2.5kWh system sizing help in South Florida

tjfpb

Hello!  First time, long time..

I'm trying to design a system to provide my home with some backup power in the event of an outage from a storm or other disaster.  Can someone here check my numbers and see if they make sense?

I've done some initial measurements of electricity needs and I'm looking at about 2.5kWh/day.

Here's what I'm thinking:

Location: Fort Lauderdale, Florida
Solar array: Qty 8, 200W Sonali panels (12V panels) wired as 2 strings of 4 panels each.  (I can get these locally and avoid freight charges)
Battery bank: Qty 8, 6V/225Ah GC2 golf cart batteries wired in series for 48V. (Should allow for approx 2 days of backup)


Also, here are the constants I used in my calculations:

Inverter efficiency: 85%
Charger efficiency: 77%  (Note: I would use an MPPT charge controller -- is this number a bit low for that?)
Rate of charge: 9.5%
Solar hours: 3.3  (Note: I would need to put the panels facing east.  My southern exposure is shaded by a giant banyan tree)
Degree of battery discharge: 50%

Based on reading other threads and learning how to calculate this stuff, this should be sufficient.  Would anyone be willing to confirm?

Thanks!!

tf.

Photowhit

I'd first look at what you want to run 2.5 Kwh will likely not run more than your fridge and a few lights, TV or radio.

In a warm environment, it might not even run the that if you have some overcast days.

That said 8 - 6 volt 225ah batteries should have about 10.8 kwh of storage, so 2.5 kwh of daily use will give you a couple of over cast days of storage to @40% SOC (3 nights+ 2 days) I think you can expect @4 hours of sun on an average day in south Florida, but I would look at your 'micro climate' do you have clouds in general during the morning? Ask a meteorologist in your area, sea breeze effect will be somewhat seasonal. in the fall and early winter fog can roll in on western coasts. I don't know how it will effect an eastern coast town.

There are some calculators that take into account which direction your array is facing, I can't look for them now... something I hope our New soft ware has room for a FAQ section for this type of thing!

1600 watt array would be under paneled for daily use, but not bad as an emergency back up. I suspect I would move rather than live in south Florida without AC!

Dave Angelini

Facing east you really are making this a challenge and the best way to succeed is 2 times the solar. In that situation you are in, I would just buy a good Honda inverter generator convert it to propane/natural gas and plug it in to the electric dryer receptacle. Turn off the main and run the iron beast.

tjfpb

I'd actually considered the whole generator route.. more on that in a moment.

According to PVWatts, I should get a minimum of 3 hours of sun on panels facing east at my location, and assuming 50% efficiency of a 1600 watt array, I should be able to generate 2.4kwh, but I could always add another panel or two.

That being said, let's assume I don't install a PV array at all as suggested, and instead maintain a bank of batteries with a charger/inverter like the Outback VFX3648.  I could run the AC input via a transfer switch that would allow me to keep the batteries topped off when the grid is available, or in the event of an extended outage I could flip the switch and use my 8500/6500W generator to charge them up.  Furthermore, if my situation changes in the future (tree comes down, solar efficiency improves, whatever) I could always add a solar charge controller to maintain the batteries while maintaining the ability to charge via grid or generator power fairly easily.

Is this reasonable?

Dave Angelini

I do not think this is reasonable. Batteries that are not being used for anything always fail when the tropical storm is 90 miles out and at night !
This issue you have has been covered a zillion times. You can make this work any way you want as long as you put the maintenance and testing in. A gasoline generator is just down the list from battery failure. A propane/Natural generator is the way to go for the ultimate reliability. It all depends on you !  I have a friend nearby and he has a small runway with a plane in the garage.... Good Luck!

Photowhit

tjfpb said:

According to PVWatts, I should get a minimum of 3 hours of sun on panels facing east at my location,

No site is going to tell you this!

They may suggest you will average 3 hours a day...

Photowhit

I would first look at your goals, I think you want to run a fridge for a couple days when the power's out?

...but perhaps it's to run everything at night so you can shut down your generator?

tjfpb

Photowhit said:

No site is going to tell you this!

They may suggest you will average 3 hours a day...

Hello, Photowhit!  With all due respect, the PVWatts site does indeed tell me this.  See attached screen shot.  In my location -- with an array facing east -- will give me a monthly minimum of 3.04 hours in the month of December and I should see more throughout the rest of the year.  Obviously, this is an average for the month itself but a minimum for the year's worth of monthly averages -- or is this what you were getting at?

Per your next post, let's talk about goals.  First some context.  I'm a tinkerer/experimenter/hobbyist.  I live in an area with a pretty stable grid and affordable rates.  I'm not living in an off-grid cabin or anything like that (as cool as that would be, I'm not at that point of my life yet.)  That all being said, here's what I'd say my goals are:

1) In the event of an outage, some quiet power in the evenings for a few LED lights, a TV and my ham radio equipment would be nice.  I don't want to run my generator all night long (or at all, if my PV array can meet the needs)
2) Even when the grid is active, I would like to take some things 'off-grid' and power them with solar -- landscape lighting, maybe run my ham radio gear and laptop via solar full-time, etc.

Does it make sense, financially?  Probably Absolutely not.  As I said, the grid here is stable and cheaper than many other places in the country.  In the spirit of growing and trying new stuff, however, I think it'd be something fun to play with and a fun learning experience to boot.

My thoughts were I could get a 10kWh battery bank setup (as I've read it's difficult/impossible to add batteries later), and using the grid combined with a couple panels to keep it topped off.  That should allow me to add panels as time goes on and grow my system, I think.

Thanks again for your reply and feedback!

BB.

Just to clarify--Those are average daily hours of sun... When you have bad weather (depending on your local weather patterns), you can go a few day of very dark weather (less than 5% of the arrays capabilities) to weeks of cloud cover (a bit better production, but still not the near the estimated average sun/electric power projections).

So--The suggestion of a backup genset is still highly recommended if you need "reliable" power during all weather conditions... Solar does not automatically substitute for a backup genset--Just reduces the hours of run-time (and fuel usage).

Where are you planning on installing this system? What are the typical weather patterns (how many days in stretch of cloudy weather)?

-Bill

tjfpb

Hi, Bill, I was hoping you would comment on this thread!  If you take a look at the first post, you'll see all the information you're looking for; I am located in Fort Lauderdale, Florida and have good easterly exposure -- and some southern exposure, depending on how high up my roof I were to mount panels.

I based my calculations off the formulas I've seen you use in many many posts on this forum.  There has not been any debate over whether I'd need a genset or other means -- I was really looking for someone to check my figures in the first post.  Dave and Photowhit have been kind enough to offer their opinions already but I'm curious to hear your thoughts as well.

Thanks!

zoneblue

I think by minimum you meant the minimum month, but as bill said 3kWh/d is the average. Ready enough confusion.

Have you considered a plan that involves a genset for your backup and a smaller system for experimenting? Say 2 or 4 GC2s, a midnite brat or kid. As many panels to suit? Youll get the learning without sinking a whole bunch of money down a hole. 

mike95490

East facing panels in Florida are good, They harvest when temps are cool, and before clouds build up.
the problem is storms.  Airborne debris that takes out the powerlines, is also likely to take out your Panels.

Photowhit

tjfpb said:

Inverter efficiency: 85%
Charger efficiency: 77%  (Note: I would use an MPPT charge controller -- is this number a bit low for that?)
Rate of charge: 9.5%
Solar hours: 3.3  (Note: I would need to put the panels facing east.  My southern exposure is shaded by a giant banyan tree)

tf.

Yes, inexpensive inverters will run about 85% efficiency.

MPPT charge controllers will run 95% + efficiency. Solar panels only produce about 75% of panel rating at Normal Operating Cell Temperature. (NOCT)

Rate of charge 9.5% is about right max.

The 'local' seller is a national concern, who often does things very right, but the '12V' 200 watt Solali panels are really 'grid tie' type panels with 54 cells They will have a VMP of around 26 volts, too high to be considered 12 volt nominal panels.

http://sunelec.com/datasheet-library/download/SS-120-200-Series.pdf

BB.

Sorry, I missed the information in the first post (has been a bit of a tough few months). Anyway...

And, yes, by the time you derate "hot panels" and a 95% eff MPPT controller--77% is a good (conservative) efficiency number).

8 * 225 AH * 6 volts batteries * 0.85 inverter eff * 1/2 days storage * 0.50 maximum discharge = 2,295 Watt*Hours

Ok, you where looking at 2.5 kWatt*Hours per day... Probably "close enough" for solar (assuming most loads are at night and/or carry through 2 very dark days).

And the two calculations for solar array size--First based on size of battery bank and second based on hours of sun per day...

• 225 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 862 Watt array minimum
  
• 225 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,724 Watt array nominal
  
• 225 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,241 Watt array "cost effective" maximum
  

So, you 1,600 Watt array looks pretty good there.

And working backward from your 2.5 kWH per day load (assuming worst case charge daytime and loads at night/dark weather):

• 2,500 WH per day * 1/0.52 end to end system eff * 1/1,600 Watt array = 3.0 hours of sun minimum (break even)

So, my first question would be is your 2.5 kWH per day a minimum needed--Or can you run a bit less in winter. In general, I would suggest that your "minimum base load" be ~65% to 75% of your "expected" solar output. Cut back on cloudy days, use more on sunny days (water pumping when sunny, even refrigerators use a bit less power on cloudy/cooler days).

Of course, solar arrays can be pretty easily damaged in heavy wind storms from flying debris.

From a solar point of view, you may wish to think about (either larger array, or expect more hours of sun in summer/fall):

• 1,600 Watt array * 1/0.75 base load derating = 2,133 Watt array
• 3.0 minimum (for 2.5kWH energy) / 0.75 base load derating = 4.0 minimum hours of sun per day (1,600 watt array)
  

That will give you some more room in December... Although, hurricane season ends around November 30th for you (I believe)--So, perhaps being a little "light" on solar panels in December is not a big issue for a backup system.

Anyway--Looks like you did a very good job calculating your base system requirement...

When you start picking equipment... Things like matching solar array to MPPT charge controller is critical... The correct number of panels in series (Vmp/Voc of panels vs Vinput max and Vbatt charging)... For example, here is a link to the Midnite calculator:

http://www.solar-electric.com/inverters-controllers-accessories/chco/misoclchco/midnite-solar-classic-sizing-tool.html

4 panels in series will work OK for Fort Lauderdale (not too cold, where Voc-cold of 4 panels will exceed controller Vinput-max rating)... For other controllers, these panels may be an issue (Voc-cold for 4 panels in series is ~141 aran8F -- Maybe an issue for some brands/models of MPPT charge controllers.

-Bill

BB.

I should also ask what size of AC inverter you are looking at... With that battery bank, ~2.25 kWatt AC inverter would be about the recommended maximum (rough rule of thumb is 100 AH @ 48 volts per 1 kWatt of AC inverter output). If you need to run a typical well pump, may be a bit on the small side.

-Bill

tjfpb

Thanks for all of the advice and feedback, guys.  I do have a generator for my backup, but wanted to play around with some solar.  I'll probably take the approach recommended by zoneblue to try and get my 'feet wet' without spending a ton of money.  I appreciate you all taking the time to reply and try to educate me.  I'm sure I'll see you all around the forum as I will undoubtedly have more questions, I'm sure!