New to RV and Solar - could use some guidance

[lennon_68]

We bought our first motorhome last summer - kind of an impulse buy off a family member that was going to trade it in - a class c 1994 Conquest 102. A friend from work was telling me that they have one and it has a solar setup on it which is really nice. A couple weeks ago I saw a good deal on some solar panels and started researching what all we would have to buy to add solar to an RV. I think I did OK on my research and purchasing... Ended up with this stuff:

2 x Grape Solar 100-Watt Poly solar panel from Home Depot (5.56 amp)
Allpowers 20A Solar Charge Controller (PWM) from Amazon
2 sets of Z-Bracket for solar panel roof mounting from Amazon
5 pair of Renogy MC4 male/female cable connectors from Amazon
10-pack of ATC in-line fuse holders + 30 amp fuses + 40 amp fuses from Amazon
Power TechON 600W pure sine wave inverter from Amazon

During my research and purchasing our motorhome was parked at our family's house for the winter. After picking up the motorhome and poking around I realized that although I figured out what to buy I actually have no idea how to actually hook it up to my RV...

Looking for advice on where to start digging and if I need to purchase anything else. What I have now on the motorhome:
2 batteries both under the hood - one for driving and one for the motorhome... Unsure of the aH of the battery
30 amp plug
Generator
I assume a converter somewhere as the generator / plug charges the battery

Goal:
- Learn about solar for a potential addition to our home
- Significantly reduce generator usage when boondocking
- Have at least some 110v A/C available without needing to run the generator (that's alternating current not air conditioning)
- Ideally be able to run a 19" LCD TV and DVD player (or raspberry pi) when boondocking (again with minimal generator usage)
- Note, we're a family of 6 that is used to tent camping... I don't anticipate that our electric needs are very high (we don't shower in the RV, aren't using laptops, etc)

Specific questions:
1. Did I completely miss on any of my purchases (did I buy something dumb or miss buying something I'll need other than the actual wire)
2. Do I need more battery
3. Where in my system do I hook this up??? (I'm assuming this is a bigger question and I'll have to figure out more about my setup but I need help there)
4. What locations do I fuse? I knew for sure I fuse before and after the charge controller and between the battery and inverter. Read recently that I might need to fuse each panel independently if wiring in parallel - sound right? Anywhere else?
5. Is 8 AWG wire adequate for all the wire runs? I'm most unsure of the wire between battery and inverter
6. I *think* I sized everything so I can add a third 100W panel at a later date


I realize I probably need a lot of help and would really appreciate any guidance you guys are willing to provide

[Sunking]

Specific questions:
1. Did I completely miss on any of my purchases (did I buy something dumb or miss buying something I'll need other than the actual wire)
2. Do I need more battery
3. Where in my system do I hook this up??? (I'm assuming this is a bigger question and I'll have to figure out more about my setup but I need help there)
4. What locations do I fuse? I knew for sure I fuse before and after the charge controller and between the battery and inverter. Read recently that I might need to fuse each panel independently if wiring in parallel - sound right? Anywhere else?
5. Is 8 AWG wire adequate for all the wire runs? I'm most unsure of the wire between battery and inverter
6. I *think* I sized everything so I can add a third 100W panel at a later date

A1. You bought a cheapo PWM controller which means you must wire your panels in parallel requiring larger and more wire. It also turned your 200 watt panel into 130 watts.

A2. Minimum size battery for 600 watt 12 volt Inverter is 400 AH with MPPT controller or 600 watts with cheapo PWM controller . Minimum size panel wattage to support battery is 400 watts.

A5 No

[lennon_68]

A1. You bought a cheapo PWM controller which means you must wire your panels in para

Edit: I see you're modifying your response. I'll check back later

[lennon_68]

A1. You bought a cheapo PWM controller which means you must wire your panels in parallel requiring larger and more wire. It also turned your 200 watt panel into 130 watts.

A2. Minimum size battery for 600 watt 12 volt Inverter is 400 AH with MPPT controller or 600 watts with cheapo PWM controller . Minimum size panel wattage to support battery is 400 watts.

A5 No

Thanks for the feedback. That just leaves me with more questions though unfortunately.

1. Based on the limited research I did it sounded like for the most part series vs parallel wasn't a significant difference. I'm guessing that it ends up being important in my case because of the cheap charge controller I bought? Is there a budget MPPT controller that people recommend (or non MPPT for that matter)? Would series be a significant improvement? If I do series what do I have to look at to size things appropriately for adding more panels in the future if desired?

2. I think I have a fundamental misunderstanding somewhere here. Why does the AH of my battery or the solar charge controller have anything to do with the watts of the inverter? Quite possibly a dumb question... I thought the AH of the battery determined how long I could run the inverter (and the solar panels determine how much AH per day I generate - again indirectly effecting how long I can use the inverter).

5. Can you elaborate? Is 8AWG enough for the runs from panel to solar controller? What is required for the run from battery to inverter?


Thanks again!

[Mike90250]

With an RV, there is a trade off, wiring panels. Series wiring (higher voltage) uses a highly efficient MPPT controller. BUT all the panels must be in full sun at all times, shade one panel, and your charging stops.

With PWM controller, you can wire panels in parallel, and if one is shaded, charging proceeds at half power.

And with mounting flat on roof, panels will seldom reach full power, you may only ever reach 50% of nameplate. So your panels will help, but not very much, running your engine, and using your 130A alternator for half an hour, will produce more power into the batteries, that a week of your solar would.

[Suprasoup]

1. PWM controllers are Current in=Current out.
MPPT controllers are Watts in =Watts out.

With your 100 watt panels the current is 5.56a and voltage is 18v at maximum power point. If you series wired them the voltages are added but current stays the same. If you parallel wire them the voltage stays the same and the current is added. In your case:

Wired in series: 18v + 18v =36v. Current stays the same at 5.56a
Wired in parallel: Voltage stays the same at 18v. Current is 5.56a + 5.56a = 11.12a

since PWM controllers are Current in = Current out if you wire your panels in series you would get 5.56a into the controller. 12v x 5.56a=66W. For parallel wiring 12v x 11.12a= 133W. So you can see why you do not wire in series with PWM controllers

MPPT controllers are Watts in= Watts out. 2 100 W panels wired in series OR parallel will net 200W. If you wire in series you get to use less wire ( 2 wires to the controller instead of 4) and smaller wire to boot. Bonus you get to drill less holes in the roof of your RV too.

[Sunking]

1. Based on the limited research I did it sounded like for the most part series vs parallel wasn't a significant difference. I'm guessing that it ends up being important in my case because of the cheap charge controller I bought? Is there a budget MPPT controller that people recommend (or non MPPT for that matter)? Would series be a significant improvement? If I do series what do I have to look at to size things appropriately for adding more panels in the future if desired?

2. I think I have a fundamental misunderstanding somewhere here. Why does the AH of my battery or the solar charge controller have anything to do with the watts of the inverter? Quite possibly a dumb question... I thought the AH of the battery determined how long I could run the inverter (and the solar panels determine how much AH per day I generate - again indirectly effecting how long I can use the inverter).

5. Can you elaborate? Is 8AWG enough for the runs from panel to solar controller? What is required for the run from battery to inverter?

OK lets take this one at a time:

A1
PWM Output Current = Input Current
MPPT Output Current = Panel Wattage / Battery Voltage.

So with your PWM controller if you wire the panels in parallel 5.5 + 5.5 amps = 11 amps. Power = Voltage x Current. 11 amps x 12 volts = 132 watts. Wire the panels in series and you really screwed because have 5.5 amps x 12 volts = 66 watts from 200 watt panels.

MPPT makes no difference on paper initially if the panels are series or parallel. No matter watt 200 watts / 12 volts = 16.7 amps or 200 watts to the batteries. Which is greater than the other 11 or 16.7 amps? You said 11 amps is greater than 16.7.

But there is another plus of MPPT you are not forced to use expensive low voltage 12 volt battery panels. You can use much higher voltage Grid Tied panels which are less expensive and wire them in series. In an RV can be a moot point because the wiring distance between panels and controllers is short. Wire has resistance based on size and length. When you run current through resistance you loose both power and voltage. You want to limit those losses to 2 to 3% or less. Do some math. Wire the 2 x 12 volt panels in series and you have 12 to 13 volts @ 11 amps. Or wire them in series and you have 36 volts (vmp) @ 5.5 amps. 3% of 12 volts is .36 volts. 3% of 36 volts is 1.08 or 3 times more head room to work with. At 1/2 the current means much smaller wire and only 1 pair of wirse to deal with instead of 2. Bottom line it takes a 300 watt PWM system to equal a 200 watt MPPT system.

A2. Batteries have minimum and maximum charge and discharge rates. Color outside the lines and you fail class. Generically the limits are C/8 and C/12 with C/10 being perfect. Where C = the battery Amp Hour Capacity, and the digit is Hour Rate. Example if you have say a 225 AH battery it needs at least 225AH / 12H = 18.75 amps. In an RV with poor orientation you want to push charge rate to the max of C/8 or even as high as C/6. With MPPT or PWM is real easy to figure out panel watta is real easy to figure out panel wattage. For MPPT Wattage = Charge Current x Battery Voltage = Example 12 volt 225 AH battery @ C/8 is 225 AH/ 8 H x 12 volts = 337 watts or you can get away with 300 watts. PWM = 225 AH / 8 H x 18 volt panels = 506 watts use 500 watts.

On the discharge side all batteries have Internal Resistance and jus tlike wire the more current you draw, the more voltage loss you will incur. True Deep Cycle batteries roughly C/8 is as far as you want top go before you exceed 2% voltage loss pluse another 2 or 3% wire losses. Otherwise you can have a fully charged battery an dwith to large of an Inverter running near maximum, voltage sag will cause your Inverter to falsely trip off-line from under voltage. There are a couple of ways to work around this by using AGM batteries or Hybrid baterie slike Golf Cart batteries that have low internal resistance and can handle as high as C4 Discharge rates. So if you have a 600 watt Inverter means it wil require at least 600 watts / 12 volts = 50 amps of current from the battery. So if you used a 225 AH golf cart battery you can make it work.

Do all the math and you quickly realize Inverter Wattage roughly equal panel Wattage.

A5. 8 AWG would be over kill between the panels and controller. 8 AWG about right between Controller and batteries, and too small between battery and Inverter depending on size. Rule. No such thing as too large, never undersize.

If you were to read the stickies in the Off-Grid Section like 12 Volt RV Systems, most all you questions would have already been answered. Look around some more and even find drawings.

[Suprasoup]

Not gonna lie Sunking. I am in awe of your typing abilities.

Edit: and your technical skills.

[lennon_68]

Thanks for the information guys. I have a lot of homework to do

Note, I did click 2 of the 3 stickied threads but they weren't very useful for what I need... Seems the one I didn't click is the one I needed haha.

[jflorey2]

1. Based on the limited research I did it sounded like for the most part series vs parallel wasn't a significant difference.

In small systems it doesn't matter too much. In larger systems you save a lot in copper, since you only have to carry the current of one panel, not N panels.

Is there a budget MPPT controller that people recommend (or non MPPT for that matter)?

Victron isn't bad. They have a 100V 20A controller for about $170 that would work with your array.

I think I have a fundamental misunderstanding somewhere here. Why does the AH of my battery or the solar charge controller have anything to do with the watts of the inverter?

Higher power inverters can discharge the battery faster. High discharge rates (compared to the battery's AH rating) results in big dips in voltage due to the battery's internal resistance. This causes unreliable operation and reduces the life of the battery. They also have higher tare discharges which will rapidly discharge a smaller battery even when there's no load on the system.

5. Can you elaborate? Is 8AWG enough for the runs from panel to solar controller?

You can do the math for this. Calculate resistance (in BOTH conductors; they add) and figure out how much power you lose (I^2R) to heating. If it's more than 3% or so, go to thicker gauge wire.

What is required for the run from battery to inverter?

Again, you can do the math. This time you care more about voltage drop under maximum load, since that's what will cause the inverter to trip.

[lennon_68]

First off thanks again for the assistance. I realize that this information is out there but as someone coming in with no RV or solar knowledge it's difficult to piece everything together and figure out what is and isn't relevant to my situation without learning EVERYTHING

I've been reading a more and have a better understanding of how the inverter interacts with the battery (thanks to solar king above, solar king's FAQ, and jflorey2 above for driving it home). It sounds like my options there are to either buy a smaller inverter or buy more battery. If I upgrade the battery to something adequate to support this inverter then I go down the system design rabbit hole and find I don't have enough solar to support the battery... which means I need more panels and an even bigger MPPT controller... which becomes cost prohibitive extremely fast. For reference I'm in for $330 right now which is the high end of 'playing around' money for me - to do it 'right' with this 600w inverter I'd be way into 'big project' money which is somewhere I don't want to go right now.

SO that leaves me with completely different questions altogether. Rather than drop all the money to size everything up to this 600W inverter can we talk instead about where to go from where I'm at

A big part of my issue is I don't truly understand the real world differences in these systems. I surely can see the cost difference but have no clue how that actually applies to me. I think I see 4 potential routes but some of these are probably dumb

1. Keep the cheap PWM controller and both panels. Accept that I'm limited to 133W and spend more on copper for the wire runs (and more fuses I believe). Target a 120aH battery? and 300W inverter (may be too big of an inverter)?

2. Keep the cheap PWM controller and 1 panel. Target 100aH battery? And 200W inverter?

3. jflorey2 recommended the Victron 100V 20A controller. Can I get away with their 75W/15A with both panels (had it in my cart yesterday for $80)? If so maybe this is a route worth considering? Target 200aH battery and 300W inverter.

4. Victron 75W/15A with 1 panel. Target 100aH battery and 200W inverter

5. Is there some other option that I should be looking at instead

6. (added later...) The amount of solar in my budget isn't worthwhile. Spend the money on a bigger battery bank and keep the 600W inverter. Charge the batteries by alternator or generator and call it a day.


If I'm limited to a 200W or 300W inverter is it even worth running an inverter?



Thanks!!!

[Suprasoup]

Seems like you are going about this the wrong way. You need to do an energy audit to determine how many watt hrs you use daily.

Do you have space under the hood for a larger battery? A group 31 battery is roughly 100ah and can usually fit under the hood. Going to 200ah and you are looking at either 2 x 6v golf cart batteries or a big honking 4D or larger battery. Pretty sure a 4D battery wont fit under the hood.

I would figure out what kind of battery you got under the hood. Make sure you have a good battery isolator to charge off your alternator and do an energy audit with your existing equipment. Once we figure that out we can size your next battery accordingly as well as how much solar you may or may not need.

[lennon_68]

Seems like you are going about this the wrong way. You need to do an energy audit to determine how many watt hrs you use daily.

Do you have space under the hood for a larger battery? A group 31 battery is roughly 100ah and can usually fit under the hood. Going to 200ah and you are looking at either 2 x 6v golf cart batteries or a big honking 4D or larger battery. Pretty sure a 4D battery wont fit under the hood.

I would figure out what kind of battery you got under the hood. Make sure you have a good battery isolator to charge off your alternator and do an energy audit with your existing equipment. Once we figure that out we can size your next battery accordingly as well as how much solar you may or may not need.

Thanks for the input! I did some digging and found that my battery is a Super Start 27DCM which I found information on in this thread suggesting it was 99Ah:: https://www.homesteadingtoday.com/th...torage.425053/

I also believe I've tracked down the battery isolator to a cheap solenoid type that currently has a blown 10A fuse in the line that engages the relay (explaining why the alternator didn't charge a dead battery last time we used the motorhome).


As far as usage I don't think any amount of maths will give an accurate answer any time soon. Basically we're a family of 6 that's used to tent camping. Some days we'll only use the lights at night and a few minutes of water pump. Other days if it's rainy we may want to use the TV/DVD player. Ultimately I anticipate we'll use the camper a total of 20 days this year.

Just to reiterate from the original post my goals are:
- Significantly reduce generator usage when boondocking
- Have at least some 110v A/C available without needing to run the generator (that's alternating current not air conditioning)
- Ideally be able to run a 19" LCD TV and DVD player (or raspberry pi) when boondocking (again with minimal generator usage)

[Mike90250]

Solar on an RV is not going to reduce generator usage on gloomy days. A small 1 or 2 kw inverter generator with auto-throttle & a 20a battery charger will be a very simple way to solve the power issue.

And fixing the battery isolator is the first step

[lennon_68]

Solar on an RV is not going to reduce generator usage on gloomy days. A small 1 or 2 kw inverter generator with auto-throttle & a 20a battery charger will be a very simple way to solve the power issue.

And fixing the battery isolator is the first step

Thanks for the input Mike. I'll definitely be replacing that fuse before we go out.