Sizing Solar Cells for Couple Full-time Boondocking

[SendIt]

Hello!
The weather is warming up here and there's no longer a foot of snow on my travel trailer, so I'm getting ready to install some solar panels. I already set up the energy storage and shore charging system. tl;dr:
- 280 Ah 24V DIY battery
- Multiplus inverter/charger
- Some other victron stuffs and a 24/12v converter

Starting in April, my wife and I will be full-time living/working from it for our honeymoon (we are both remote workers). I've completed a power audit [attached].

The original plan was to charge with a portable generator, but after doing some testing I've decided I'd like to supplement that with a yet-to-be-determined amount of solar. The generator I have is somewhat noisy and it takes most of the day to charge 280 Ah at 0.2C. The trailer is a 26' long Keystone travel trailer.

Some assumptions:

• There plenty of open roof space (26' x 8' mostly minus a few vents and an A/C unit), but no built-in ladder, so I'm leaning towards not tilting the panels. If I can figure out a good solution to tilt them and carry a portable ladder I may reconsider though.
• We will need to bring the portable generator as a backup in case we have a lot of bad weather or if we need to run the A/C for any extended period.
• We don't have a microwave or toaster and the fridge runs on Propane or AC.
• The water heater and cabin heater both run off propane.
• We will be in the Southwestern United States April-May, mostly camping off-grid.
• The power audit is relatively conservative.
• The main goal of adding the solar system is to reduce the time/frequency of running the portable generator. Ideally we'd only need to run it every other day or less.
• The rest of the system is Victron, so going with one of their charge controllers.

Power Audit Summary if you don't want to download it:

Full Load W-hrs.	5593	W-Hrs.
Full Load Watts	1741	Watts
AC Watts	557	Watts
AC Surge wattage	na	Watts
AC Watt-Hrs	3988	W-Hrs.
DC Watts	1184	Watts
DC Watt-Hrs	1605	W-Hrs.

Questions:

1. Does anyone have good information about what the range of full-sun hours I can expect are in Utah or Arizona during April - May?
2. Given the power audit numbers I'm working with, what size system would you go with (MPPT charger capacity and solar cell wattage)
3. Has anyone lived out of a travel trailer for an extended period of time and have any other sage advice?
4. Is there anything I haven't mentioned that I should consider?

Thanks in advance, you rock.

 

[Rednecktek]

Do a search for Blurb under my username, it'll help point you in the right direction.

Doing the power audit right out the gate is a great start! Most people skip that step and regret it later.

 

[Rocketman]

I have a 36’ class A - full time nomad.

On solar my advise (espically if not tilting), is to put as much as you can on the roof.

On our 36’ I have two arrays a 800w going into a Victron 100/50, and a 600w going into a Victron 100/30 (this one is very overpanelled). These give me 4 to 4.5kwh of power a day, which is all I use and brings my battery (525ah @12v) up to 100% (with good sun). I can tilt - but usually only have to do so in December & January in AZ. (We usually range from Az in winter, through Ut and MT/Id in summer (or Utah mountains).

The biggest issue is roof space.

If you are going full-time in that rig, ( and don’t think you will change rigs), I would make a frame of aluminum and have the roof completely covered. That way you have panels over the vents and A/C’s. This will keep the sun off the roof (cooler and no sun damage) and you can use larger (cheaper per watt) panels.

I wish I had gone that way from the beginning… but I used 200w panels (cost about $200each), installed the first array. That worked fine, then added residential fridge - so added a second array of 400w. Then added Starlink so added another panel to the 400w array, so we would get more power when conditions are not ideal.

I think your power audit may be a bit high, but you won’t know until you are using the rig.

Good Luck with your project.

 

[SendIt]

Thanks @Rocketman. Good perspective.

I think your power audit may be a bit high, but you won’t know until you are using the rig.

I'll work in there a few days off the battery this week and see how accurate my guesses were.

I used 200w panels (cost about $200each)

Where did you find those prices?

I would make a frame of aluminum and have the roof completely covered.

Interesting thought. Have you come accross any plans for this on the forum or elsewhere for inspiration?

Do a search for Blurb under my username, it'll help point you in the right direction.

Thanks, also will check that out.

 

[Rocketman]

Continuous Resources.com is my solar and Victron dealer. The High-Tec 200w panels are now $185 (shipping included). However if it possible to use larger 330w (or so panels) they can be cheaper. Look local for panels from solar companies…

 

[Rocketman]

To cover the roof, the best example is one of our friends new camper - they have eight 215w panels on a truck camper! Plus they tilt! They just did a walk through of their camper on YouTube.

At about the 20 min mark they talk and show the solar. They don’t show it flat - but I think you can get the idea of what it looks like. The solar panels sit 1” above the air conditioner (if I remember correctly).

 

[Rednecktek]

Blurb time!

Well, I'll start the default answer to these questions and we can work from there. Here's you To-Do list:

1: Power audit! This will give you some important information on how big your inverter needs to be as well as how much battery capacity you'll need. There is a link in the FAQ section (I think, or someone here will post it shortly) so fill in the blanks and see what it comes up with. You'll probably need some sort of Kill-A-Watt to get accurate measurements. Are you going to be running a 12v system? 24v system? 48v system? What are the specs on your solar panels? VoC? Vmp? Being as this is a new build, throw together a wish list of what you want and estimate on the high side.

1a: Where do you live? Speccing out a system for Scotland is a LOT different numbers than Arizona due to the amount of light you actually get. Someone here can post the link to the PVwatts.com or JCR Solar Uber-Sun-Hours calculator sites to help figure out how much you'll have to work with. That will be a box in the Power Audit form.

2: Parts list: You don't need a make & model list, just a parts list to start from for reference. You'll need an inverter, a MPPT charge controller, fuses, shunt, buck converter, batteries, wire, etc. Once you have a basic list it can be fine tuned to make & models after that. If you're looking at the All-In-Ones check for correct voltage outputs (120v or 240v Split Phase for North America, 220v Single Phase for European type areas) and make sure it has enough capacity for a little bit of growth and fudge factor.

3: Budget!: Steak is great but doesn't mean anything if your wallet says hamburger. Figure out what you're able to spend now vs what you'll have to cheap out on now and upgrade later.

4: Tape measure! Figure out where you're going to stick all the stuff you'll need. A dozen 3000AH batteries sounds great until you're sleeping on the floor because there's no room left for a bed. Is there a compartment that can house all this stuff? Will the server rack batteries fit? Are you going to have to make space? Physics can be pretty unforgiving.

5: Pencil out what you think you need and throw it at us so we can tell you what you've missed (because we ALL miss stuff the first go-round ) and help figure out which parts and pieces you're going to want to get.

Well that's the thing about solar systems, there is no 1-Size-Fits-All answer. Your system will need to be designed to fit YOUR needs. When you design and built the system, it's not going to be the perfect system for me, or Will or 12vInstall or anyone else, but it Will be the right system for You and that's the goal.

As for where to get started, let me throw my standard blurb in here to help point you in the right direction. There's going to be a lot of math and research involved, but that's going to be a LOT cheaper than just buying parts off of someone's list and finding out that it doesn't do what you need.

Don't panic on the Power Audit, you'll actually be doing that a few times. When you do the first pass put in ALL the Things that you might want. AirCon? Sure. Jacuzzi? Why not. MargaritaMaster-9000? Go for it.

The second pass will be the "I Absolutely Need This To Survive" list that isn't going to have much on there.

The third pass will be the "This is what is realistic" audit that you'll use to design the rest of the system.

The Power Audit is going to tell you 3 primary things: 1: How big does your inverter need to be to power your loads? 2: How much battery bank do you need to last $N number of days with krappy weather? and 3: How much solar panel will I need to install to refill those batteries in a 4 hour day (the average usable sun hours rule-of-thumb).

Once you know what you Want and what you Need and what your budget can Afford there will be somewhere in that Venn diagram where those three things meet.

After that, THEN you can start looking at parts.

Yes, it's a long drawn out process, but it's worth it in the end. Not every house has the exact same floorplan, not every vehicle is the same make & model, and not every solar system is designed the same.

 

[littleharbor2]

I'd avoid 12 volt panels. Too expensive. Look at Santan solar for great pricing on new and used panels.
Different cell count/format panels are different sizes You might look into which particular format fits your roof best. Example, 60 cell panels and 120 cell perc, 1/2 cell panels are generally in the 62 - 64 inch lengths. The 72 cell/144 1/2 cell panels run in the 77 - 79 inch range. The 60 cell panels voltage range seems more flexible for the 150 max voc. MPPT controllers.

 

[RoadTurtle]

Go with max panel layout, even if you don't add them all at the same time. I was very constrained both in height and roof area. Cut out pieces of cardboard and lay them out on the roof. I left space for two additional panels, but adding those two compromises some other stuff too. If it turns out I need them, then they have a home.

 

[SendIt]

I found a local supplier that has 330w residential panels for $318. I haven’t found anywhere selling for less online factoring in shipping costs. I’m be got a few more places to call, but thinking I’ll go with 3-4 of those unless I can find a better deal.

Can anyone recommend any good reading on sizing the MPPT charger?

 

[Rocketman]

At almost $1/watt those big panels are on the expensive side. I would go with the 200w for $185 if all else is equal. (They fit as well). They will be easier to handle and get up on the roof. Also look at Craig’s list. About where do you live?

If you use the 200w panels you will need to have at least two in series to get the needed volts. How many panels are you thinking of putting on the roof?

As far as sizing which mppt. Victron 150/45 would probably handle four of those 330w panels (I believe- I would want to see the specs of the panels first). If possible maybe put them in a 2s2p array. If you have room and may want more power in the future a Victron 150/60.

Three of those in 3p, four in 4p or 2s2p, five only in 5p, and six in 2s3p. (There maybe other configurations once we know the exact specs of the panels and how many you are thinking for now and max in the future.

 

[SendIt]

About where do you live?

I’m in western Montana.

If you use the 200w panels you will need to have at least two in series to get the needed volts. How many panels are you thinking of putting on the roof?

I makes sense. I’ve got a decent amount of space to work with: 8’x23’ ish. Depends on the size I go with.

I’ll keep looking for used or cheaper online.

Thanks for the MPPT sizing info. If I find panels that don’t take up all my space, I’ll size it to handle extra capacity case I expand later on.

 

[HRTKD]

Without a microwave or toaster oven, your load would be a lot less than mine. I don't think I could live without the microwave. After finding that I had plenty of PV and LiFePO4, I added the toaster oven. It's a great way to reduce the amount of propane I go through.

I have 640 watts (2 x 320) on the roof and the same that I can deploy on the ground. When properly oriented, the ground panels easily exceed the output of the roof panels. I'm in the Flat Panel Society and we're accepting new members at this time. Yes, you can create some cool mechanisms for tilting panels on a roof. But if it is a tedious task, it won't happen. When I was looking into tilt mechanisms for the roof, I found numerous posts (throughout the Internet) of people that installed tilt mechanisms, used them a few times and then never again.

Ground deployed panels are a great alternative. I don't put mine out that often and sometimes leave them at home. They are not fun to store/deploy, but I used panels that are too large.

With regard to which panels to use, I had originally planned to use four 200 watt panels. But when I created a CAD drawing and tried to place the four panels I found that there wasn't room for four. I switched to the two 320 watt panels. My local supplier sold me the 320 watt panels for $185, but that was in 2020. My local supplier was GreenTechRenewables, formerly CEDGreenTech. You can see what they carry on the website, but you'll have to call them to get pricing and availability. It was easier than it sounds.

I would use new panels for the roof and used for the ground if you're going to do that. You don't want to have to replace roof panels anytime soon.

 

[SendIt]

I'm in the Flat Panel Society and we're accepting new members at this time.

Haha, I was a mechanical engineer in a former life... A tilt mechannism seems like a lot of hassel and cost when I have this much roof real estate. For the price of the actuator and aluminum, I could get double the panels and not have to spend the time designing/building.

Thanks for the tips.

The best I'v found is $0.68/watt I've found so far is this amazon deal in a two-pack. It seems like a pretty good option or am I missing something?

Specs:

I'm not sure how to read the specs -- Can I do three of these per string or only two? With flat panels, I doubt i'll be hitting the max current any time, especially because the top of the trailer is slightly curved so they won't ever all be normal to the sun.

 

[Rocketman]

My issue with 100w panels is you need a lot of panel mounts. It costs half as much to mount a 200w panel than two 100w panels.

You have 24v batteries. You size the solar charge controller to the amount of panels you are or may put on the roof.

So a Victron mppt 150/45 can handle just under 150v Voc. (You need to leave room for the voltage to increase when the panels are cold in the winter). You could have 6s with that SCC. 6 times 21.6 = 130v

 

[Scar]

Rather than recommending an amount of solar I suggest drawing up a schematic of your available roof space and going onto Santan Solar and finding which panel gets you the most wattage from your roof space. Shipping is expensive but the panels are cheap, I did 6 of their 455w panels on my class C.

As far as the panels you linked, they're rated for 600v per string so no worries there, although I would be leery of any product where optimum is spelled opitmum on the label. Jokes aside I'm sure those panels are fine.

 

[SendIt]

My issue with 100w panels is you need a lot of panel mounts. It costs half as much to mount a 200w panel than two 100w panels.

Agreed. My plan is to place unistrut tracks down and hook into that with custom L-brackets. It's more labor, but the parts cost is pretty minimal. I'll keep an eye out for 200W panels.

So a Victron mppt 150/45 can handle just under 150v Voc. (You need to leave room for the voltage to increase when the panels are cold in the winter). You could have 6s with that SCC. 6 times 21.6 = 130v

Isn't the 15A series max series Fuse rating going to be an issue? I think the 14 AWG wire is probably the limiting factor here.

Rather than recommending an amount of solar I suggest drawing up a schematic of your available roof space and going onto Santan Solar and finding which panel gets you the most wattage from your roof space.

The snow is almost melted up there, so I'll go measure it out soon.

 

[HRTKD]

Sendit, I agree with what Scar said. Find panels that fit your roof.

Boseman is probably too far from you, but maybe Spokane isn't. There are GreenTechRenewable.com offices in those locations. If you're looking for larger panels it might be with the drive.

 

[Rednecktek]

If you're willing to hit Spokane, Platt Electric is now selling large panels. Order online and pick up near you. Not sure how far east they have stores.

 

[SendIt]

Based on availability, roof real estate, and prices, I went with 3x residential 545 watt panels and a used/heavily discounted SmartSolar MPPT 150/100.

If I’m wiring the panels up in series how should I go about sizing the wires?

 

[Ampster]

If I’m wiring the panels up in series how should I go about sizing the wires?

Size the wires so that voltage drop is acceptable. Since this is a vehicle mounted system distances should not be an issue so if the panels are in series the current should be small as long as you do not exceed the voltage of your controller or inverter as mentioned below by @Rocketman

 

[Rocketman]

What are the specs on the back of the panel? You need to make sure the Voc times three plus cold temp increases are below the max voltage of the mppt.

If you are unsure post the spec sheet and we can help.

Which mppt did you actually get?

 

[HRTKD]

If you bought the 250/100 then you should be able to wire the panels in series. But not knowing the actual specs of the panels, the answer is, "I don't know."

 

[SendIt]

Sorry, lasted the wrong line on the controller invoice. I went with a 150/100.

I’ll get panel specs tomorrow when I go look at them. Supplier is being flakey and hasn’t gotten them to me yet. Can swap out the charge controller if needed, he’s got several others.

You need to make sure the Voc times three plus cold temp increases are below the max voltage of the mppt.

Thanks, this is what I was looking for!

 

[Rocketman]

Can you fit another panel on the roof?

This is just guessing… (that’s why we need all the panel specs).

With a mppt 150/100 you may not be able to hook three in series. If you can fit a fourth panel, then hook them up in 2s2p (two in series and then those two in parallel). The issue is you may not have enough for the cold temp increases.

You could go with a 250/100 - but if they fit the four panels would be better. (Especially for poor solar days).