Newbie battery question

stoneunhenged · June 2009

Hey, guys.

I'm in the process of designing a PV system for my Florida home. On the average I use about 500 kWh per month. I'm looking at roof-mounting about two dozen 210W panels, tying them into a battery bank, then pumping into the grid any surplus I don't use in the house. Cost, while a consideration, is not a determining factor. Reliability, performance, and low maintenance are all more important considerations.

Here's what I'm wrestling with now: Given the system demands, what is the best battery configuration for this system? What battery system would provide the most maintenance-free and reliable storage for a system this size?

Thanks for your help.

Oh, you should also know that I am a fan of NA Wind and Solar. They helped me design and build a great solar-powered water well.

icarus · June 2009

Re: Newbie battery question

I'll write more later,,, but the best battery configuration for this system is none!

Adding batteries to a grid tie system basically doubles the cost,, you are way further ahead to double the number of panels,, grid tie,, sell excess, buy when needed,, and use a generator for emergencies.

Tony

stoneunhenged · June 2009

Re: Newbie battery question

Call me crazy, but having sat on my sweaty butt for two weeks waiting the for the lights to come on after a hurricane --more than once-- I'm willing to spend the money on a battery system.

Incidentally, I have a big propane fired generator. Nice in pinch, but loud as heck.

TnAndy · June 2009

Re: Newbie battery question

24-210watt panels will be a NICE system, for sure.....and I don't think you're crazy at all. Personally, I think a system like this makes a WHOLE lot more sense than a whole house backup generator that takes nothing off your power bill, and just sits there depreciating, requiring regular exercise, and consuming fuel when you do use it.

Batteries come down to cost and how much you want to maintain them. Regular ole wet cells are the cheapest, but you have to do some maintenance. The good thing is with them primarily being for backup, you won't be charging/discharging them much until that hurricane hits, and the rest of the time, they will just trickle charge to keep them topped off. I have a similar system ( 3.1kw in size ) and find about once every 6 months, I need to add a little water.

But if you don't want to do that and spend a bit more, the AGM sealed batteries are another way to go.

icarus · June 2009

Re: Newbie battery question

Please don't take this the wrong way,,, but there are extensive conservations (threads) on this forum on this very subject. Using batteries for backup, (especially when you have a genny already) adds a huge expense to your solar system.

In my opinion,, let's just say that to buy a battery system to power the normal loads that your panels would provide, will cost about twice as much as the panels themselves. They also have a limited life span depending on how they are used. So for example, a 2 kw pure grid tie system could be installed for ~$15-20,000 before rebates. (rebates and tax credits reducing that cost considerably in many cases) The same 2kw system,, grid tied,, with battery a battery back up of similar capacity might add another $15k. This for a battery system that has an average life expectancy of ~5-10 years,, a battery system that will drop overall system efficiency from ~95% to ~50%,, making the effective system even more expensive on a per net AC watt basis.

This for a system that in the worst case is likely to be needed less than 2 weeks per year,, and likely less.

For the same $15,000, I would buy some propane for my generator,, plus a sound enclosure. Then I would buy another, 1.5kw of panels, so that at the end of the day I would be generating a much greater portion of my power from PV.

I suggest that you read some of the following links to learn a bit more about batteries and battery technology.

http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries
http://www.batteryfaq.org/

PS To TNAndy,

The reality is that gensets are very cheap on a KWH basis relative to battery power. In fact if quality is purchased they don't depreciate much,, and the newest generation of Inverter generators are VERY fuel efficient. $5000 spent on a good quality generator, will in 10 years still have some significant value. The same $5000 spend on batteries will be worth scrap metal value in 10 years. In addition you, can indeed use the electricity generated during the exercise cycle if you plan carefully. I have wired stand by gennies to solar water heater tanks,, to heat water during the cycle,,, or to pump large quantities of water to a storage tank once a week to be used during the week. In my case I use the gennies to run the shop,, so I do in one sense,, kill two birds with one stone.

As an aside note,, I have a 1948 Onan 3 kw gasoline genset still in use today,,, and is in fact sought after as a reliable fuel efficient (relatively) quiet rig. I also have 1950's vintage Lister 5 kw diesel that is very fuel efficient. Both these units were in daily service until I began to solarize our remote site about 10 years ago. They both still start on the first crank!.

stoneunhenged · June 2009

Re: Newbie battery question

TnAndy wrote: »

24-210watt panels will be a NICE system, for sure.....and I don't think you're crazy at all. Personally, I think a system like this makes a WHOLE lot more sense than a whole house backup generator that takes nothing off your power bill, and just sits there depreciating, requiring regular exercise, and consuming fuel when you do use it.

This is my thinking exactly. Like I said, cost isn't the driver in this decision. I spend almost a thousand dollars a month paying the local utility for electricity. Scrapping a $5,000 battery system every ten years isn't a deal killer. And, I would use it more often than two weeks a year. I would use it every night when the sun goes down.

So, please take a shot at describing your fantasy battery system. Any additional advice would be sincerely appreciated.

niel · June 2009

Re: Newbie battery question

tony is also forgetting that this backups system with pv can go towards some of your normal power needs and can even be sold to the grid as some of the inverters allow for this. in this manner you won't have everything just sitting there waiting for the outage to happen as it will go towards your regular electric needs too and is called a battery backed grid tied system. the system you propose is quite large and are you sure you need that many? i think a 3-4kwh might be better and that still accounts for all of your normal electric usages. 500kwh/30days=16.7kwh per day. 16.7kwh/5hrs full sun per day = 3.4kw system. you can go up slightly more for losses, but i'm sure this would be heaven even if halved during an outage with some conservation being done.
don't forget that pvs make good sails in hurricane force winds and that flying debri hitting the pvs can also make for a dissaster on your survival plans.

stoneunhenged · June 2009

Re: Newbie battery question

Good points, including the one about the panels acting as impromptu wings in hurricane-force winds. We live about 40 miles inland and the max winds we've experienced are about 50 mph. The area where these panels would be mounted is actually fairly sheltered from buffeting winds. But, we live in a heavily-treed part of the world and it is very common for us to lose power even in a heavy thunderstorm when trees clip the power lines. And, frankly, I just like the idea of being energy independent.

icarus · June 2009

Re: Newbie battery question

Perhaps I am not being clear in my opinion.

I am not suggesting that having a battery back up system is silly. What I am suggesting that a whole back up system for the rare event of power outages can indeed be had for a lot less money,,, making any PV system more efficient.

What Stoneunhenged MAY be missing is the concept of net metering. (Help me out here Bill!) With net metering,, you sell your excess power back to the grid at a prescribed rate during the day, usually when your personal loads are likely to be the smallest and your generating capacity is the highest. You then buy back power during hours of no sun from the utility. At the end of the day/week/month/year depending on how the utility is set up your final bill is the difference between what you sold and what you bought. In some utilities with time of day metering,, you can sell power at the high peak rate, mid afternoon, and buy it back at night at the low demand rate. If you plan your usage so that your high draw items, such as water pumping, dryer use etc is done at the low rate hours,, your net bill can be less than zero EVEN if you buy more power than you sell.

My argument against using a whole house battery back up system,, powering your house off the battery bank at night is a very inefficient way to do it. Right off the bat, you lose a significant percentage of your Pv potential just for system losses that you don't have with a battery. Assuming similar inverter efficiencies,, the greater losses come from charge controllers that might have ~5-15% loss depending on circumstance. Couple that with the basic Perkett effect of battery charging of ~20% (That is,, it takes about 120 amp hours of charge power to replace ~100 ah of draw on a battery) add in greater wiring losses because you are running lower voltages, and you are left with a system efficiency that is at least 25% less than that of a conventional grid tie system. (Maybe more)

So by all means build yourself a battery based UPS. Even build an of grid house with grid back up,, but understand the limitations.

As an addition to a previous post,, rebates and tax credits for battery based systems may not be as lucrative as those that are grid tie.

Tony

stoneunhenged · June 2009

Re: Newbie battery question

Tony, I understand your points and they're well taken. I'm aware of net metering, but again, economics aren't driving my decision here. You seem to know a lot about these systems. If you had an unlimited budget for batteries and related equipment, what would you build?

icarus · June 2009

Re: Newbie battery question

In all honesty the only batteries I have used are Trojan l-16 and t-105. I don't have a lot of experience buying anything where money was no object. I hear good things about Rolls Surrettes, but I will let others who have more experience with large battery based systems. There are some folks here who have some very large off grid systems that use fork lift batteries.

Good luck,

Tony

BB. · June 2009

Re: Newbie battery question

Much in the way of solar RE comes down to personal needs and wants...

So--a good start for what you want is a Xantrex XW (or equivalent) system.

The XW is a "hybrid" system. It supports Grid Tie and Off-Grid operations (with solar, wind, genset as alternative energy sources).

Normally, for an off grid system, we use the 6x rule of thumb (3x for three days of no sun, and 50% maximum level of discharge).

For an "emergency" system like yours... You can probably go with the minimum recommended battery configuration.

So:

1. Decide how much solar panels you want (typically, for Net Metering, max is ~your to supply average daily load--net zero daily usage)
2. Build out a Xantrex XW system (with as many inverters as you need in 6kW chunks).
3. Battery for your minimum required daily load * number of days of backup power desired without genset * 1/50% max battery discharge.

For the XW it is roughly 400 Amp*Hours minimum for an XW 6,000 watt hybrid inverter system (recommendation per Solar Guppy).

A 400 AH * 48 Volt * 50% max discharge = 9,600 Watt*Hours of useful energy.

500 kW * 1,000 W/kW * 1/30 days per month = 16,700 Watt*Hours per day

9,600 WH of storage / 16,700 WH per day = 0.58 days = 13.8 hours

So, a minimum batteried XW system (6kW inverter) would supply a little ove 1/2 your normal daily load.

You can decide if that is enough, or more than you need (you shut down all except a few essencials--fridge, lights, radio/tv, only use a little A/C, etc.).

So:

1. Decide how much solar panels you want (typically, for Net Metering, max is ~your to supply average daily load--net zero daily usage)
2. Build out a Xantrex XW system (with as many inverters as you need in 6kW chunks).
3. Battery for your minimum required daily load * number of days of backup power desired without genset * 1/50% max battery discharge.

You either build a system that meets your daily average need (based on season when you are most likely to loose power). Or build out a system that meets your minimum power needs (emergency power) such that the solar panels will supply your minimum seasonal loads and only use the genset when clouds or other emergency need arises.

Either system would be well within what many people would make sense for their needs vs co$t$.

So, what are your emergency daily power needs (may be your normal power needs, you decide), where do you live (roughly), what season/months do you generally loose power in, and how much do you want to run the generator to reduce costs--if any (fewer solar panels, fewer batteries).

-Bill

PS: Add link for XW Hardware site, and to NAWS webstore for basic pricing information.

icarus · June 2009

Re: Newbie battery question

Bill says it so much better than I!

Tony

sub3marathonman · June 2009

Re: Newbie battery question

stoneunhenged wrote: »

This is my thinking exactly. Like I said, cost isn't the driver in this decision. I spend almost a thousand dollars a month paying the local utility for electricity.

Now that is what I don't understand. Previously stoneunhenged stated that he uses 500 kwh / month. Is it really 5000 kwh / month?

niel · June 2009

Re: Newbie battery question

you are quite right that it doesn't add up for a $1000 billing and if that is an accurate billing then some severe conservation efforts are warranted. even if he acquires a 2kw system for example, he could still reap some very good power to run a frig and some cfls. some standard white leds would make some good night lights for a minimal amount of power used. i suppose if one has tons of money and tons of room for the stuff needed that anything is possible, even to wipe out a $1000 bill, but that is bordering on ridiculous for i would feel quite guilty with that much electric power while others suffered.

Dapdan · June 2009

Re: Newbie battery question

hey stone,

If I had unlimited funds and wanted a good set of batteries I would buy the agms made by GNB. I would get the Absolyte range of cells. These cells are made with RE use in mind. As an example I got some old one (about 10yrs unused) from a telecom company and I have been able to improve their performance over the past 4months to the point where they perform like new batteries. They have a float service life of 20yrs. The cycle life will depend on your dod (depth of discharge). With my cycle usage according to manufacturers info I can expect 4000-5000 cycles. For my energy use profile I can expect at least 11yrs of service.

Cheers...
Damani

stoneunhenged · June 2009

Re: Newbie battery question

Thanks for taking the time to impart all the very helpful info. As I build the system I'll post updates so you can see how it's coming along.

We live on an organic farm and have slowly been converting to alternative energy. We have a windmill-powered water system and a solar well for those windless days. Here's a picture of the well. The panels drive a Simple Pump.

And windmill (with my funky photoshopped coloration):

colderthan · June 2009

Re: Newbie battery question

The ultimate battery? Check out this bad boy, the hup solar one.
1990 ah @ 48 v
6200 lbs
only 24 cells to maintain
10 year warranty
And only $18000

niel · June 2009

Re: Newbie battery question

as far as batteries go, there are advantages and disadvantages to both agm and standard lead acid batteries. you can see many posts on them here on the forum, but in a quick overview i can say standards are cheaper and agms are more efficient with no gassing if properly charged and shorter warranties than standards. of course it goes more in depth, but i see you already have a sunxtender so have you tried a standard to know how they might compare in your application? if using standards in large systems you must be sure to stop any accumulations of gas and vent the gas or risk fire or explosion.

rplarry · June 2009

Re: Newbie battery question

Stone
I have used several types of batteries FLA and AGM, and for me, and what I do, the AGMs are by far the best. Very efficient to recharge and absolutly maintenance free. I agree with Dapdan, the GNB absolytes are the best I have had.
HTH
Larry

stockoneder · June 2009

Re: Newbie battery question

I live in Florida and just put in a 4.3 kw hybrid system. I went with Xantrex xw 4548 and 20 sharp 216 watt panels. My battery bank is 4 12 volt trojan 8d AGM batteries wired in series. I am getting a Honda eu2000i generator soon. That and 25 gallons of gas should do nicely if the grid goes down and my batteries get low due to cloudy days.
Here is link to pics. http://righttoreason.blogspot.com/2009_03_01_archive.html
Waiting on my $17,200 state rebate and will take advantage of my $11k plus federal tax credit this year.

BB. · June 2009

Re: Newbie battery question

stockoneder,

Looks nice--agree that this is better investment than a $42k SUV.

One minor point... Watts and KWatts is a Rate (you are currently sending 2.48 kWatts out to the grid through your meter).

kWatt*Hours (kWH) is the actual (total) work... You have sold/produced 4.28 kWH out to the grid that day (so far)... kWH is what you pay the utility for ($0.10 per kWhr or whatever the price is).

-Bill

Cariboocoot · September 2011

Re: Newbie battery question

If you're going for battery back-up for emergency situations the first thing you do is determine and reduce your critical loads (those things that must be kept running in the event of a power outage) to the bare minimum. That way you don't have to spend so much money on equipment that most of the time will not be in use.

Building a battery-based back-up system that can handle the typical needs of a whole grid-connected house is possible, but it would be very expensive and quite impractical.

That's what icarus/Tony is getting at.

The Original Ralph · September 2011

Re: Newbie battery question

stoneunhenged - i'm still designing my system, and being electically illiterate, have been researching here on this forum as well as getting to speed on what's available, equipment wise and what works or doesn't.

I started out with the same goal as yours, but the more i researched, the more i have reduced my battery backup goal.

1st - i'm in central VA, in an area where if you sneeze and say thunderstorm right after it, the power seems to go out. I run a business out of my home, so having power is more crucial than a normal homeowner scenario. It was 1999 when i purch'd a 13.5KW tri-fuel gen, and just a few months later, hurricane isabell came thru and we lost power for 9.5 days - that gen drank 20-22 gallons gas a day.

BTW, as far as noise abatement on that generator, a simple solution is to get a six foot section of flex exhaust line from JC Whitney http://www.jcwhitney.com/flexible-exhaust-tubing/p2006251.jcwx?filterid=j1. Go with a size that's at least twice the ID of the gen's muffler exit opening to eliminate or diminish any back pressure issues. My 20 HP engine's muffler had a 1" ID exit port, so i went with 2.5" ID flex line. Have someone weld a short pc of exhaust tubing to the muffler over the exit opening, so you can slip the flex line over it when needed. That six foot length alone will surprise you at how much it'll quieten the exhaust note. then for the final touch - i run the end of the six foot flex line into the back end of a spare muffler i had - muffler came off a V8 exhaust system, real free flowing system. Where before you couldn't hear yourself think inside the house, now you can hold a normal level conversation standing 8-10 feet away and don't even hear it inside the house.

back to point - i understand the need for battery back up - even with a quiet gen, i'd rather conserve my fuel - in that 9.5 day power outage, i had to drive 20 miles to find a gas station that had power and was pumping gas - and wait in line to get it.

I've reduced my load or useage considerably, have a smaller gen (4800 cont. wattage Makita, and it burns about 3 gal per 7 hours, or 11-12 per day, without the A/C on.

my normal running load, without A/C is 900W - 1880W (when refrig comes on).

I'm still going to do a battery backup, but only four 6V batteries in series, which should give me 4.8 kWh useable (i think the accepted wisdom is to not discharge the batteries more than 50%, which means i can run the batteries for 12-16 hours without approaching that 50% charge level, then run the gen, which after the normal load of max 1880W, even the smaller generator will still have enough power to recharge the batteries.

what icarus suggested about propane powered gen isn't a bad idea, at least propane doesn't go bad in storage. and you don't have to drain the carb etc, because of the ethanol blended gas we're forced to use these days, here in the US.

One point in support of a battery backup goal is the fuel conservation it allows (whether propane, gas or diesel) it allows, and saving trips to find fuel in prolonged outages.

Another point in support of your battery backup goal, and what also makes it appealing to me, it gives me a chance to svc the gen while the batteries are carrying the system. I change the oil on my generator every 25-50 hours - at most, that's 2 days of operation. Without the battery backup, i'd be svcing the generator with the power off, and again, i run a business from my home, so that's fairly out of the question.

I only mention these points or considerations to balance the opinions that seem to run totally negative towards the idea of any battery backup, considerations that i don't think have been taken into acct in the negative opinions. Even on a reduced battery backup power system, it's expensive but that expense is worth it to me because of the above points, especially in prolonged (3 - 10 day) outages, when you factor in :

a) less generator fuel consumption which also means less time spent driving to find fuel
b) less time spent svcing the generator
c) a "backup" power source to the generator
d) in short duration outages ( 2- 8 hours) it means automatic backup power transition, with no need to drag out the generator, fuel it up & get it running
etc, and then the subsequent putting it into storage (draining tank & carb) after the outage.
e) while this last one isn't as important, it's still a valid convenience factor; the ability to service the generator without shutting down power to the house

Steven Lake · September 2011

Re: Newbie battery question

stoneunhenged: Ok, I'm obviously late to this discussion, and while I didn't see it, someone may have already mentioned this. As for solar powered off grid setups (yours is a bit different from mine, as you want grid tied with battery backup and I want completely off grid, but still similar enough to comment) my suggestion is this.

In one of your posts you mentioned that you pay old Ma Electric a grand a month for power. Given all the research and information I've come across in my research, that's a monstrous red flag. Even on a grid tied system. The first goal you should always shoot for is getting your usage down as low as you can absolutely go. I'm presently at 250kwh a month (that was my last bill) and working towards reducing everything to about 100kwh a month.

Now, given that you're in Florida, and on an organic farm, 100kwh might not be practical for you, or even achievable. (I live in farm country, and I know how much power one of those can eat) But, if you can get things down a bit further, it'll help make your solar farm considerably cheaper.

Newbie battery question

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