Off-Grid "Tiny House"

[Animag771]

Hi everybody.

I am currently in the process of purchasing a 2 acre lot with very good solar exposure and as soon as the well and septic are installed the plan is to live on-site in a 5th wheel camper while we build our 432sqft house. So because there is no electric on-site the idea is either use a generator or make a temporary shed-like building to house the batteries and put some panels on that while we build... or both. I have searched all over the place to get as much information on the subject, but I still have some questions about sizing the system, battery types, and battery storage.


Ok, so as far as sizing the system... Because the house has not been built yet and I only own 1 of the appliances that will actually be going into it I have pretty much had to use the Energy Star rating of everything and divide the yearly Kwh usage by 365 to come up with a daily usage for each appliance. That being said I do believe that Energy Star assumes an average household of 4 people. I will only have myself and my wife.I have come up with a total of 8.8KW for per day based on my estimates. According to my math based on a 25% DOD and including 15% for AC inverter loss, I need to size my system at 12,320W (8800 x 1.4 = 12,320). So that means I need a minimum of 12.3KW, right? Well here is the big question...

Question 1: What about the hours when the sun is out? If I size my solar/wind array large enough to support my usage during the daylight hours plus 15-20% and I sleep for 8 hours every night then wouldn't I only be on battery power for about 10 hours daily? So do I really need that that much battery capacity? Not that having extra is ever a bad thing, especially because you wont always get clear blue skies.

So far I have been looking really hard at either using Trojan L16-RE or multiple strings of Trojab T105-RE batteries. It seems like the T105's are actually a better bang for your buck, but you would need a lot more of them.. For a system my size I would only need 6 L16's in series (12V -> 13,320W -> $2118) and for the T105's you would need 12 total batteries 3 parallel strings, 4 batteries per string (24V -> 16,200W -> $1,800). It's kind of looking like the T105's are the better buy. The L16's have 64% more capacity, but are 135% additional cost.

Question 2: When it comes to the cost over the entire life of the battery, which one is really the better investment?


Now to battery storage... I REALLY want to put my charge controller, batteries and inverter all in a subfloor-like area (it's actually more like a closet under a closet) I designed into the house with a hatch to access it. In this scenario I'm not really too concerned with cooling, because the batteries will be sitting on the concrete slab and it will be an on the interior of the house. The thing that worries me about this spot is venting.


Question 3: The space is in a corner along 2 exterior walls, could I just add some sort of ventilation to the outside through the wall or should I still be concerned about gasses getting to the rest of the house?

If not: What about using a vented DIY freezer root cellar for battery storage? Might not use a freezer, but you get the idea.




I know I just threw a bunch of stuff out there, but please correct me if I've made any mistakes.

[Mike90250]

I would look into building a little battery shed/solar roof and not stuff the big heavy batteries down in a pit where you won't be inclined to maintain them.
here's an idea, charger & inverter gear on one side, batteries on the other side.


pwrCtr_1.jpg
pwrCtr_2.jpg

[Wy_White_Wolf]

... will only have myself and my wife.I have come up with a total of 8.8KW for per day based on my estimates. According to my math based on a 25% DOD and including 15% for AC inverter loss, I need to size my system at 12,320W (8800 x 1.4 = 12,320). So that means I need a minimum of 12.3KW, right? Well here is the big question......

You need to divide 8.8KWH by the insolation available at your location. With off-grid you need to use the lowest monthly average. So if you have 2.5Hours in December then:

8.8KWH / 2.5H * 120% = 2640watt array.

Battery bank should be 44KWH of storage or

1056AH @ 24V
528AH @ 48V

...Question 1: What about the hours when the sun is out? If I size my solar/wind array large enough to support my usage during the daylight hours plus 15-20% and I sleep for 8 hours every night then wouldn't I only be on battery power for about 10 hours daily? So do I really need that that much battery capacity? Not that having extra is ever a bad thing, especially because you wont always get clear blue skies.
...

You should only use about 20% of a batteries capacity in a day. That gives you the best battery life per cost and allows for 2.5 days autonomy before hitting 50% discharge which is where irreversible damage starts. So a properly sized battery bank will cover 2 days of cloudiness and get you through a third night before needing charged with the back up genny.

...So far I have been looking really hard at either using Trojan L16-RE or multiple strings of Trojab T105-RE batteries. It seems like the T105's are actually a better bang for your buck, but you would need a lot more of them.. For a system my size I would only need 6 L16's in series (12V -> 13,320W -> $2118) and for the T105's you would need 12 total batteries 3 parallel strings, 4 batteries per string (24V -> 16,200W -> $1,800). It's kind of looking like the T105's are the better buy. The L16's have 64% more capacity, but are 135% additional cost....

You should buy battery batteries in a size that allows you to run a single string. That will allow for the best balance of the batteries when charging. Multiple strings also means more battery interconnects and more cells to maintain. The only option between those 2 that makes since for you would be 8 L-16 RE batteries that would be about 410AH at 48V. Still smaller than ideal so I would look for some other option. For some reason my head thinks the L-16RE were closer to 530AH. Maybe a different manufacturer.

...Question 3: The space is in a corner along 2 exterior walls, could I just add some sort of ventilation to the outside through the wall or should I still be concerned about gasses getting to the rest of the house?

If not: What about using a vented DIY freezer root cellar for battery storage? Might not use a freezer, but you get the idea.....

If you place them in the floor like that I think you're going to want to power vent the area with a fan. Otherwise hydrogen can collect in the higher areas of the space creating a hazardous situation. I think using a non-working freezer for your battery bank would be a good idea. Easily vented and can be solar heated in the winter. It's what I plan on doing when we build the final system for our cabin.

WWW

[Animag771]

Thank you very much for taking the time to read and respond.

WOW!! I made a huge math error. Now I see that I need 44KWH, something I may have missed if I hadn't asked.

Looks like my best option is the L16's, which are actually the L16-RE 2V which are 1110AH. So it would be 53,280W of storage which should give me 3 days of autonomy (if I add a wind turbine, even more) and based on the worst month of 4.8 hours of sun I'll need a 2,200W array. Sounds about right?

[wes@SH]

The sunlight should really be a secondary consideration, right now you have been advised above and are calculating for trickle charge.

(kWH per day) x (days autonomy) x 1/.9 (inverter efficiency) x 2(for 50% discharge of FLA) = Battery Bank Size

Battery Bank Size/48 = AH@48 volts(Figure out Batteries)

Batter Bank Size x 1/.89(panel derating) x 1/.95(CC derating) x59(charging Voltage) x.1(c/10 rate) + kWH per day/24(running load) = Array Size

the size of the array you get here will probably be close to or larger than winter month calculation by irradiation if you added all the deratings you should for the system(more like 50% then 20%).

I recommend looking at stickied posts in the off-grid section of these forums, most of the information you will want has been provided.

[Mike90250]

I think you want to re-look at your load estimate. It seems high for a tiny house, unless you are running air conditioning. Days I don't pump water (+6KWh) I only consume 7Kwh, running a house, 3 fridges and 2 laptops and 2 hours of TV on a LED tv.
For sure look at a 48V system for that load, and ease of battery configurations.

Also, where do you get winter sun hours of 4.8 hours, are you on the equator ?

Don't spend too much on wind power, unless you have to tie your hat on, you don't have enough wind to bother with.

[Animag771]

I think you want to re-look at your load estimate. It seems high for a tiny house, unless you are running air conditioning. Days I don't pump water (+6KWh) I only consume 7Kwh, running a house, 3 fridges and 2 laptops and 2 hours of TV on a LED tv.
For sure look at a 48V system for that load, and ease of battery configurations.

Also, where do you get winter sun hours of 4.8 hours, are you on the equator ?

Don't spend too much on wind power, unless you have to tie your hat on, you don't have enough wind to bother with.

The load estimate is tough for me because as I mentioned earlier, I only have 1 of the appliances that will be going into the house. To answer your question, yes I will be running air conditioning along with washer/dryer combo (line drying as often as possible)

I got the 4.8hr winters from the site below for Ft. Worth, TX.

http://www.solardirect.com/pv/systems/gts/gts-sizing-sun-hours.html

I won't be spending much at all on wind. Probably going to convert my brother's old washing machine into a turbine. It will be a fun project and if it keeps me from having to run the generator when I get a few days of wind and rain, that's even better. Most people say they get about 100W out of them when using washing machine motors, but I haven't been able to figure out what wind speeds they are getting to get those numbers.

[ButchDeal]

I won't be spending much at all on wind. Probably going to convert my brother's old washing machine into a turbine. It will be a fun project and if it keeps me from having to run the generator when I get a few days of wind and rain, that's even better. Most people say they get about 100W out of them when using washing machine motors, but I haven't been able to figure out what wind speeds they are getting to get those numbers.

I wouldn't spend anything on wind. A single module can get you double that output

The load estimate is tough for me because as I mentioned earlier, I only have 1 of the appliances that will be going into the house. To answer your question, yes I will be running air conditioning along with washer/dryer combo (line drying as often as possible)

the physical size of the home doesn't matter. What is not going to fly with an off grid smaller system like this is a AC and electric clothes dryer.

[wes@SH]

http://solarelectricityhandbook.com/...rradiance.html Fort Worth
Average Solar Insolation figures

Measured in kWh/m²/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
2.78	3.49	4.51	5.46	5.89	6.50
Jul	Aug	Sep	Oct	Nov	Dec
6.72	5.84	5.08	3.99	2.99	2.56

I think you are looking at your project the wrong way at this point. Figure out what it takes to store the energy you need daily, then figure out what it takes to keep that healthy. You should look at the stickied posts in off-grid forum about sizing for off-grid.

[Sunking]

Question 1: What about the hours when the sun is out? If I size my solar/wind array large enough to support my usage during the daylight hours plus 15-20% and I sleep for 8 hours every night then wouldn't I only be on battery power for about 10 hours daily? So do I really need that that much battery capacity? Not that having extra is ever a bad thing, especially because you wont always get clear blue skies.

So far I have been looking really hard at either using Trojan L16-RE or multiple strings of Trojab T105-RE batteries. It seems like the T105's are actually a better bang for your buck, but you would need a lot more of them.. For a system my size I would only need 6 L16's in series (12V -> 13,320W -> $2118) and for the T105's you would need 12 total batteries 3 parallel strings, 4 batteries per string (24V -> 16,200W -> $1,800). It's kind of looking like the T105's are the better buy. The L16's have 64% more capacity, but are 135% additional cost.

OK you are way off on Sun Hours. No place on earth gets 10 Sun Hours. For off grid you have to use worse case which is winter months. Depending on where you live, orientation, and shade issues can be as lkow as 1 Sun Hour up to a extreme 4 to 5 Sun Hours. You have to look at the tables for your area.

To determine Panel Wattage = [Daily maximum Watt Hour x 1.5] / Sun Hours.

That formula is if using a quality MPPT controller. So lets say in Winter you receive 3 Sun Hours and need 10 Kwh per day. Panel Wattage = [10,000 wh x 1.5] / 3 = 5000 watts.

As for batteries get model number out of your head. You want to use a single string of batteries, NO PARALLEL Batteries. Also forget cost for now because you are only looking at initial short term cost. You need to look at long term. Your way is way too expensive.

First thing I can tell you is FORGET 12 Volts. You will be operating at 48 volts. So if you need 13.32 Kwh of batteries means you are looking at 13320 wh / 48 volts = 277 AH battery. Those are going to be 6-volt cells and you need 8 of them in series.

A Trojan T-105 is not the same quality as a LR16RE battery. The T-105 is a 2 to 3 year battery. A LR16RE is a 5-6 year battery. Even better a Trojan INDV is a 7-9 year battery. One thing you have already missed is a Trojan T-105RE a 225 AH Deep Cycle 5 year battery. T-105 and T-105RE are not the same quality.

Do the math. You could use 225 AH batteries. Either a T-105 or T-105RE. A T-105 will cost you $130, a 105RE wil cost you $150. The 105 will last 2 years and at 25% DOD per day will cost you 55-cents per Kwh in battery cost or 5 to 6 times more than you can buy it from the Power Company. The T-105RE on the other hand will cost you 30-cents per Kwh or roughly 3 times more than the power company or half that of a T-105.

Look at warranties, not cycle life. Trojan has 7 levels of quality that range from 1 to 8 year batteries.

The T-105 is the Signature Line and carries a 2 year warranty. The T-105RE is a Premium Line and carries a 5 year warranty. INDV is Industrial line and carries a 8 year warranty. Ignore cycle life vs DOD, you will never experience those numbers. You wanna know Calendar life, that means warranty. The Industrial Line is the best Long Term investment. The Signature line is the highest cost. So don't let initial cost get in the way, or you will screw yourself.

In Ft-Worth you only have 3.64 Sun Hours

[Sunking]

I got the 4.8hr winters from the site below for Ft. Worth, TX.
http://www.solardirect.com/pv/system...sun-hours.html.

Whoever told you that is full of Chit. Ft Worth December is 3.64 Sun Hours. Go to PV watts. Enter

DC Power = 1000 watts.
System Losses = 33%
Tilt = 20 degrees
Orientation = 180 degrees.

December = 3.64 Kwh or 3.64 Sun Hours.

[Sunking]

Another blocked post.

[SunEagle]

Another blocked post.

Its the damn software not allowing links in anyone's original post or someone making a quote on that post.

At least it keeps me thin with all the clean up work.

[Sunking]

Its the damn software not allowing links in anyone's original post or someone making a quote on that post.

At least it keeps me thin with all the clean up work.

Can you say

ROLL BACK

[SunEagle]

Can you say

ROLL BACK

Actually I was thinking of just quitting the forum and taking a vacation from all this chaos and turmoil.

[Sunking]

Actually I was thinking of just quitting the forum and taking a vacation from all this chaos and turmoil.

Come on down to paradise. I got lots of room. Bring your planes, lots of places to fly. Got any climate you want down here as long as it is either year long Spring, or Tropical. I live where it is Spring 365 days a year. Forcest is super easy where I live. High 85, low 65, with a slight chance of a shower in the morning or late evening.

Or on the north side, Hot, Humid, with Rain Showers throughout the day. Americans are treated like royalty here. No taxes, half price energy (7-cents per Kwh forr Ex Pats) , and duty free. $80K buys a huge house and no Property Taxes with all the bells and whistles. Everything is dirt cheap. If you come, bring me some beef.

I just bought a Cessna 182 and come pick you up in Panama City.