Sizing and Recommendations

[WildcatSolar]

Could someone help me size this and make recommendations? I am going to cover some basic needs at my home for a couple of rooms. Minimum 2500+ Watt-Hours per day. Would consider some higher for better cost benefit.
This would operate: a 547kw per year refrigerator rated by Mfg., 1/2hp 240v well pump up to 1 hour per day with one startup to fill a 500 gal tank on a hill to gravity feed, Misc. Small fluorescent lights and a radio or TV for a few hours per day.
I want a MPPT controller to handle a 24v system with approximately 50% +- expandable reserve capacity. (Recommendation?)
2000 Watt Pure Sine Wave Inverter 120v

[Mike90250]

When you add batteries, you loose 30% of your efficiency. If you have reliable AC power, I would strongly suggest simple Grid Tie, with a small inverter generator (eu2000) for reliable backup power.

If you insist on a battery system, you will have some re-wire to move loads to a backed up panel, and also a Transfer Switch.

I also submit that you will need a 48V system, which might seem daunting at first, but it's simpler to have 1 series string of batteries, then lots of parallel strings. Could be 4, 100A 12v batteries, and if you need more power, change to 8, 200A, 6V batteries, and no parallel loads & fuses to worry about.

Xantrex XW is only 240V inverter, otherwise you have to stack inverters.

What's your location roughly? You can use the PV watts website to predict your harvest. http://www.pvwatts.org/

If going off-grid, you will not have backup, so you need to plan for generator usage for 2 winter months, or else oversize the PV for 10 months.

Refine your plan a bit, and then ask again, and I'll figure out the #'s you gave earlier.

Mike

[WildcatSolar]

Mike,

I am trying to prepare 2 or 3 rooms in my house for solar in case the grid goes down. I live in the country and may have to depend on my on abilities if that grid goes down. I believe this country is on the path to repeat the lessons of the Weimar Republic. I want a backup that will still be useful no matter what. My earlier post said the upstate of South Carolina for the sun availability. We would have 5+- hours daily on average.

A 24v system with all 4 Trojan L-16 RE 320AH 6V wired in series = 1280AH at 100% Charge. The house wiring requirement is 120v except for the 1/2hp 240 well pump which I will run under 1 hour per day and probably far less than that to meet the needs to fill the 500 gal tank on my hill. A step up transformer can handle this and it should cost around $500 or so for that piece.

The refrigerator is estimated to use 547kw/year so that is 1.5kw per day on 120v so that is only 12.5 AH per day and that is the greatest load. The 4 batteries described above should be overkill and last for years with the discharge per day needed.

2500-3000 watt hours per day should be sufficient for my needs. I would like opinions on the MPPT Controller brand, amps to be held in reserve for expansion down the road, a good 2000-2500 pure sine wave inverter recommendation, and the step up transformer recommendation. I am open to suggestions on anything. I thought about going grid tie with everything needed for taking it off grid if needed. Batteries don

[Mike90250]

I'll use Columbia, SC and a 1KW array. Outputs are raw DC power, not derated after battery charging

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.81, 89, 7.21
2, 4.48, 93, 7.53
3, 4.98, 113, 9.15
4, 5.95, 127, 10.29
5, 5.66, 122, 9.88
6, 5.81, 117, 9.48
7, 5.65, 117, 9.48
8, 5.41, 113, 9.15
9, 5.38, 109, 8.83
10, 5.71, 125, 10.13
11, 4.54, 99, 8.02
12, 3.80, 87, 7.05
"Year",
5.10, 1310, 106.11

one Problem in planning, is the fridge, in winter. The sensor is in the freezer compartment, and you open the fridge door all the time. Soon, the fridge is warm, and the freezer is still cold. Everyone has this problem, I will just split the power by 12, to figure your winter usage, when you are more likely to use lights and entertainment.
547 / 12 = 45.58 / 30 days = 1.6 KWh daily


add you other loads
pump 700W x 1 hr = .7 KWh
laptop 100W x 2 hr = .2KWh
TV 120W x 2 hr= .24KWh
Lights 35W CFL x 5 x 6 hr = 1.1kwh
radio (Short wave w/wall wart PS) 15W x 10hr = .15KWh
total = 3.99KWh daily

December is the lowest production month @ 87KW / Mo. (2.8KWh / day)
On an Average, you only want to discharge battery bank no more than 10% daily, so you can squeeze 3 or 4 days of storm / no sun. The light discharge will make batteries last 5-10 years. Deeper discharges give shorter lifetimes. 50% discharges - maybe 200 - 300 of them.
So your battery needs to store about 3.0 KWH each day, for a total size of about 30KWH. (10x your daily use) at 24v that's 1250AH. 48V that's 625AH
a single "group 27", 12v battery holds about 105ah. (1 ton PU truck battery)
This would end up being 2, 48V battery banks of 8 each, Trojan L16RE-B 6V. Pretty large, but thats what it takes to make a long life battery bank.
370A x 2

To charge it, the rule of thumb is 10%- 15% of the battery capacity amps (740A).
10% = 74A @ 55V = 4070KW harvest

A PV array will deliver about 80% of it's nameplate wattage. Like a car gets "27MPG". You would need an array of about 5,300 watts. (27 or 28 200w panels)
This is achievable, given that PV watts says a 5KW array would give 14KW daily, December. (Grid Tied)

This allows to recharge after a 3 or 4 day storm, and enough power to properly equalize the battery bank on a monthly basis.

You will have extra power in the summer - run a fan, you'll need it.

You could easily cut this into 2 systems. 2.5KW arrays each, 1 battery bank, and charge with a 60A MPPT controller. ( like the Monrningstar 60A MPPT, Xnatrex makes a good one also, 60A.
I would use the Xantrex XW 4548 inverter charger, easily adaptable to stand alone or grid tie w/ battery float. This also archives your 240V for the pump. http://store.solar-electric.com/xaxw12hyin.html

You might think a simple step-up transformer could do that, and if you size the transformer to the starting current, it would likely work. 1/2 hp pump likely has a 20- 25A starting current. A 25a/240v transformer would be really large. That's needed because a transformer cannot pass any overload current. When the amps saturate the core, it can't pass any more power, and a small transformer would cook both your pump (low starting voltage) and the transformer.

Additionally, a 24V system may not be able to deliver the starting current from the batteries.
240V @ 25A = 6,000 w (well within the 10x, 10sec surge of the XW)
6,000w @ 24v = 250A
6,000 @ 48V = 124A Whis is still a lot, even for 5 seconds, but more likely to actually work.


Hope this helps

Mike

[WildcatSolar]

Mike,

I thank you for your time and efforts. I continue to learn and you have helped in that process.

I believe there are other things to consider. I have two refrigerators. I would only need one as a necessity. One is in a heated living space at the moment, but would not be heated normally under the scenario I descibed. The other is in an unheated garage at ambient temperature 5' away. The temps during our winter are mild by some standards but the lower sun availability coincides with the weather in the 30-50 degree range or lower. I could take advantage of natural cooling at night to carry the refrigerator portion if not the freezer portion during the winter months and use no electricity for that need at all much of the time. I have also been thinking of a superinsulated containment box based on the underground ice boxes I saw at Jefferson's Monticello for the storage of meats and fish if one must keep ice. I could easily pull the ice from my swimming pool and pond when it forms to keep such a box. I also could do what my grandparents did in their old smokehouse to preserve meats. I used to help do all those things on the farm and can again.

I would not run the well pump every day. I could probably push the pump requirements to two hours per week by filling the 500 gallon tank each time. I have also been studying a manual pump or alternative power source that uses no electricity or fuel.

The number of lights, wattages, hours and the like are all likely to be less than those calculated. I know the number of hours would be reduced. One or two 7 watt lights (40w incand. equiv) at night on at any one time would work for these rooms. I would run a blower on my woodstove backup that I made from a couple of low wattage computer fans. This is how I would heat the living area along with some solar collector systems I would use during the day to get the temps up on some thermal mass storage units. I am working on other ideas and regardless of the grid condition, I am helping my power bill so this is fun trying to see what is possible. It is a challenge and I love challenges.

Let me ask a simple question and not take too much more of your time. If I can only use 3000 watthours a day or less and not factor in storms, using the four batteries I descibed and a 2000w pure sine wave inverter for 120vac, how many panels of approximately 200 watts would I need with the available sun worst case-best case? I have an idea, but I want a second opinion. Thank you so much for your time and help.

[Mike90250]

Let me ask a simple question and not take too much more of your time. If I can only use 3000 watthours a day [3KWH mike] or less and not factor in storms, using the four batteries I descibed and a 2000w pure sine wave inverter for 120vac, how many panels of approximately 200 watts would I need with the available sun worst case-best case? I have an idea, but I want a second opinion. Thank you so much for your time and help.

The calculated consumption I gave you above, in bold total = 3.99KWh daily is not much larger than you are asking.

I laid out the math, and the way to calculate. The laws of battery life dictate that lightly discharging batteries, gives them WAY more lifetime, then deep discharging them. When the S.H.T.F., it's unlikely you will be able to expand the system.
6v x 320ah = 1920wh (1.9KWh)
50% discharge = 960wh available. Winter days, expect <3 usable hours of charging light (per PV watts, each 1KW array = 2.8KWh harvested)

So in winter, you have 3 charging hours, maybe 3 neutral hours (consume as much as you harvest) leaves you 18 consuming hours, for only .960 KWh of storage. You can spend the storage any way you want, Lights, tv, fridge, computer, water pump.
Your batteries will last 2years at the most, with the heavy loading.

Panels: 10% of your battery AH spec, 32A charge current.
32A @ 24v = 768wats harvested. At 80% of panel label output, you would need 998.4w of PV (200w panels = 5 panels)
Look to www.SunElec.com for best panel prices. Look for deals in the blems.
I still suggest the Morningstar 60A MPPT (or for the 1KW array, 45A MPPT)
as your charge controller.

Good luck getting a transformer to start the well pump. search the web for success stories.

Search for "chest freezer conversion refrigerator" You may save some power there. Avoid the designer "off grid" solar refrigerators. They are small, very expensive, and a modern Energy Star fridge does as well as the special yuppie solar fridge.

[WildcatSolar]

Thank you. Great teacher.

[BilljustBill]

Protection of Your Array?

Mike,

In view of Wildcat's planning and your insight, I do understand that once things start going down hill, getting more solar items would be limited. Ideas about dealing with storm damage or possible vandalism/theft to PV systems seem to be next in being considered.

At garage sales and monthly flea markets, I've found a retractable/rollup steel bed cover for pickups and several 32" wide rollup stainless steel covers with 1" channel aluminum spaced and riveted in 1/2" spacings. The latter covers seem to be known as way-covers that protect the bed surfaces of large CNC milling machines. I paid under $7 each for these. Also, at a surplus store, for $100, I bought a 6-1/2 ft wide by 10' long version used to cover large chemical tanks made in the same way. While the smaller covers have tension springs like a window shade, this large one uses a compressed air motor with sprocket and chain drive to coil it back. (An employee of the store said it came from a prison in Arizona...I had to laugh when I got it home, and with the data plate, went looking on the Net for information. Turns out it's a $9,000 item made by this company....
http://www.gortite.com/category.php?id=15 )

After going through replacing a double pane picture window due to a neighborhood "Tween" and his B-B gun, and some years later, a solid 30 minutes of golf ball size hail, putting these panels in view of the big sky makes me a bit concerned. How would you protect a PV array?

Bill

[Mike90250]

a solid 30 minutes of golf ball size hail, putting these panels in view of the big sky makes me a bit concerned. How would you protect a PV array? Bill

Move.

Well, for the hail, the PV's do have a spec for some resistance, and if they are at an angle, that helps too. (my array is going up at a 50 deg tilt, to harvest winter sun)

Roof tops and pole mounts don't get a lot of windblown debris that high, but if a storm is expected (hurricane) and worries about wind blown stuff, I'll either plywood cover it, or unmount it for the duration.

Onset of hail and tornadoes is too rapid to do much about it. A roll over cover, you have to be sure the width is secure to not flail against the PV.

Vandals. 2 legged varmints can be controlled with shotgun and rock salt. Used to work for grandpappy's watermelon patch. Eli with his machete looks to be effective too. (movie 1/2010)

Dealing with "Midnight Solar Parts" company, creative signage may be the best answer.

If panels are installed with theft proof hardware, they may get smashed out of frustration. Maybe a good insurance rider is the answer.

[BilljustBill]

Semi-Bargain AIMS inverter

Thanks for the input, Mike. I was watching the last few hours of an auction while researching a 1500 watt pure sine wave inverter... It's a 48v Aims with a remote button...and sold as brand new... I bid on it with a $152.00 bid plus $25 shipping... It's "in the mail".... Not a MSW 1500w Xantrex for $45, but seemingly good deal....

Hunted for a controller... Found the Morningstar's Tristar 60 amp that adjust from 12v, to 24v, to 48v Shippiing and all, it was under $200.

Before wire went up again, I bought a 500ft reel of #6 strand copper single run for $85.00 from Lowes... A flea market had welding cable, looks like 1/0, to do the battery cables...

Now, I'm on a learning curve for batteries.... Brand, voltage, AH size?

Should the 48v bank be:
Four 12v, or six 8v, or eight 6v?

Best Prices and Places??

What are you using for your new battery bank?

Thanks, Bill

[Mike90250]

Batteries, you need to know your loads, and then size your system accordingly. Generally, you only want to deplete your batteries about 15% on a daily basis. Then you size your solar to be 5-10% of your battery bank size.
I'll be using L-16 size, 6V batteries ( 395AH) for my power, 8 in series. If I needed more amps, I'd use 4V or even 2V L-16 case batteries. I really don't like batteries in parallel, and prefer to keep just one series string, and cases that I can pick up. (no 1200lb forklift battery - I've got no forklift)

[WildcatSolar]

QTY DESCRIPTION
1 XW System Control
1 Xantrex XW60 MPPT 60 Charge Controller
1 Xantrex XW4548 120/240-60 Pure Sine Inverter 48v
16 Trojan T105 6V 225AH (Tax and Shipping Included)
1 Set 10

[Sunking]

I have not read all the replies or ran any numbers but you need to be careful because I detect the numbers might be off because I do not see any allowance for AC power factor.

If you will use 3 Kwh per day, your panels have to generate a minimum of of 4.5 Kwh with UNITY PF hours per day in December and January and 5.7 Kwh with an average PF of .8

Minimum recommended battery capacity is 4.5 Kwh x 5 days = 22.5 Kwh at UNITY PF and 28 KWH at PF .8 Just be aware Trojan batteries quality has fallen off the cliff with cost cutting measures in production and they are trying to find a buyer. I agree with Mike you should be looking for a battery with the Ah capacity to make a single series string. At 22.5 Kwh @ 24 volts you are looking for a battery with a 950 AH rating. A very good candidate would be a Surrette 6 CS 21P. The Surrette is a true 10 year battery (4000 cycles when discharged 20% DOD and 5000 cycles @ 10% DOD), unlike the Trojan that will have to be replaced every 2 to 4 years.

Last comment is I notice you have selected Evergreen solar panels. Be aware Evergreen Solar is in very bad financial shape and I will be very surprised if they are still in biz by the end of the year. Something to keep in mind for warranty replacements and future expansions. Not likely to be around and available if you need them.

Just my two-cents worth.

[WildcatSolar]

Sunking, I have been convinced to go with a 48v system and not the 24 as I first started. The 3kw system would be understood to be used at 2kw or less during the winter months. I have a way to do this. In SC, even in the winter, I would average 8x210x3.3=5544wh of energy max. If one factors in losses, ineficiencies, etc., that should still work. In the summer with 6hrs plus, I would have plenty and would be able to do a little more. This is a minimal backup system that I would have to manage accordingly. At the worst case for a week or more, it would have to provide about 1kw a day for my minimal needs. The Evergreen corp may be in trouble. I do not doubt that. Plenty of companies are going through strains. GM even failed. Many will die or be absorbed over a 25 year warranty period. I would be pushing 70 then. By then, new technologies will have emerged and I would replaced them with something much more efficient. The 16 t105 Trojan batteries would give me 450ah at 48 volts. Two parrallel strings I admit, but plenty of people do it and I can get them for $121 a piece. That is the cheapest cost per AH I have found. Trojan may go under too, but I doubt it. They may be absorbed by another, but they won't fade away to nothing or at least probably not. Who knows these days. The Surrettes would be better. Agreed. If I do want to use a XW6048 inverter rather than a XW4548 as Mike recommended and I can do it for the same price, is there a downside if I may increase capacity later?

[Mike90250]

for the price, I see no downside to a 6048 vs 4548. It's mostly a software tweak internally, and a handfull more FET's installed, likely on a universal PCB.
You might look to see if Trojan has 2V or 4V batteries, to make your string a single string, but it's not a big deal to have 2, be sure you have a fuse or breaker one EACH battery string.