volt brain please.

Stick with 12v unless you have a very good reason to build a 24v system.

If you have that reason, and can state it here, then we can work you thru the solutions.

 

[John61CT]

But understand we're talking about the bank, load circuits, charging voltage

The **panels** can be high, say 40-60V even higher, with MPPT that's more efficient, nothing to do with the SC charging output voltage, which will be 14+V

 

[Oneleggedcowboy]

I am starting to get this. So poly and mono. So I should get mono panels because they are higher amp?

Is my 245 watt 24 volt mppt setup to my 12 volt battery bank equal to 490 watts of 12 volt panels???

 

[Trebor English]

I am starting to get this.  So poly and mono. So I should get mono panels because they are higher amp?

Is my 245 watt 24 volt mppt setup to my 12 volt battery bank equal to 490 watts of 12 volt panels???

100 watt poly is 100 watts.  100 watt monocrystaline is 100 watts.  It might be smaller but it's still 100 watts.  

245 watts is 245 watts.  It is not 490 watts.  

Multiply amps times volts to calculate watts.  A 120 volt 60 watt incandescent light bulb is 0.5 amps.  A 12 volt 60 watt incandescent headlight is 5 amps.  12 times 5 equals 60.  120 times 0.5 equals 60.

 

[Oneleggedcowboy]

Confused again. I have see my 245 watt 24 volt panel put out 14 plus amps on my Eco Worthy controller to my 13 volt bank. However the panel on the back says something like 7 or 8 amps, I can't see it anymore because it's mounted to the roof. So that's the mppt effect right? If I had 245 watts of 12 volt panels hooked to the Eco Worthy how man amps would I get?

I thought the 24 volt broke down in the mppt thing to give you bonus amps?

Thanks yall, I am.trying to get this figured out because I want a lot of solar on the roof of my gooseneck horse trailer. I got plenty of room and no weight problems.

 

[tx2sturgis]

When you use the terms 12v or 24v we will assume that you are referring to battery voltages. So my answer was, and is, you should use a 12v battery system, unless you have a specific reason to use a 24v battery system. 

A solar panel for charging 12v batteries directly (as in PWM) will usually be around 18 to 22v but is sold as a '12v' panel.

If you are asking if you NEED 12v panels, for a 12v battery, then the answer is NO...you can use panels of much higher voltages, or panels in series, IF you use an MPPT controller.

 

[tx2sturgis]

Confused again. I have see my 245 watt 24 volt panel put out 14 plus amps on my Eco Worthy controller to my 13 volt bank. However the panel on the back says something like 7 or 8 amps, I can't see it anymore because it's mounted to the roof. So that's the mppt effect right? If I had 245 watts of 12 volt panels hooked to the Eco Worthy how man amps would I get?

Quite possible to get about 15-20 amps, IF the controller can provide that, and the battery needs it. 

I thought the 24 volt broke down in the mppt thing to give you bonus amps?

Bonus amps?

No, the power provided by the panel is the determining factor. The MPPT controller can deliver most of that power under certain conditions, but there is no magic, no 'bonus amps'.

 

[John61CT]

OK, so you have a 245W panel, is that right? For example.

Say that produces average 150W for four hours a day, in theory maximum at 14V is (600 wH ÷ 14V) 42.8AH per day.

If it's a "12V" panel rated at say 21Voc, and a PWM controller outputs charge current at say 34AH, a more efficient MPPT might give you 38AH.

If the panel is rated say 45Voc instead of only 21Voc, the difference would be so much greater, say 18AH for PWM compared to 41AH for MPPT, that MPPT really becomes the only choice.

 

[John61CT]

PWM just drops the power output by the higher voltage, the MPPT "bonus" is the buck conversion and optimization at higher voltages.

The latter gives a higher % of the panel's potential, the former is wasteful.

At say 18Voc panels, very rare these days, then the difference is not so great.

 

[frater secessus]

Confused I am but I feel the need to understand this. First off I read somewhere that one 60 cell panel cannot charge a 24 volt battery bank???  But that a 72 cell could?

36 cell = 12v nominal
60 cell = 20v nominal <-- here's the problem
72 cell = 24v nominal

60 cell panels (single or parallel) do not have enough voltage to consistently charge 24v banks.   You could run the 20v or 2v panels in series to get sufficient voltage.

I agree with others that there needs to be a compelling reason to deviate from 12v banks.

 

[John61CT]

Some solar controllers can boost / convert voltage up.

Not that I'm suggesting that is relevant to OPs case, just saying for general info on the relative-voltage topic.

 

[frater secessus]

Good point.

 

[wmyers4u]

Mppt will convert extra voltage above battery requirement to amps and charge the battery quicker. I'm not feeling the math today so I won't but those are your bonus amps

 

[Oneleggedcowboy]

Thanks I fully intend to use mppt controllers.

So would 2 of the 245 for a total of 490 watt panels be equal to 500 watts of 12 volt panels? The reason I ask is its a lot easier to munt and run cables for two big panels than to munt and run cables for 5 of the 100 watt panels.

 

[tx2sturgis]

10 watts difference. 

Not enough to matter in most cases.

 

[John61CT]

Thanks I fully intend to use mppt controllers.

So would 2 of the 245 for a total of 490 watt panels be equal to 500 watts of 12 volt panels? The reason I ask is its a lot easier to munt and run cables for two big panels than to munt and run cables for 5 of the 100 watt panels.

Watts are watts, always independent of voltage.

Only amps vary with voltage.

 

[Weight]

Keep in mind. Solar panels do not have a fixed output. The output varies with the amount of light and the temperature. The ratings are tested under laboratory ideal conditions. Conditions that are never experienced in the wild.
Your important factors would be your battery bank, voltage and AH rating. Then your controller must be sized to handle the maximum voltage from the panels, and the expected amps for charging.
Most modern panels will have the same efficiency, and so have the same foot print. A 300 watt panel will be very close to the same real estate as three 100 watt panels. That is very simplified and it depends on type of panel and other stuff, but it is a consideration.