Small cabin setup, am I on the right track?

boomadge

Some advise would be great,

Weekend cabin in New york usually 2 days max stay at a time.Usually from march untill
november (dec thru feb its shut down) I calculated a bit on the high side of usage.

mini fridge (132W) 1.1A/hr at 24hr =27A/hr
microwave (600W) 50 A/hr at 15min = 13A/hr
coffe pot (800W) 66.6A/hr at 30min = 34 A/hr
19'LCD TV (23W) 1.91A/hr at 2hr = 4 A/hr
light bulbs (4)x(25W EACH) 8.3 A/hr at 5 hr = 42 A/hr
Small box fan (35W) 2.91A/hr at 2 hr = 6 A/hr
Flojet pump (DC) 5 A/hr at 2hrs day = 10A/hr

136 A/hr day
1632 W/hr day

Do the numbers look correct?

I was looking into four 100 watt panels (400W total).
Panels each are photovoltaic (22.10 volts Voc) (5.91 Amps Isc)
(18.00 Volts Vpm) (5.56 Amps Imp)
(Module efficiency % 15.7) Plan to setup in 12V connected to TrakMax 40 MPPT Charge Controller
charging four 6v Trojan T105-RE battery's (225A/hr). Series/parallel for 12V 450A/hr.

The invertor I would use is Magnum energy MM1512AE 1500 watt 120V Modified Sine Wave
with built in transfer switch for hardwire of generator. Also has 70 amp AC side built in charger
with battery temp connector..

Am on on the right track? Would this setup supply my weekend power needs?
Should I go 24 volt route or is 12 volt going to work?

Any help would be appreciated.

Thanks

Cariboocoot

Re: Small cabin setup, am I on the right track?

Welcome to the forum.

First, do yourself a big favour and get a Kill-A-Watt meter. Measure the loads you want to use. Why? Because those numbers on the tags are often way off for the amount of power really used.

Second, you're looking at about 2kW hours AC per day, which is quite normal for the size of the application. However 400 Watts of panel is unlikely to supply this. Think about how much sunlight you get there, and understand that not all of it would fall directly on the panels. What you could expect in the shorter days would be more like 800 Watt hours. That's pretty short of your goal.

You will also probably find that 100 Watt panels are going to cost more per Watt than higher Wattage panels, even though the latter will have "incorrect" Vmp for the system. Even with an MPPT controller they can work out to be cheaper.

By all means go with a 24 Volt system. The only time you should use a 12 Volt system is if there is a specific need for 12 VDC for something. The four Trojans can be configured for 24 Volt even more easily than for 12, and will work slightly better in that configuration.

In fact you'd have about 1200 Watt hours @ 25% DOD, counting on all power being pulled from the batteries. If some of the load can be used during daylight after the batteries are charged efficiency will improve above that.

But to recharge those batteries you'll want more like 700 Watts of array. If you check my signature you'll see this if very close to my cabin system, which is capable of running a full size refrigerator as well as the satellite Internet system, 120 VAC water pumps, and even occasional microwave blast. BTW, a "600 Watt" microwave actually uses more like 1kW; the rating is for cooking power. My system can actually produce around 2.4 kW hours + in good light conditions, partly due to better panel efficiency at high altitude and careful load management. You may actually be able to use a smaller system.

boomadge

Re: Small cabin setup, am I on the right track?

Thankyou for the information it is much appreciated.

The cabin is completely off grid, previous owner
has it setup with 145 watts of crappy HF panels
on 4 12v car batts through a HF 1500 watt invertor.

Needless to say not enough power to test anything with Kill-A-Watt meter.
For a starting point I had to use equations.

This is all new to me, and I have been researching so much it got to a point
my brain was overloaded.

I will look into 24 volt system, thanks again.

Cariboocoot

Re: Small cabin setup, am I on the right track?

You may as well buy an inverter-generator now to give yourself power to test things with and serve as a back-up for your system once built.

That HF equipment is basically junk, but you know that already.

rich

Re: Small cabin setup, am I on the right track?

That charge control is more efficient at 24 volts,and quite capable....hope this helps.

boomadge wrote: »

Thankyou for the information it is much appreciated.

The cabin is completely off grid, previous owner
has it setup with 145 watts of crappy HF panels
on 4 12v car batts through a HF 1500 watt invertor.

Needless to say not enough power to test anything with Kill-A-Watt meter.
For a starting point I had to use equations.

This is all new to me, and I have been researching so much it got to a point
my brain was overloaded.

I will look into 24 volt system, thanks again.

icarus

Re: Small cabin setup, am I on the right track?

A couple of notes.

First, why does your pump run so long?
Second, consider making coffee with a French press or. Stove top Mr. Coffee. Resistance electric heat his a big draw in a small off grid system. Next, what is with 25 watt light bulbs? 10-15 watt CFLs will provide as much light as a 40-60 watt conventional bulb. Using 3-5 watt LEDs will save even more power.

Also look at your mini fridge. These re notorious inefficient. You might consider a Propane fridge instead.

As a general rule of thumb, all design considerations stem from the loads. Define your loads as best you can, and that will define the battery bnk and inverter size, and then that will determine the PV size.

My rule, for off grid is this. Take the name plate rating of the PV, divide in half to account for all submultiple system loses, then multiply that number by 4 to account for the averqge hours of good sun, per day over the course of the year. So your 400 watt system might yield numbers like this:400/2=200*4=800 Wh per day. As a side note, we live off grid, it's 400 watts of PV, and we routinly use 5-800 Wh of power per day.

Two other notes. First, people more often than not underestimate thier loads while at the sme time over estimate thier solar harvest, leading to chronic over draw/under charge. Second, loads ALWAY grow with time. We have moved from ~300 Wh/day to nearly twice Tht, and are slowly moving to near a kwh on some days.

Good luck, and keep in touch,

Tony

petertearai

Re: Small cabin setup, am I on the right track?

I would consider a pure sine inverter. Much better for your fridge and all other appliances. Yes go 24 volts, my setup is 12 volts, and I wish I had started with 24 volts .

ramloui

Re: Small cabin setup, am I on the right track?

icarus wrote: »

Second, consider making coffee with a French press or. Stove top Mr. Coffee. Resistance electric heat his a big draw in a small off grid system.

I agree with Tony. I, too, was hesitant in abandoning my coffee maker. But someone suggested the Coleman stove top like the one in the picture. I bought it for around 65$. It works GREAT!!! I am really impressed! It is like having coffee at home. It is a bit slower but now, I think it is the only way to go for an off-grid cabin.
Attachment not found.
Cheers

niel

Re: Small cabin setup, am I on the right track?

icarus wrote: »

Resistance electric heat his a big draw in a small off grid system.

actually any electric heat is a big draw on small off grid systems and not just resistance. nobody can change the physics of this as 1wh=3.4btus be it resistance or induction.

Cariboocoot

Re: Small cabin setup, am I on the right track?

niel wrote: »

actually any electric heat is a big draw on small off grid systems and not just resistance. nobody can change the physics of this as 1wh=3.4btus be it resistance or induction.

And the induction may actually be worse because whereas the heat transfer is more efficient the induction coils themselves are less so. So 100 Watts is ends up being the equivalent of 95 Watts out.

NorthGuy

Re: Small cabin setup, am I on the right track?

Cariboocoot wrote: »

And the induction may actually be worse because whereas the heat transfer is more efficient the induction coils themselves are less so. So 100 Watts is ends up being the equivalent of 95 Watts out.

Unless the elctric energy is transferred to some other form (such as mechanical or light), all the lost energy is coverted to heat. Electrical heating is therefore 100% efficient.

Cariboocoot

Re: Small cabin setup, am I on the right track?

NorthGuy wrote: »

Unless the elctric energy is transferred to some other form (such as mechanical or light), all the lost energy is coverted to heat. Electrical heating is therefore 100% efficient.

Nope.
Because not all the heat goes where you want it too.

Biggest problem with off-grid electric heat is the mass of batteries you have to have to store it and the huge array needed to replenish it. Batteries are only about 80% efficient (energy in vs. energy out) and whereas that 'lost' energy is going to heat it is not going to heat the pan on the stove. PV's with their pathetic <20% efficiency rate are the biggest problem with solar.

Let's face it; the main reason society is addicted to fossil fuels is because the energy density is incredible compared to other forms. We can use vast amounts of power to pump the stuff out of the ground, refine it, ship it, use it ... and it is still more economical. The conversion waste on oil must be something awful, greater than 50% of potential perhaps, and yet it still beats solar on cost. Some of that is fudging the economics (tax breaks, unrealized costs such as pollution), but the same can be said about solar subsidies (and often is).

Curiously the national grid wastes up to 50% of produced electric in transmission. No matter what the source, we need improvements in efficiency. Hence the mantra: conservation, conservation, conservation.

NorthGuy

Re: Small cabin setup, am I on the right track?

Cariboocoot wrote: »

Nope.
Because not all the heat goes where you want it too.

Once you have a certain amount of electricity and decided to convert it to heat, it is 100% efficient.

The problem is only that electricity (espcially off-grid electricity in most places) is much more expensive for heating than natural gas (or propane), for example. But this can change in a blink of an eye.

Cariboocoot

Re: Small cabin setup, am I on the right track?

Yep; 100% of the energy that goes into heating the food goes into heating the food.

niel

Re: Small cabin setup, am I on the right track?

Cariboocoot wrote: »

Yep; 100% of the energy that goes into heating the food goes into heating the food.

not quite as there are losses to the air. only a microwave will only put heat into that which you want heated, but there are other losses involved when talking radio waves. electric heating basically sucks and is expensive.

Cariboocoot

Re: Small cabin setup, am I on the right track?

niel wrote: »

not quite as there are losses to the air. only a microwave will only put heat into that which you want heated, but there are other losses involved when talking radio waves. electric heating basically sucks and is expensive.

Yes; I think people don't realize that these induction cookers operate at 25kHz+ and generate RF.

But no matter how you slice it, nothing is ever 100% efficient. Ever. Of course the Internet is full of fools and con artists who will try to convince you they have things that are >100% efficient. :roll:

boomadge

Re: Small cabin setup, am I on the right track?

Picked up a Kill A Watt meter, took readings and will post them below. Going to answer some questions
first.

I actually use instant coffee (propane stove) but enjoy a cup of perk once in a while.
2 hr runtime on the water pump is best guess (rather it be on high side than low side)
Pure sine wave is what I have decided to go with.

Hardest part was fridge calculation. It runs for 6 minutes and then shuts off for 15-20 minutes, almost like clock work. So its is really only
drawing power 24 minutes out of every hour. I calculated it high at 30 minute per hour (12 hours run time per day).

mini fridge (84Watts) .89 A/hr x 24hr/2 = 10.68
microwave (714Watts) 6.61 A/hr x 1hr = 6.61
coffe maker (940Watts) 8.7 A/hr x 30min = 4.35
19'LCD TV (22Watts) .28 A/hr x 4hr = 1.12
light bulbs (6)x(14Watts)=(84watts total) .18 A/hr each x 6=1.05A/hr x 6 hr = 6.3
Fan 1 (38Watts) .30 A/hr x 8 hr = 2.40
Fan 2 (42.7 Watts) .61 A/hr x8 hr = 4.88
Radio (7.7 Watts) .07A/hr x 6 hr = .42

Water pump is DC and based on 2 hrs run time.
Flojet pump (DC) 5 A/hr at 2hrs day = 10A/hr

46.76 A/hr day
Max possible load (all items on at same time) 1938 Watts

With these numbers running 24 volt system
Would 400 watts of panels be enough?
40 amp mppt controller work ok?
4 x 6volt 220 a/hr batteries be good for 2 day backup?
Cotek ST2000-124 2000/4000 Watt 24 Volt pure sine inverter W/30 amp transfer switch hardwired to 2000/2200 gen backup supply seem good enough?
I am in zone 5 NY (sun hours 4.2)

Any and all suggestions greatly appreciated..

Thanks.

Cariboocoot

Re: Small cabin setup, am I on the right track?

Okay, first you should express the power usage as Watt hours. Never mind the Amp hours because that won't be determined until a system Voltage is fixed. When you shift Voltage Watts remain constant; only the Volt:Amps ration changes.

I'm not surprised your 'frige runs about 1/3 of the time; that's quite normal except in extreme use conditions. So you would say "84 Watts * 8 hours per day = 672 Watt hours per day".
If you really use the microwave for 1 hour a day (more likely a fraction thereof) and it really draws only 714 Watts (pretty low) then that would be 714 Watt hours per day.

At this point you're hitting 1386 Watt hours.

940 Watt coffee maker for half an hour: 470 Watt hours.
TV @ 22 Watts for 4 hours: 88 Watt hours.
84 Watts of lights for 6 hours: 504 Watt hours.
One 38 Watt fan for 8 hours: 304 Watt hours.
One 43 Watt fan for 8 hours: 344 Watt hours.
8 Watt radio for 6 hours: 48 Watt hours.

Total: 3144 Watt hours (does not include water pump or inverter power). Call it 3200.
On 12 Volts that's 267 Amp hours. On 24 Volts it's 134 Amp hours. Minimum battery bank would be that plus conversion loss, inverter power, and DC pump demand times at least two. As you can see becomes quite a few Amp hours.

A big savings could be had with a smaller microwave use allotment. For example at 15 minutes per day the 714 Watt hours goes down to 179. That's half a kilowatt hour saved per day, or a reduction of 44/22 Amp hours (12/24).

To make that kind of power from panels using the Icarus formula you get 3200 Watt hours / 0.52 (over-all efficiency) / 4 (hours of equivalent good sun) = 1538 Watt array.

See what you're getting in to?

BB.

Re: Small cabin setup, am I on the right track?

I am not sure, but I think you are "off" on the power calcuations... Amp*Hours needs to include at "what Voltage" to be a "complete" description of what power/energy is needed.

boomadge wrote: »

mini fridge (84Watts) .89 A/hr x 24hr/2 = 10.68
84 watts * 24 hours * 1/0.50 duty cycle = 1,008 Watt*Hours per day
microwave (714Watts) 6.61 A/hr x 1hr = 6.61
714 watts * 1 hour = 714 WH per day
coffe maker (940Watts) 8.7 A/hr x 30min = 4.35
940 Watts * 1/2 hours = 470 WH per day
19'LCD TV (22Watts) .28 A/hr x 4hr = 1.12
22 Watts * 4 hours = 88 WH per day
light bulbs (6)x(14Watts)=(84watts total) .18 A/hr each x 6=1.05A/hr x 6 hr = 6.3
84 watts * 6 hours = 540 WH per day
Fan 1 (38Watts) .30 A/hr x 8 hr = 2.40
38 watts * 8 hours = 304 WH per day
Fan 2 (42.7 Watts) .61 A/hr x8 hr = 4.88
42.7 Watts * 8 hours = 341.6 WH per day
Radio (7.7 Watts) .07A/hr x 6 hr = .42
7.7 Watts * 6 Hours = 46.2 WH per day
Water pump is DC and based on 2 hrs run time.
Flojet pump (DC) 5 A/hr at 2hrs day = 10A/hr
5 AH * 24 volt Battery * 2 hours = 240 WH per day

46.76 A/hr day
Max possible load (all items on at same time) 1938 Watts

===========================================
Add bolded lines together = 3,751.8 WH per day

Assuming a "typical" 4 hour a day minimum for 9 months of the year in a reasonably sunny location:

3,751.8 WH * 1/0.52 system efficiency * 1/4 hours per day = 1,804 minimum solar array

With these numbers running 24 volt system
Would 400 watts of panels be enough?
40 amp mppt controller work ok?
4 x 6volt 220 a/hr batteries be good for 2 day backup?
Cotek ST2000-124 2000/4000 Watt 24 Volt pure sine inverter W/30 amp transfer switch hardwired to 2000/2200 gen backup supply seem good enough?
I am in zone 5 NY (sun hours 4.2)

The problem is (I think) is you mixed up Amp*Hours at 120 VAC (kill-a-watt) readings and AH at 24 VDC... Basically, the AH at 120 VAC need to be multiplied by 5x to get AH at 24 VDC.

Before you have a heart attack--3.75 kWH per day of power usage for a full time energy efficient small home with a refrigerator and other "modern" appliances is not a bad estimate. It is probably pretty close.

Note that the refrigerator, if you have hot summers in your kitchen, may consume 50% or more power during hot weather (check the Energy Star website--if it is still up at this time).

In any case, this is not a "small" system. Battery bank size (1-3 days of storage, assume 2 days and max 50% discharge for longer battery life):

• 3,751.8 WH per day * 1/24 volt battery bank * 1/0.85 inverter eff * 2 days * 1/0.50 max discharge = 736 AH @ 24 volt battery bank

Assuming 5-13% rate of charge solar array for that size battery bank:

• 736 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,256 Watt array minimum
• 736 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,511 Watt array nominal
• 736 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 3,264 Watt array "cost effective" maximum

So, you would need around 1,804 Watt to 3,264 Watt array, with ~2,511 Watt array as a "healthy nominal.

An 80 Amp charge controller on a 24 battery bank:

• 80 Amp * 29 Volts * 1/0.77 panel+controller deratings = 3,013 Watt array maximum "cost effective" for an 80 amp controller...

Is this making sense (the math)? And why we suggest conservation and limiting cooking/heating with electricity (using other fuels, if available).

-Bill

boomadge

Re: Small cabin setup, am I on the right track?

I think I see what your getting at correct me if I am wrong. Amp/hr for battery and watt/hr for pv.

I can see how removing the microwave,coffee maker, 1 fan,reducing the lights to 3 on max at a time,tv time to 2hrs, would cut the watt/hrs basicly in half therefor
should put me in managable 800 or close to watt array. This would inturn cut amp/hrs to approx 67 amp/hrs.

Is this correct to think like that?

Thanks

boomadge

Re: Small cabin setup, am I on the right track?

Wow didnt think for a weekend cabin I would need 8 plus 270 watt panels and 80 amp charge controller, 12 6 volts s/p . Time to trim some fat off the power suckers..

Thanks, so far I am getting a small grip on the conversion.

BB.

Re: Small cabin setup, am I on the right track?

boomadge wrote: »

I think I see what your getting at correct me if I am wrong. Amp/hr for battery and watt/hr for pv.

If everything is, for example, a 12 volt load--Then working everything in Amp*Hours is OK...

However, when you have 12 volt and 120 volt loads--It is easier to work in Watts and Watt*Hours--And only do the Amp and Amp*Hour conversions when needed (like figuring battery capacity).

And--just to keep the units correct... It is Amps and Amp*Hour... Not Amps/Hour or Watts/Hour.

Amps and Watts are already "rates" (like Gallons per Hour). When you multiply by Time (Hours in our math), then you have Amp*Hours and Watt*Hours (like gallons pumped).

We are so used to seeing Miles per Hour and such--We confuse that into Amp/Hr etc...

I can see how removing the microwave,coffee maker, 1 fan,reducing the lights to 3 on max at a time,tv time to 2hrs, would cut the watt/hrs basically in half therefor
should put me in manageable 800 or close to watt array. This would in turn cuts amp*hrs to approx 67 amp*hrs.

Yes, conservation, conservation, and a bit more conservation. Of grid power is not cheap (around 10x the cost of utility power over the life of the equipment). It is almost always cheaper to conserve power than it is to generate power.

Regarding Amp*Hours vs Watt*Hours--Because I am not sure where the "67 AH" is at--I don't know if 12 VDC or 120 VAC (as an example), I really cannot comment on the specific number:

67 AH * 12 volts = 894 Watt*Hours
67 AH * 120 volts = 8,940 Watt*Hours

The first one is a small cabin with a few loads... The second is my three bedroom suburban home's worth of power (I use natural gas for heating/cooking). It is really less confusing to work with Watts until you need to convert for Amps (battery AH capacity, Amps flowing through wires, etc.) when needed. Especially when doing the first paper designs of your system. We are doing lots of conversions/checks to see if 12/24/48 volt battery bank, etc...

-Bill

Coach Dad

Re: Small cabin setup, am I on the right track?

BB. wrote: »

.

In any case, this is not a "small" system. Battery bank size (1-3 days of storage, assume 2 days and max 50% discharge for longer battery life):

• 3,751.8 WH per day * 1/24 volt battery bank * 1/0.85 inverter eff * 2 days * 1/0.50 max discharge = 736 AH @ 24 volt battery bank

Assuming 5-13% rate of charge solar array for that size battery bank:

• 736 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,256 Watt array minimum
• 736 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,511 Watt array nominal
• 736 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 3,264 Watt array "cost effective" maximum

So, you would need around 1,804 Watt to 3,264 Watt array, with ~2,511 Watt array as a "healthy nominal.

An 80 Amp charge controller on a 24 battery bank:

• 80 Amp * 29 Volts * 1/0.77 panel+controller deratings = 3,013 Watt array maximum "cost effective" for an 80 amp controller...

Is this making sense (the math)? And why we suggest conservation and limiting cooking/heating with electricity (using other fuels, if available).

-Bill

Bill and Coot- I'd like your opinion on the following...
Since this is a weekend cabin... He stated in his first post that he is there for "usually 2 days max stay at a time. "
Would it be feasible to use 1 day of battery storage instead of 2 days for these calculations and use a generator as needed?

The reason for that question is:
I believe we determined in other threads that it is ok for batteries to cycle down to about 60% SOC every so often as long as they don't stay there too long and too often. I believe the consensus was that they need to be brought back up to 100% SOC every week or so..
If the sun is shining while he's at the cabin... no problems... If it's not, he can bring the batteries up to a reasonable state ( 75% SOC) with the genny and let the PV do the rest when the sun comes out.

What is your expert opinion?

BB.

Re: Small cabin setup, am I on the right track?

It can be fine in terms of battery life... For "weekend" cabin use, the batteries will probably "age out" rather than wear out anyway (cycling 50-100 times a year vs cycling 365 days a year--aging will probably become the dominant cause of battery failure--Assuming no "oops" of discharging to dead, forgetting checking electrolyte levels, etc.). That can also allow the use of a smaller solar array too (nice to reduce costs and limit losses if theft is an issue).

But then a backup genset (for bad weather) and "extra loads" (power tools when repairing cabin, etc.) may become a "requirement".

Need to figure out the loads on the batteries to ensure the system will still maintain "base loads"... Also, as you discharge batteries faster (say 10 hour rate instead of 20 hour rate), the batteries appear to have "less capacity". And, of course, surge power capacity of the battery bank is 1/2 as large.

May or may not be an issue, but you have to be more careful to ensure the basic power requirements are still met.

-Bill

Cariboocoot

Re: Small cabin setup, am I on the right track?

One of the trickiest aspect of off-grid is power management. Your over-all system efficient can go way up if you can make use of the power from the panels available after batteries are charged that would otherwise go unrealized. This is where the DOD calculations start to get fuzzy, as in basic planning we rely on all the power coming from the batteries during a day.

So if you increase the DOD to 50% over two days you have to look carefully at how much PV you've got for recharging. If you can put back 50% of the first days 25% battery usage then technically you enter Day Two at >75% SOC and you're good. If not you end up with the problem of going below 75% SOC over the weekend and possibly not getting back above it before 'Tuesday'. This means the batteries spend a longer time at a lesser SOC and that will increase the sulphation rate. This may not be an issue if you buy cheap batteries and are okay with replacing them sooner rather than later.

It can be negated by gen use if you're okay with that; quick Bulk charge in the morning or before you leave and let a minimal array take over afterwards. You could wind the array down to 5% charge rate or even a maintenance rate then, trading off generator run time for array size.

It does mean you will spend more time looking at the system to know what state it is in (or spend inordinate amounts of money on an AGS set up to do the work for you).

The thing to beware of in this case is that not only do loads grow over time, so does usage. Next thing you know you'll spend a week there and want that power daily. Then it will be six months. As time spent there increases you'd want to reduce gen run time and increase the array. Therein can be a trap as next year the same panels may not be available or you may have bought a small controller and you could end up having to buy a completely new array, controller, and wiring. But if you plan ahead you can avoid that trap (except for the matching panels problem). I know because I went through exactly this, and the only problem I ran in to was that my original panels were discontinued. If I want to add array now I either have to do some math and buy more expensive per Watt standard panels or swap out the array on the MPPT for all new panels and put the old ones on a different controller. It can be done, but it won't be simple.

Coach Dad

Re: Small cabin setup, am I on the right track?

Cariboocoot wrote: »

One of the trickiest aspect of off-grid is power management. Your over-all system efficient can go way up if you can make use of the power from the panels available after batteries are charged that would otherwise go unrealized. This is where the DOD calculations start to get fuzzy, as in basic planning we rely on all the power coming from the batteries during a day.
.

Yep... I agree 100%... With this statement...

Also:
I think that I'm in the same boat as boomadge... where I only use my cabin on the weekends... So I will give my opinion based on my limited (1 1/2 year) off-grid Solar experience.

I normally don't go to my cabin when I know the weather is going to be bad... So I see sunny days almost every time I'm there and my batteries are fully charged before noon on a typical day. This allows me to pretty much use any electricity that I want... I use table saw, air compressor, planer and any of my hand held power tools (one at a time of course).. I use this stuff all day long and the batteries stay at 100% or real close to it.
When night time comes around, I might have a light or 2 on, the refrigerator, a ceiling fan and the TV. In the winter when the days are shorter, we use more night time electricity but we still use most of the big stuff during the day when the sun is out.
The reality is that most of the electric usage for us weekend cabin guys is used during the day and during nice sunny days.

All of the above came as a pleasant surprise to me after I went live with my off-grid system. I designed it based on the worse case scenario and found that I had tons of capacity to spare... I did all the right things... I measured all my loads and planed for 2 days storage but I ended up with more than I could use on a sunny day... So I've been adding loads. So my loads grew because I had the capacity.

So I think there are 2 sets of equations that can be used for a PV system...
One is for the full time off-grid home that Bill did his calculations for and the other is for the weekend off-grid user which should be based on a 1 day battery storage with enough PV to charge them properly (5% to 13% charge rate).

Of course both off-grid users must have a back up generator and be ready to use it.

So if I was boomage, I would do Bill's calculations for the 1 day battery storage and use those figures but pay attention to the warnings that Coot points out.

Cariboocoot

Re: Small cabin setup, am I on the right track?

I did a similar thing when re-roofing the cabin; ran the gen first thing to bulk the batteries, then let the panels take over. This enabled me to leave the air compressor active all day with the PV keeping everything up even though the compressor firing would momentarily pull it out of Float.