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Old 04-22-2018, 08:46 PM   #1
Mini-Skoolie
 
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What amp hr battery for the solar wattage?

Hi 🌞 I recently bought 4-290w panels. The price was right.
I'm not sure what amp hr batteries to buy, what wattage inverter and what controller. I read Mppt was better than pwm? The heaviest appliances I run are my freezer (on & off) and mini water heater-that one is 120v. That is only on about an hr a day. Also an ac but that's a 5000 btu plugged into shore power. I've read so much and doing online calculators but I just feel more and more confused.
Plz advise. It will be greatly appreciated 🙏💮

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Old 04-23-2018, 01:57 AM   #2
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6-800AH should be plenty, if weather isn't consistently sunny, for 1200W could even go to 1200AH but that's getting very heavy.
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Old 04-23-2018, 01:58 AM   #3
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Electric heat is very wasteful, consider propane or diesel.
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Old 04-23-2018, 02:00 AM   #4
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MPPT for sure, look at Victron's SmartSolar line.

If partial shading, consider one 100/20 per panel, or even 75/15 to save money, just wastes some panel output, but only in peak conditions.
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Old 04-23-2018, 07:58 AM   #5
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The learning curve for solar/RV electric is fairly steep but it all comes together pretty quickly. Fear not!

The best way to go is typically to establish your energy budget. This is done by measuring the power used by the various items/accessories and then adding in the time factor. You will then have a pretty good idea of how much power you need each day. You can then factor in the amount of reserve power you require and finally determine the battery bank capacity that you want/need.

The size of the inverter depends on the 120 VAC accessories that you plan to operate. If you have three items that each consume 500 watts (for example), you will need at least a 1500 watt inverter.

With 1200 watts of panels, you are likely to get a maximum of about 1000 watts under ideal conditions. If you have a 12 volt battery bank, you will need a solar charge controller (MPPT) that can handle 70-75 amps. If you have a 24 volt battery bank, it will need to handle half that.

I run my 11 gallon electric water heater (Solar to Power the Water Heater? - JdFinley.com) from solar/battery but it consumes a huge amount of power (when heating the water - not so much to maintain) so your solar/battery/inverter has to be able to handle that load.
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Old 04-23-2018, 09:15 AM   #6
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Quote:
Originally Posted by JDOnTheGo View Post
With 1200 watts of panels, you are likely to get a maximum of about 1000 watts under ideal conditions. If you have a 12 volt battery bank, you will need a solar charge controller (MPPT) that can handle 70-75 amps. If you have a 24 volt battery bank, it will need to handle half that.
That is talking about output amps, not input.

At 295W it is likely the panel Voc rating is higher than with nominal 12V panels, so need to first confirm the SC is rated for say 15-20% over that rating on input voltage.

If that happens to be say 40V, then input current is well under 8A each.

At the charging voltage, simplifying, output amps may sometimes get up to 18-19A, but average will likely be under 15A, which is why the Victron 75/15 at only $100-120 each would be a bargain.

But if you wanted to extract every watt in even peak conditions, the 100/20 version is ~$160 each.

Also, getting a single controller to handle all four panels is not optimal for shade handling, and the 150/85 at nearly $700 won't even save any money.

But yes less mounting space, cheaper wiring, and you can do temperature correction with a sense wire.

To really save wiring costs over a long distance, serial all four panels together with the 250/85 at ~$850, that is assuming they are indeed higher voltage.

OP, please link to the detailed electrical specs on these panels?
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Old 04-23-2018, 10:47 AM   #7
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That is talking about output amps, not input.
Yes, it is. In the case of the Morningstar MPPT charge controllers, the input amps are irrelevant (within reason), the maximum input voltage is 150 volts. The charge controller output rating is the maximum amps that you will get out of it, regardless of output voltage or input (voltage or amps). So, the TS-MPPT-45 will output 45 amps at 12VDC or 45 amps at 24VDC or 45 amps at 48VDC. This is one easy/convenient way to avoid having to use multiple charge controllers. Of course, it depends on your battery bank voltage.

At that rating (290 watts), the panels are almost guaranteed to be higher voltage panels which will require an MPPT charge controller. Wired in parallel, shading won't be near the problem (basically lose the output from the shaded panel(s) but not the others). Since they are 40+ volts to begin with, you can still run much smaller gauge wire (from array to controller).
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Old 04-23-2018, 10:27 PM   #8
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I can get 4-900ah. Would that work? When buying them I must make sure they are 12v? Or would that be determined by the controller?
I also need to know what size cables. I plan to set everything up relatively close, on the same side of the bus. Panels in parallel between roof hatches and everything going down right behind the driver. Batteries in a storage under the bus, same side.
I've read your emails over and over. They're incredibly helpful. Thank you!
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Old 04-23-2018, 10:28 PM   #9
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I can get 4-900ah. Would that work? When buying them I must make sure they are 12v? Or would that be determined by the controller?
I also need to know what size cables. I plan to set everything up relatively close, on the same side of the bus. Panels in parallel between roof hatches and everything going down right behind the driver. Batteries in a storage under the bus, same side.
I've read your emails over and over. They're incredibly helpful. Thank you!
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Old 04-23-2018, 10:39 PM   #10
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Also, regarding the Morningstar, can I do 2 controllers and what how many amps each?
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Old 04-24-2018, 01:05 AM   #11
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I can get 4-900ah
Sorry, please clarify what you mean. Ideally with a link?
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Old 04-24-2018, 01:11 AM   #12
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What is the battery voltage of that battery? A 6 volt golf cart battery is about 225 amp-hours at 6 volts. An 8d battery is essentially two GC batteries, 225 amp-hours at 12 volts. A 900 amp-hour 12 volt battery would be almost 4 8d batteries, something like 600 pounds of lead and acid.
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Old 04-24-2018, 02:07 AM   #13
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It is important to be able to lift your batteries.

4 pairs of GCs would be a good value 900AH bank.

But for a mostly-solar system, few rigs have the roof space to fit the panels needed to make it worth carrying that much lead around.
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Old 04-24-2018, 02:13 AM   #14
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Originally Posted by Yorkitator View Post
When buying them I must make sure they are 12v?
Pairs of 225AH 6V makes 12V in series, same as one 225AH 12V unit.

Same with 6x 2V, or 3x 4V or 4x 3V.

Once you get to 12V in series "strings", from then on you parallel them to add total AH capacity.

Two string is 450AH, 3 makes 675, 4 gives you your 900AH if you really need that much.

The controller usually detects the bank voltage and automatically adjusts its settings when it powers up.

Some require a manual switch to say 24V if you change to that (don't).
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Old 04-24-2018, 06:38 AM   #15
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Originally Posted by Yorkitator View Post
I can get 4-900ah. Would that work?
Not sure what you mean. If asking about a 900 ah battery bank made up of four batteries, you probably can't do it with any sort of typical "house" use battery. The whole discussion about startingn vs deep cell vs other battery types can get confusing. The important thing to remember is that a starting battery is not the right choice. Four typical 6 volt golf cart batteries will yield a 12 volt 450 ah battery bank. Obviously, 900 ah would require eight batteries. Very doable but a lot of weight and space.

I went with lithium batteries for big power in a small package. They are still expensive so not for everyone. If you just gotta have big power storage, they are the way to go.

Quote:
Originally Posted by Yorkitator View Post
I also need to know what size cables.
You need to determine how much power is going to be passing thru those cables and size for a 3% (or less) voltage drop (there are lots of charts and calculators online). Again, not for everyone, but I've found a 24 volt battery bank to be a very good thing. It allows running smaller gauge cables/wires and allows (in my case) the TS-MPPT-60 charge controller to pump out twice the power than it could in a 12V configuration (basically 120 amps at 12VDC). At a little more than $500, being able to avoid a second (networked) charge controller is a pretty nice deal. Of course, there are always negatives. You would need one or more 24VDC-12VDC step down converters to get the 12VDC power required for many accessories/components. Those are available pretty inexpensively to over $100. I use a 30 amp Samlex that is about $125. The other negative, it eats a bit of power even when doing nothing. Clearly there is no free lunch. Plus, keeping things simple clearly has some value.
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Old 04-24-2018, 07:27 AM   #16
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Just a +1 on buying a battery you're able to lift. I've lifted my 200AH 12V 4D batteries a few times and I do not recommend it. I'm a fan of using 12v over 6v but everything else being equal if I was buying today I'd get 2x 6v instead of 1x 12v.
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