Help design my solar system for living fulltime in a travel trailer

kevin85 · April 2016

Ok. I think I follow you BB. Let me work on posting my loads then maybe you can better help me understand. You make me think I won't be able to run my microwave off my batteries, which is a bummer but I guess I can work around this.

BB. · April 2016

There are 800-900 Watt microwaves (fixed)... Also, I try to design a system "conservatively". You don't want the "microwave" to function correctly the first year (new batteries)--But randomly die later (older batteries, discharged some during winter, works when the sun is up, fails in cloudy weather/at night, etc.).

Others here (that run microwaves) can tell you if, in their experience, 4x golf cart batteries are enough to reliably run a small microwave.

-Bill

Photowhit · April 2016

I've regularly run a smallish microwave off of 4 golf cart batteries for @4 years. but the 4 batteries were setup as a 24 volt system, 1800 watt inverter with a low peak of under 3000 watts.

I'd just about say if someone isn't willing to left their hair dry naturally or cut their hair, they aren't ready for the compromises that they will have to make going to an off grid solar setup. A bit blunt, but small off grid as all about compromises.

You should remember that off grid requires you to produce MORE energy than you use on a regular basis. It's often easy to under estimate your loads.

kevin85 · April 2016

Thanks Photowhit. The microwave is really the only concern. Hairdryer not so much, she rarely uses it at home now. Like I stated most loads are optional or for convenience, we can get by on very little when necessary

kevin85 · April 2016

BB. Correct about the microwave, I will have to look again but I think ours is 800w

BB. · April 2016

Kevin,

I/we do not want you to feel like you are living in a "solar prison"--We just want to make sure the system you eventually decide on will meet your needs. (to the best of our predictive ability)

-Bill

kevin85 · April 2016

Right on brother. I would be lost without you guys. The knowledge you have shared is greatly appreciated

kevin85 · April 2016

I am revising my loads data and was wondering if it would be better to calculate for watt hours or amp hours? Or does it not make a difference?

BB. · April 2016

If everything is 12 vdc, you can do in ah.

If you have different voltages, then wh is less confusing.

If you do in ah, remember to tell us what voltage is being used.

-Bill

kevin85 · April 2016

Ok thanks BB.

TucsonAZ · May 2016

Just to toss in a quick note here as I was once in your situation with some of these choices. I had 1,430 watts on the roof and a 400 amp hour battery bank which worked well for me with a flat roof and some of my conditions. I had 6 panels set up as three in series banks with each bank on a separate breaker. Ran this into a MidNite Classic 150 (has the battery monitor built in so that was an instant $150 savings) and out to my 24v battery banks. I was EXTREMELY pleased with this and am very thankful I went 24v with the system. If you're building from the ground up a 24v system should be really easy for you as there are SO many 24v DC options, lights, fans and pretty much anything else you would need. I ran a 1500 PSW 24v inverter made my Cotek which I was very pleased with. If you contact ABC electronics in Minneapolis they may have some left, they were under $200 shipped and I bought two just for redundancy. I was able to run a micro, back with a toaster oven (endlessly if I wanted when the sun was shinning), run an AC when the sun was out and on the panels, run tools for repairs, run my chest freezer (just had to make sure it wasn't on when other things were) and do anything I really needed. I was very happy with the setup and I think if you go with something along the same likes you will too. You would really just need an extra panel to give you to solar banks and you will be fine.

BB has some really solid advice for sure!

PS Keep those panels a solid 3 or better inches off the roof! Mine BAKED me, actually bubbled the paint of the roof under them with the glass only 2 inches off and the frame an inch above.

kevin85 · May 2016

TusconAZ. Thanks for the excellent example. Nice to here real world experiences. Good tip on panel distance from the roof too. My roof will be rubber, not sure if that would be a pos or neg in that regard.

BB. · May 2016

A good engineering rule of thumb... For every 10C (18F) increase in temperature, the "thing" will age 2x faster.

So--for your roofing, anything you can do to lower temperature is a good thing.

With solar panels you kind of have two issues--One is that it is possible for solar panels to "trap heat" underneath them. But they also "shade" the roof.

I am tending towards the "shade the roof" reduces temperatures more than "close fitted" solar panels will raise roof temperatures.

In any case, solar panels with good ventilation run cooler. And that raises Vmp-array (a bit), which increases output. And cooler panels should last longer too (less peak temperatures, less thermal cycling).

-Bill

kevin85 · May 2016

Here are my AC & DC load calculations. I'm including the math I used in hopes that someone will check this for me. I was a little confused about the phone & laptop chargers. Also most of these items had only the amps listed so I had to calculate the wattage.
The DC loads should remain pretty consistent from day to day. As for the AC loads, they will fluctuate a bit. I have listed the upper range of expected usage.

DC Loads @ 12v
Water pump 7.5a 90w × .5hr = 3.75ah 45wh
Roof vent 1a 12w × 2hr = 2ah 24wh
Bathroom vent 1.5a 18w × 1hr = 1.5ah 18wh
Compost toilet fan 0.2a 2.4w × 24hr = 4.8ah 57.6wh
Phone charging 5v 1a 5w × 2hr × 2phones = 1.7ah 20wh
Laptop charging 19.5v 3.3a 65w × 2.5hr = 8.3ah 162.5wh
LED lighting .25a 3w × 4hr × 4bulbs = 4ah 48wh
Total 26ah 375wh @ 12v

AC loads
Microwave 6.7a 800w × 20min. = 2.2ah 267wh
TV 0.54a 65w × 4hr = 2.2ah 260wh
Dvd player 0.08a 10w × 4hr = 0.32ah 40wh
Total 4.72ah 700wh @ 120v

Please point out any mistakes

kevin85 · May 2016

So what do you guys think, does this system seem feasible? Balanced?
4 - 6v 225ah t-105 batteries wired series/parallel for 450ah bank at 12v
3- 285w panels wired in parallel for 855w total
Morningstar mppt 60a charge controller
Xantrex prowatt sw2000 inverter ( 2000w to cover microwave surge which is probably around 1500w)

Will this work with my load information?
Should I add one more panel?
Would 24v battery bank be any better?
If there is a better way please let me know

BB. · May 2016

Especially, for 120 VAC loads... You really need to measure them using a Kill-a-Watt type meter. Name plate is usually not very close--And cycling equipment (refrigerators, pumps, etc.) are difficult to estimate vs just measuring (DC AH/WH type meter).

1,000 WH per day--Looks like a good starting point.

-Bill

kevin85 · May 2016

BB. Thanks again, I would love to check things with a meter. Only problem with that is I don't actually have most of the items on my list. I have spent a lot of time researching & selecting items I plan to buy later. I hope this is enough to get me in the ballpark. I do need to get a feel for the major solar/electrical components needed so I can determine their location, necessary framing/blocking, & wiring routs in the trailer. Also to get an estimate of what these major components are going to cost.
When you get the chance, would you mind answering the other questions from my previous post?
I'm really interested in if I decide to expand later on that I will be at my limits with this 12v system.
Thanks for your time

BB. · May 2016

Yes, 4x golf cart batteries are fine for 1kwh per day loads. As well as 8xx panels.

If your ac loads (solar) are low, a 300 watt Morningstar 12 vdc inverter is great.

If you are going towards 1,200+ watt loads on ac or dc, I would go 24vdc battery bank.

I would also suggest that many of your dc loads would work well on 120 vac inverter.

The wide 10.5 to 16+ vdc of a deep cycle battery bank can make many native 12 vdc loads "unhappy".

Once you go 120 vac on many of your loads (chargers, tv, computer, etc.), the whole 12 vs 24 vdc question becomes less of an issue.

-Bill

BB. · May 2016

300 watts at 120 vac is less than 3amps ac.

Small awg wire and easy to send power over 100 feet.

300 watts at 12 vdc is already near 30 amps dc... Much more difficult to wire around the trailer. And long wire runs over a few dozen feet require much heavier cable.

-Bill

kevin85 · May 2016

Thank you BB. You have given me more to think about.
So you are saying I should use my "sensitive" electronics through the inverter to protect them from fluctuations in bank voltage. Which no doubt should extend the life of said electronics. Makes sense, I believe I read that elsewhere.

kevin85 · May 2016

I think I'm going to stick with the 12v system. After a brief research session I found out 24v components jump up a lot more than 12v. Also 24v components are harder to find than 12v.

With the dc loads being so small individually & the longest wire run being about 25ft I think 12v would be ok.
Shouldn't need a huge wire for these loads right?

BB. I think at most I would add a fourth panel if needed later on. This would give me a 1140w array. This would be right under your 24v threshold being 1200+ right?

westbranch · May 2016

a rule of thumb is you will need 110% of the Ah/Wh used to fully charge your battery,( there are losses in the whole system from PV to point of use) and that, on average over a year, you will get ~ 70% of the name plate Watt rating of your panel replaced....
hth
PS you are so close to the limit, go 24V rather than 12V, I have both now and started with 12 and very small loads...house or cabin, 24V minimum.

BB. · May 2016

The separate inverter just lets you place the high powered cycling loads on one inverter and separates the other smaller electronic loads to a second inverter.

The wire length for 12 volt loads depends on how much current too... For example, say 25 feet, 0.5 volt maximum drop, and 14 AWG wire--How much current? Trial and error gives me ~4 amps:

http://www.calculator.net/voltage-drop-calculator.html

http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=12&phase=dc&noofconductor=1&distance=25&distanceunit=feet&amperes=4&x=60&y=21

Voltage drop: 0.51
Voltage drop percentage: 4.25%
Voltage at the end: 11.49

So, does this work for you on a 12 volt system?

-Bill

kevin85 · May 2016

TusconAZ. Good tip on the cotek inverter, probably the best daeal I could find. Thank you. Again I appreciate your opinion on going 24vdc. I'm really debating on it.

Westbranch. I appreciate your input and bias towards 24v. Honestly, this is all a little confusing to me but I'm getting a grasp on it, thanks to some help. I have no problem going 24v if it is the better option. I have no experience, so I need you fellas to put me straight & tell me if I'm doing something foolish.

BB. I didn't realize you were talking about running two inverters. That seems a little redundant for me, I understand the logistics of it. I think I am comfortable with stepping down to a 1500w inverter, as you stated from a previous post I only need about 1000w of inverter. I do think I could make 12v "work" for my situation, even if I need a larger wire for a couple components.
Although, I don't want to be wishing later that I had chosen 24v instead. I suppose power loads do tend to climb

I have been doing more research on 24v components and I'm fine with everything except I can't find a single 24v roof vent. They are all 12v.
What would be the best way of dealing with this? Run 24v to it then wire in a 24v - 12v resistor/converter (for lack of proper word) right before the fan?

If I do go with 24v I should probably add a fourth panel to wire them in series parallel correct?

BB. · May 2016

It is up to you--Running a 24v to 12v DC to DC converter is OK. Or pick 12 volt bank (if you stay "small") and use a second smaller 120 VAC 300 Watt AC inverter. Just run the microwave off the 1,500 Watt inverter (only turn it on when using Microwave/Power tools).

The rest of the loads can probably run just fine on the 300 Watt inverter.

There are (possibly) other options--Drop the 1,500 Watt AC inverter and get a "12 VDC microwave"--They are out there (I have no idea how will they work--At least one is around 660 Watts). An FYI link (I have no information about the website or microwave):

http://www.roadtrucker.com/12-volt-microwave-wavebox/12-volt-microwave.htm

-Bill

kevin85 · May 2016

BB. That is an interesting microwave. I still prefer to keep the larger inverter incase I need to use some power tools or whatnot.

On another note. Does anyone have any experience or advice on connecting a 12v vent fan to 24v instead. Is this a bad idea? Seems like it would work to me but I don't want it to fail prematurely. Probably would make the rpms twice as fast right? Not sure if that would be a good thing. Any advice is appreciated.

BB. · May 2016

In general, connecting a 12 motor to 24 volts is not going to give you happy results.

You can get a 24 to 12 VDC converter (they are not that expensive--Can even find on EBay). (you can pay a lot for one too--bidirectional charging, etc.).

Or if you can fit it, a 24 VDC computer fan will work well too.

-Bill

kevin85 · May 2016

Ok scratch that idea. Will probably just use a dc to dc converter

kevin85 · May 2016

TusconAZ
I'm leaning towards the 24v system.
Were you in an rv?
Where did you find 24v fans? I've seen computer type fans but can't find an rv roof vent in 24v.
How was the midnite classic for you? No need for a separate monitor sounds nice. I was checking out midnite & outback. They end up about the same price as a Morningstar & seperate monitor.

vtmaps · May 2016

kevin85 said:
Where did you find 24v fans? I've seen computer type fans but can't find an rv roof vent in 24v.

I don't know the answer, but when I researched bathroom vent fans for my house, I discovered that the most efficient fans were powered by 120 V AC, but had low voltage DC motors. Each fan has a power supply to make the DC.

The point being: DC power supplies can be very cheap and efficient. If you need to, just make 12 volts from 24 volts with a DC to DC converter, or make 12 volts from 120 V AC with a power supply.

--vtMaps

Help design my solar system for living fulltime in a travel trailer

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