Electrical & solar system basics

CharlieRN

Let me preface everything by saying that most of this is new to me, and that my understanding of electrical power consumption & storage along with solar systems, is pretty rudimentary. Please don’t be shy in pointing out errors in my calculations or assumptions! Let me also add that a lot of what's below was covered, or at least touched on, by some of the responses in my previous newbie post.

Here’s the question: Is the stock solar system of the Boondock adequate for off-grid camping under the following circumstances:

·         Time off grid – 3 days

·         Electrical usage (daily) – 500 Wh

·         Factory-supplied 133 W solar panel & Victron controller

·         Upgraded battery system - dual  6 volt Pb-acid batteries, nominally rated at 116 Ah (232 Ah total)

Assumptions/calculations:

Power consumption: Refrigerator uses 360 W per day (per factory for Isotherm CE 65) and the balance of 140 W is the total used by all other devices.

Batteries:  232 Ah converted to Wh is 2,784 (232Ah * 12v) - adjusted to 50% min safe state of charge gives 1,392 Wh usable [(232Ah*0.5) * 12 volts].

The factory solar panel for the Boondock is nominally rated at 133 watts; adjusted to 93 effective watts using 70% (133W * 0.7 ~ 93W).

For 3 days of boondocking, if I want to keep my battery’s SOC at >50%, I need to limit average daily power use to 464 watts (1,392 Wh / 3 da) OR ensure that I get at least a little help charging from the solar panel

Takeaway: Three days of boondocking looks pretty doable with a 232 Ah battery system.

Sharon_is_SAM

I believe the 2021 320s with solar only has a 105 watt panel.

DougH

So starting with your 1400Wh of battery juice, subtracting 1500Wh of fridge and other uses for 3 days, then adding back on 900Wh (300Wh *3) of solar if you're mostly cloud-free, and yup, three days of boondocking is doable.

P.S.  A 100W panel yields 280Wh a day in the PNW, 450Wh in the desert Southwest, 380Wh in Florida, 325Wh in NewEngland.  So add another panel, keep your utilization the same, and you might be able to boondock for much longer.  Again, assuming smoke and clouds don't cut your input severely.

VictoriaP

Math makes my head spin. 😁  @jkjenn said on one of your previous threads that 2 days with a pair of 6v + the factory panel was already pushing things to the limit with the Isotherm’s consumption. And she has a ton of previous experience boondocking with a Tab, I would trust her judgment on this.

jkjenn said:

I cannot get more than 2 days from 225ah of battery (dual 6v, AGM) and the 105w factory solar. I have an additional 100w solar panel on order, now, to hopefully give me a little more breathing room. 

One possible thing to double check, and one variable you cannot control:

- Most recent posts I’ve seen indicate the current factory solar panel is actually rated at 105 watts, rather than 133, so you may want to confirm this with the factory.

- Your actual energy collected by a solar panel can vary widely by location and time of year. Case in point: I have a 100 watt Renogy panel that is considered very efficient. In mid June, just after the solstice, and at midday in full sun, the absolute best I’ve seen it produce is 81 watts per the Victron MPPT controller. The faintest wisp of high clouds, or a change in time of day, or facing the wrong direction/at a less ideal angle, and it’ll drop down to around 60 or less. On a bright but overcast day? I’ve seen it drop to 10 watts. Add in that most boondockers tend to park in the shade where possible because you don’t have the option of using A/C to cool the trailer, and you simply cannot count on replenishing all that much with a single trailer-mounted panel.

Personally, I think a single ~100w panel is fine for long weekends with a 3-way fridge on propane, but I tend to think if I were having a 2021 built, that 12v fridge means I’d be adding a second solar panel as well as upgrading the batteries.

nhmike

I'd relax and try it, with a contingency plan.  It seems we learn with every trip we make and enjoy the learning.  I've had disastrous off the grid experiences and learned from the experience.  A cloudy day changes everything.  I do my best to flow with what we have, adapt for next trip and prioritize having a good time.  I try to plan ahead, and part of my planning is to accept that plans change.

Verna

The 2021 T@B 320 does have 105 watts from one solar panel. 

– watts corrected by Verna—OOPS!

VictoriaP

Verna said:

The 2021 T@B 320 does have 105 watts from one solar panel. 

@Verna 105? Really? Awesome if true!

Verna

Good Grief!  I fat fingered it👎👎👎👎👎

105 WATTS, one hundred and five watts on the 2021 T@B 320 solar panel!

I am so sorry!

VictoriaP

Verna said:

Good Grief!  I fat fingered it👎👎👎👎👎

105 WATTS, one hundred and five watts on the 2021 T@B 320 solar panel!

I am so sorry!

Aw...I was excited for a minute there. 😁

Verna

@VictoriaP, I grabbed my iPhone and not the stylus. I have the stylus in hand now! So sorry to get your hopes up!

ontheroad

So having come from T@gland where we started/ played with 80 watts it wasn't enough to keep up with our Norcold cooler..upped it to 120 watts but never played with it after...decided when we upgraded to the CS-S which we did order as Boondock so it has the 105 watt (wish it was the 130 watt)..we still will be bringing the additional 120 watts with us...and yes we also will be having the 2 6 volt batteries onboard...we are going to start playing with some overnighting somewhere...even if it is in the back 40...finally will be picking up next week...lots of time to play before the snow flurries start!

elbereth

We have the older Norcold NR740 in our CS-S, and a 235 Ah battery bank.  If we have overcast days with no meaningful charging, 3 nights is about our boondocking limit.  Sun definitely extends that, and your fridge is at least nominally more efficient than ours. I think you’ll be fine.

Try it out this fall and learn what your real world experience is! 

If you find you’re using way too much and you’re worried, you can always turn down the fridge, or even turn it off at night - especially if you keep some frozen water bottles in the freezer compartment.  Then it’s basically an upright cooler.  

CharlieRN

Perhaps it's an error, but the section of the invoice for my Boondock that details what is included in the package states " . . . 133 Watt Rooftop Solar Package."

Is it possible that nuCamp has made a change to the solar system for the 2022 model year and and the cut-in has occurred early enough that the new system is part of my recent order for a 2021?

Verna

NüCamp changed from a 133 watt last year to a 105 watt this year. I did verify that on my order. 

Sharon_is_SAM

@CharlieRN - I just confirmed with the nuCamp technician that the TaB and TaG 2021 models solar package comes with a 105 watt panel.  If the invoice was generated by your dealer, I would check with them.  You can also run it by nuCamp by providing your VIN # or order #.  No word about the 2022 models yet.

CharlieRN

Sharon_is_SAM said:

@CharlieRN - I just confirmed with the nuCamp technician that the TaB and TaG 2021 models solar package comes with a 105 watt panel.  If the invoice was generated by your dealer, I would check with them.  You can also run it by nuCamp by providing your VIN # or order #.  No word about the 2022 models yet.

Thanks for this - the invoice was definitely dealer-generated.

It's not a big deal but it does seem to reduce capability slightly unless the 105 W panel is more efficient. Do you happen to know what make the new solar panel is?

Verna

@CharlieRN, no, I do not. As I’ve been saying, I’ll know more next month when I receive mine. 

rfuss928

In the spirit of "Electrical & Solar Basics" it is important to consider that successful extended off grid systems are designed around battery cycling between 50-80% capacity.  The long, slow absorption cycle does not lend itself to the limited time available from the diurnal cycle, especially after bulk recovery.  Batteries can generally take near their maximum charge rate in the 50 -80% range and can make the most of all the solar energy available.  It is exceptional conditions or an over-sized system that will reach 100% charge during active use.  Some different battery chemistries have a lower discharge limits but most charge much slower during the 80-100% capacity portion of the cycle 
The take away is that you need at least enough battery capacity to meet you daily needs on about 30% rated battery capacity and enough solar to recover that much on your expected "average" day.  Also, become comfortable with 80% SOC.  Reserve for additional days without power requires battery capacity to be increased accordingly.  
The OP stated that they were able to manage for three days with no recharge.  That seems like appropriate battery capacity.  It is unlikely 100w solar will recover the ~500Wh needed daily to maintain indefinitely. 
IMO - An additional 100w portable could be deployed as needed for extended outings.  SUNT@B has 136w on the roof and an 80w portable available as needed.  Also similar battery capacity and usage profile as the OP.  Many years experience would support this opinion.
Have fun and stay safe.
Bob