Small Cabin PV System

vach · January 2015

Hi all. I've read this forum for a couple years and think it is outstanding. This is my first post. I am planning a small PV battery and generator system for my cabin in Northern WI. I have a quote from NAWS who have been very good, but wanted to check on here with others to be certain that I am getting what I really need. I have installed 2 small simple RV systems, a 100 watt panel on one and two 120 watts on the other. I probably know just enough to be dangerous, so want to make sure I get good advice.

I think my needs meet most peoples definition of small RV rather than small cabin. I have calculated my daily usage at about 1000 watts / day, mostly lighting, a fan, very small TV, some small battery recharging and small coffee maker. I have a 3000 watt Honda generator for any large loads, and any extra battery charging I may need. I can use the generator when needed, but want to minimize use of it. My area receives about 2 KWH/m2/day in November, my cabins time of highest use. I like the Kyocera 255W panels that NAWS recommended. They have about 2 sq. meters of surface area Doing rough math, I think they will produce about 800 watts/day full sun. If my math is correct, I think 2 panels be on the small side, but adequate. Am I close?

For batteries I'm looking at two, 220 AH 12 volts. At 50% discharge, this gives me 220 AH to use which I think is equal to about 2640 watts available, equal to 2.6 days at average use. Is my math correct and if so is my sizing OK? For this size system, does if pay to go 24v vs 12v?

It was recommended that I get a separate battery charger to connect to my generator for charging during high use or low sun days. I haven't sized the inverter or charge controller yet but believe I can use a 600 -1000W pure sine wave inverter and a buy a charge controller that has room to add panels in the future. I haven't seen an invert/charger small enough for my needs and think it might be best to buy them separately.

Thanks in advance for any help, it's greatly appreciated.

Cariboocoot · January 2015

Re: Small Cabin PV System

Welcome to the forum.

The main difference between a cabin and an RV is that cabins move around less.

I think your 1kW hour per day figure will be low when you look at what you want to run. Run a 100 Watt TV for a 2 hour movie and that's 200 Watt hours; 1/5 the total estimate. Coffee maker? Instant problem; although not run for long it will demand high power while it is on.

Batteries are what you base an off-grid system on, and batteries are based on load requirements. 220 Amp hours @ 12 Volts is barely 1kW hour AC stored capacity @ 50% DOD. Will it work? Maybe, depending on how much of your load requirements are overnight and how much are during the day. The overnight loads are battery-dependent, the daytime loads are panel-dependent.

Two 255 Watt panels is 510 Watts, which is more than enough for 220 Amp hours @ 12 Volts. If I were you I'd consider a slightly larger battery bank here, or else slightly less PV. That's something I rarely say.

And I would recommend going to 24 Volts because it is slightly more efficient and inevitably you will be using more electric once you have any.

Do you have a budget on this? It helps to know all the constrictions in planning.

vach · January 2015

Re: Small Cabin PV System

Thanks for the reply. Coffee maker is a one cup for the wife rated at 500 watts for 5 minutes. TV is maybe an hour of news in the evening. I think your probably right that my electric use will probably grow once I have it, but I think it will be less than most folks.

Is my math wrong on the batteries? Two 220 ah batteries equals 440 ah total and 220 ah at 50 % dod. 220 ah x 12 volts equals 2640 watt hours. Correct?

I thought 24 volt was probably best but wasn't sure. I was hoping for a budget of about $1000, but hope doesn't always match reality. Will spend what I have to to get quality equipment and meet my needs.

Cariboocoot · January 2015

Re: Small Cabin PV System

vach wrote: »

Thanks for the reply. Coffee maker is a one cup for the wife rated at 500 watts for 5 minutes. TV is maybe an hour of news in the evening. I think your probably right that my electric use will probably grow once I have it, but I think it will be less than most folks.

Is my math wrong on the batteries? Two 220 ah batteries equals 440 ah total and 220 ah at 50 % dod. 220 ah x 12 volts equals 2640 watt hours. Correct?

I thought 24 volt was probably best but wasn't sure. I was hoping for a budget of about $1000, but hope doesn't always match reality. Will spend what I have to to get quality equipment and meet my needs.

Two 220 Amp hour what Voltage batteries? Usually that is a GC2 capacity: 6 Volts. You need two of these in series to achieve 220 Amp hours @ 12 Volts.

If you have got 220 Amp hour 12 Volt batteries, then you're putting them in parallel which can be a problem but also will require more PV capacity than 510 Watts. Vis: 440 Amp hours would like 44 Amps peak current and that means (44 Amps * 12 Volts / 0.77 efficiency) 687 Watt array minimum.

I've tried several times to design a $1,000 system. Despite falling PV prices it just isn't happening. This was the last equipment list for it:

One 280 Watt Solarworld panel $335 (17.9 Amps 8% charge rate)
Kid controller $285
Two GC2's $266
PST-600 $206
Total = $1092

That's without the miscellaneous hardware and wiring, nor taxes and shipping charges.

vach · January 2015

Re: Small Cabin PV System

My bad, Crown CR 220 ah 24 volt batteries. Will I be able to get by on two of these? Is 510 watts of 24 volt panels enough to charge these batteries?

Cariboocoot · January 2015

Re: Small Cabin PV System

vach wrote: »

My bad, Crown CR 220 ah 24 volt batteries. Will I be able to get by on two of these? Is 510 watts of 24 volt panels enough to charge these batteries?

No, I don't think they are 24 Volt batteries. Crown's CR-220 is a 6 Volt 220 Amp hour battery. Put two in series and you get 220 Amp hours @ 12 Volts.

If you have 220 Amp hours @ 24 Volts you'd have four of these batteries. And you'd need more than 510 Watts of panel for best recharging: 22 Amps * 24 Volts / 0.77 = 686 Watts of PV just the same as 440 Amp hours @ 12 Volts would.

510 Watts would provide peak current of 33 Amps on a 12 Volt system or 16.5 Amps on a 24 Volt system. This would be 7.5% peak rate on 440 Amp hours @ 12 Volts or 220 Amp hours @ 24 Volts respectively. It would work, but it's marginal and marginal leads to trouble (like premature battery failure).

vach · January 2015

Re: Small Cabin PV System

How about I get out of my own way and ask you good people for your recommendations for equipment to meet my needs. 1000 wh daily usage with room to grow. How much PV and battery capacity? After I get that settled and get my brain unscrambled, then I can work on charge controller, inverter and battery charger. Thanks for tolerating my bumbling approach to this point.

westbranch · January 2015

Re: Small Cabin PV System

You were not too far off, just that the capacity of the batteries was wrong and it all went from there.

Note: you should only use 1/2 of the battery capacity or you run a great risk of killing you batteries, this is the 50% degree of discharge DoD or state of charge SoC you read about.

Cariboocoot · January 2015

Re: Small Cabin PV System

What I would do for 1kW hour per day:

24 Volt system: 220 Amp hours (four GC2's in series). At 25% DOD this gives just about 1kW hour AC all from battery (1320 Watt hours DC reduced to about 1122 when converted to AC). Any added from PV during the day is a bonus.

To get 22 Amps @ 24 Volts for recharging you need about (22 * 24 / 0.77) 686 Watts of PV. You will never hit that number spot on, and it's better to have a bit more rather than less. A couple of options:

Four Grape 185 Watt panels; 740 Watts total. Could be run as four in parallel with a combiner box & breakers (PWM or MPPT controller), or two parallel strings of two in series without (using an MPPT controller). They're about $212 each, $848 for just the panels.

Two Kyocera 325 Watt panels: 650 Watts total. Just a tad low on peak current, but viable. Vmp is around 40 so needs an MPPT type controller for maximum output. $367 each, $734. Shipping may make these too expensive, and they are quite large and not easy to handle.

There are smaller panels available with '12 Volt' configuration, but they tend to cost a lot more per Watt (like nearly double).

Good choice in charge controller would be the MidNite Kid @ $285. It is MPPT and can handle 30 Amps output.

Keep in mind you can squeeze the power requirement out of a smaller, 12 Volt system. But I think you would soon be disappointed with the result. For example if you take 220 Amp hours @ 12 Volt down to 50% daily it would work, but the battery life would be short. Or you can increase the battery capacity by buying larger batteries, but they will cost more per Watt hour stored capacity (the GC2's are usually the cheapest).

You might want to read through this thread to get a handle on battery capacity choices: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

vach · January 2015

Re: Small Cabin PV System

Finally had a chance to do some reading. Your reply and the links were a great help in understanding this all a bit better. Still some more reading and planning to do. I thank you very much. Always amazed at the amount of good info out there, just a matter of finding it sometime. (And sorting some of the chaff). Thanks again!

softdown · January 2015

Re: Small Cabin PV System

Interested in something similar for a cabin that I have. A concern that I have is theft. A lot of people have never been hit. Others get hit a few times/year. You might consider special fasteners for your panels. Batteries probably get taken more than panels. Possibly argument for an extremely heavy battery.

This is assuming that a cabin is often empty of course. I was just reading about solar equipment theft today....

Many of our best posters are Canadians. They generally don't have to worry as much about such things.

icarus · January 2015

Re: Small Cabin PV System

There are a couple of "rules" about off grid. The first is that most people over estimate thier solar input potential, while at the same time underestimate thier loading. The second is that load ALWAYS grow with time. For example, we started with a simple car battery powering the radio and a reading light. Having the light available, caused us to gt a few more. Then we added the telephone (ISP, and the modem) paddle fans, fridge ignightors (and lights) bed warmers, water pump, tool batteries etc.

We still use less than 1 kwh/ day usually around 800 WH, but thaqt is up from 500 a few years ago. Design your system to take these factors into account, give your self room to grow into your system.

Good luck and keep in touch,

Tony

706jim · January 2015

Re: Small Cabin PV System

Reading about your 500 watt coffee pot reminded me of a story regarding energy use at my island cabin.

I had to cut 32-2 x 6's from an old deck and used my Skil saw. While I never timed it, I figured 3 minutes to do the lot; perhaps 5.

1500 watts (or less) for 5 minutes = 0.125Kw-hr.

Our coffee pot runs 1000 watts and makes 10 cups in about 10 minutes.

This would be about 0.17Kw-hr.

People don't think much about the power needed to brew a pot of coffee, but who would willingly hand saw 32 boards?

My point is that your batteries can do a lot of work when used properly or can take a real beating with thermal devices such as coffee pots, toasters, ovens etc.

Alaska Man · January 2015

Re: Small Cabin PV System

Rule Number One, Nobody uses my system to make heat of any kind. If Mo'lady wants to use her hair dryer I make her go start the genny. Period.

Coffee Pot!!! I'll cut your hands off. But I'm an A$$ so.............

Audiomaker · February 2015

Re: Small Cabin PV System

Vach,

Could you tell us a bit more about your plans for the cabin? Are you going to live in it full time or just occasionally? If not full time, what season(s) do you expect to be in it the most?

About how big is it (how many rooms...etc).

Will you be mostly indoors, or is it a base for outdoor activities (hunting, mining gold...etc).

Is it near other sources of energy (a windy area, or near a river/stream?)

Normally I'd echo the other posters in "No electric heating (resistive loads) allowed", but depending on what you want (and how you need to size), you might be planning on not having propane and need to use a microwave here and there. There is always wood. Is there a wood stove?

What I'm getting at is that there's a difference in planning between having a "getaway" cabin, and a long term "bug out" cabin.

Photowhit · February 2015

Re: Small Cabin PV System

Well...

I'd go the other way, I'd make a test system, I'd try to stick to your budget;

I'd buy

4 - 100 watt Sonali panels Should run about $500 shipped (I used Green Bay, they say $108 UPS, I think these are small enough to ship UPS)

1 - Simple 25 amp Morningstar sun saver CC about $100 shipped

2 - 6 volt GC batteries from Sam's Club (Green Bay or Milwaukie) about $100 each, less if you have an exchange and they have been liberal on exchanges in the past.

and either a Xantrex ProWatt 900watt inverter for $238 delivered (true sine wave)

...or a Schneider/Xantrex/Freedom (yes they use all three) 1000 watt inverter charger, it gets poor reviews and I haven't seen a review here yet. It's only a 20 amp charger, so perhaps people where expecting more. I like it since it's sorta' hardwire able. It has AC out lugs. $300 delivered.

This would be a bit over your budget, but a reasonably balance system for a weekend use cabin. You would still need to devise a mounting system and add some fusing, but in a 12 volt system, I might go with a fused distribution block for the panels and a marine breaker for the inverter... MC4 wiring can be found reasonable, I'd be glad to help you hunt...

Small Cabin PV System

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