# Battery Sizing Question .

Registered Users Posts: 9
Good Day everyone,

Came across this forum and i like the way topics and suggestions are done here.
I was hoping I could benefit from those who are knowledgeable on renewable energy technology here.
I am a newbie here and I have a simple question.

I need some advice on sizing a battery for a solar powered LED pole lighting system .
The LED light is rated at 210W, 24V DC. The solar panel is 250 Watts, 24VDC, and I would like to find out what size 24 V battery I need
[in terms of Ahs] to power that lamp for 12 hours and have a backup power capacity for at least two rainy days.

I live in the Caribbean, so there' lot of sunshine all year round.

Welcome to the forum SW!

210 Watts of LED lighting is a lot of energy--Assuming you have done as much conservation as possible... For a full time off grid system, assuming a minimum of 5 hours of sunlight per day (you can use this site to get your seasonal variations of sun for a more accurate answer):

Battery size, assuming 2 days of backup power and 50% maximum discharge:
• 210 Watts * 12 hours usage * 2 days storage * 1/0.50 maximum discharge * 1/24 volt battery bank = 420 AH @ 24 volt battery bank
And you have two calculations you need to make for your solar array--One is based on the size of your battery bank (larger battery banks need more solar panels to charge completely). 5% to 13% rate of charge is the typical rule of thumb:
• 420 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 791 Watt array minimum (weekend/seasonal use)
• 420 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,582 Watt array nominal (daily use)
• 420 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 2,056 Watt array "typical cost effective maximum"
And then there is based on the amount of sun per day you get--By season (sun position, weather conditions, etc.):
• 210 Watts * 12 hours * 1/0.61 end to end system efficiency * 1/5.0 hours of sun per day minimum average = 826 Watt solar array minimum
That is the basics.

Notice that a single 250 Watt solar panel is not enough. Also look at the exact ratings of the 250 Watt solar panel. Many "24 volt" solar panels have Vmp (voltage maximum power) around 30 Volts--in hot weather, that can fall to as low as ~24 VDC--Not near enough voltage to recharge a lead acid battery bank (need 29-30 Volts minimum). A true "24 volt panel" has Vmp in the range of 35-40 volts (for use with a PWM type charge controller). For higher voltage panels/arrays, you can have higher Vmp-array voltages (as high as 100 VDC standard test conditions) with a high end MPPT type charge controllers--But those are not cheap.

Questions?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 9
BB. wrote: »
Welcome to the forum SW!

210 Watts of LED lighting is a lot of energy--Assuming you have done as much conservation as possible... For a full time off grid system, assuming a minimum of 5 hours of sunlight per day (you can use this site to get your seasonal variations of sun for a more accurate answer):

Battery size, assuming 2 days of backup power and 50% maximum discharge:
• 210 Watts * 12 hours usage * 2 days storage * 1/0.50 maximum discharge * 1/24 volt battery bank = 420 AH @ 24 volt battery bank
And you have two calculations you need to make for your solar array--One is based on the size of your battery bank (larger battery banks need more solar panels to charge completely). 5% to 13% rate of charge is the typical rule of thumb:
• 420 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 791 Watt array minimum (weekend/seasonal use)
• 420 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,582 Watt array nominal (daily use)
• 420 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 2,056 Watt array "typical cost effective maximum"
And then there is based on the amount of sun per day you get--By season (sun position, weather conditions, etc.):
• 210 Watts * 12 hours * 1/0.61 end to end system efficiency * 1/5.0 hours of sun per day minimum average = 826 Watt solar array minimum
That is the basics.

Notice that a single 250 Watt solar panel is not enough. Also look at the exact ratings of the 250 Watt solar panel. Many "24 volt" solar panels have Vmp (voltage maximum power) around 30 Volts--in hot weather, that can fall to as low as ~24 VDC--Not near enough voltage to recharge a lead acid battery bank (need 29-30 Volts minimum). A true "24 volt panel" has Vmp in the range of 35-40 volts (for use with a PWM type charge controller). For higher voltage panels/arrays, you can have higher Vmp-array voltages (as high as 100 VDC standard test conditions) with a high end MPPT tpe charge controllers--But those are not cheap.

Questions?

-Bill

Thank you very much for the advice Bill !

And thanks too, for the link to the solar Irradiance calculator page, it is a very useful page, and I understood it.
I also understand what you've said, so maybe I should choose a smaller wattage LED lighting unit.
Maybe a 110 Watt LED unit might be more practical for the pole lighting application.

Interesting stuff about the voltage ratings on the panels you've mentioned there.I will have to consider it next time i'm purchasing.

With respect to the charge controller, I was thinking of purchasing a simple, cheap unit.
I've heard it said that the MPPT type is better than the PWM type, but I guess it depends on the application.
Are you suggesting that I should go with the PWM type for this particular project?

It depends... PWM with "12 volt" or "24 volt" panels are good for smaller systems (cheap controller+more expensive panels). For larger systems (typically 400 to 800 watts or larger, many times a more expensive MPPT controller + "cheap" Vmp~30 volt panels are less expensive.

You should really figure out the size of the system first, then select components to make the system work.

Also--Are you sure about the LED lighting is 210 or 110 Watts? It almost sounds like you are talking about 110 VAC or 220 VAC lighting voltage.

Are you looking at street/security lighting, or what? Do you have a link to the lamps you are thinking of using?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 252 ✭✭
Not sure what you want the lighting for, but an electrical engineer friend of mine recommended this for me: http://www.samsclub.com/sams/security-light-outdoor/prod12250342.ip to go on the homestead. The intent is to keep The Sasquatch out of the chicken coop. ;-)

• Registered Users Posts: 9
BB. wrote: »
It depends... PWM with "12 volt" or "24 volt" panels are good for smaller systems (cheap controller+more expensive panels). For larger systems (typically 400 to 800 watts or larger, many times a more expensive MPPT controller + "cheap" Vmp~30 volt panels are less expensive.

You should really figure out the size of the system first, then select components to make the system work.

Also--Are you sure about the LED lighting is 210 or 110 Watts? It almost sounds like you are talking about 110 VAC or 220 VAC lighting voltage.

Are you looking at street/security lighting, or what? Do you have a link to the lamps you are thinking of using?

-Bill

Hi,

Sorry about he long delay in my response to your post.Lots of projects going on.

Yes, the LED lighting fixture I'm thinking of using is 110 Watts, not volts.
And to answer your other question, the project is a type o street lighting. Actually, the lighting of a private driveway, off grid location.
I also have a pic of the LED lamp. Let me see if I can locate it and upload.

Thanks very much for your help so far!

• Registered Users Posts: 9
Not sure what you want the lighting for, but an electrical engineer friend of mine recommended this for me: http://www.samsclub.com/sams/security-light-outdoor/prod12250342.ip to go on the homestead. The intent is to keep The Sasquatch out of the chicken coop. ;-)

Do you have a link for that light? I would be surprised if it is 100's of Watts.

Anyway, are you looking for it to run ~10 hours or so per night, or would you be OK with using motion detectors to turn on the light when needed?

A 100+ Watt LED light is usually large for an off grid security application. If you do need that amount of light, I would highly suggest a motion detector based system. It will save you a tremendous amount of electrical power--Plus, I am a big believer that a bright light that turns on when there is motion is a bigger deterrent than a security light that is just on over night (bright light turning on attracts attention, a light on all night just provides lighting to attract attention to what you are trying to protect).

Or you can try a mix--A few 5 watt fixtures on over night, and a large light with motion detection.

Very roughly, off grid solar power costs something like 5-10x the cost of utility power--You really want to look at conservation when deciding on your loads.

As an option, you could try installing the lights and using a small genset to power them. Once you are happy with the layout/type/amount of lighting, you can then size the off grid solar system to power those lights (getting a few different LED fixtures is much cheaper to experiment with vs building out a "worse case" solar power system to power an unknown set of loads).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,509 ✭✭✭✭
Thats a lot. I wonder how many realize this before taking the plunge?

I could see a board dedicated to making solar more affordable. The power companies want your first born for installing power poles. That used to be much cheaper I believe.
BB. wrote: »

Very roughly, off grid solar power costs something like 5-10x the cost of utility power--You really want to look at conservation when deciding on your loads.

-Bill

First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
softdown wrote: »
I could see a board dedicated to making solar more affordable. The power companies want your first born for installing power poles. That used to be much cheaper I believe.

I hope we are that board dedicated to making off grid solar more affordable. :-)

And alerting people that they need to plan out their loads and systems before they buy anything.

Back in the Rural Electrification Days, the government did subusidies and required utilities to subsidize running of power lines. It was a chicken and egg situation--Very few customers (and utilities) could afford to bring lines to the middle of nowhere... Which kept prices high. If the subsidies worked, then you would have enough customers to justify the electrification. [h=3]Rural Electrification - New Deal Network[/h]
Although nearly 90 percent of urban dwellers had electricity by the 1930s, only ten percent of rural dwellers did. Private utility companies, who supplied electric power to most of the nation's consumers, argued that it was too expensive to string electric lines to isolated rural farmsteads. Anyway, they said, most farmers, were too poor to be able to afford electricity. The Roosevelt Administration believed that if private enterprise could not supply electric power to the people, then it was the duty of the government to do so. Most of the court cases involving TVA during the 1930s concerned the government's involvement in the public utilities industry.
In 1935 the Rural Electric Administration (REA) was created to bring electricity to rural areas like the Tennessee Valley. In his 1935 article "Electrifying the Countryside," Morris Cooke, the head of the REA, stated that
In addition to paying for the energy he used, the farmer was expected to advance to the power company most or all of the costs of construction. Since utility company ideas as to what constituted sound rural lines have been rather fancy, such costs were prohibitive for most farmers. [ footnote]
....

We are sort of reversing those policies now (for better or for worse).... And who knows what will be "sustainable" in the future.

Anyway--Conservation is the best/first place to put your money. If this is a big project, modeling/buying a few lights to test/borrow a genset/use a Kill-a-Watt type meter to measure the power/etc. are all good places to start.

We probably spend as much time here (or possibly more) on conservation/loads as on actual solar power design questions. And everyone comes here first talking/asking about the details of their solar power system design.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 245 ✭✭✭
BB. wrote: »
Welcome to the forum SW!

210 Watts of LED lighting is a lot of energy--Assuming you have done as much conservation as possible... For a full time off grid system, assuming a minimum of 5 hours of sunlight per day (you can use this site to get your seasonal variations of sun for a more accurate answer):

Battery size, assuming 2 days of backup power and 50% maximum discharge:
• 210 Watts * 12 hours usage * 2 days storage * 1/0.50 maximum discharge * 1/24 volt battery bank = 420 AH @ 24 volt battery bank

And you have two calculations you need to make for your solar array--One is based on the size of your battery bank (larger battery banks need more solar panels to charge completely). 5% to 13% rate of charge is the typical rule of thumb:
• 420 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 791 Watt array minimum (weekend/seasonal use)
• 420 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,582 Watt array nominal (daily use)
• 420 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 2,056 Watt array "typical cost effective maximum"

And then there is based on the amount of sun per day you get--By season (sun position, weather conditions, etc.):
• 210 Watts * 12 hours * 1/0.61 end to end system efficiency * 1/5.0 hours of sun per day minimum average = 826 Watt solar array minimum

That is the basics.

Notice that a single 250 Watt solar panel is not enough. Also look at the exact ratings of the 250 Watt solar panel. Many "24 volt" solar panels have Vmp (voltage maximum power) around 30 Volts--in hot weather, that can fall to as low as ~24 VDC--Not near enough voltage to recharge a lead acid battery bank (need 29-30 Volts minimum). A true "24 volt panel" has Vmp in the range of 35-40 volts (for use with a PWM type charge controller). For higher voltage panels/arrays, you can have higher Vmp-array voltages (as high as 100 VDC standard test conditions) with a high end MPPT tpe charge controllers--But those are not cheap.

Questions?

-Bill

I really like your explanations. It is easy to understand and learn about it.
Thank you for the kind words Johann... We try very much to discuss solar power as a "system" and design all of the elements such that the design (and loads) are "balanced"... No large battery bank with small solar array, or giant generator running a small AC battery charger, etc.

It does, at times, become difficult to try and relate the individual design rules vs the overall design--It can be vary confusing. The rule of thumbs based designs I usually do here keeps the process simple and (usually) gives a cost effective/workable/reliable solar power system.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 9
Thanks so far guys.
I have another question, dunno if I'm allowed to ask it in this thread or if I should start another thread.. Anyway this is for the same project.

I am looking for a good 5 Kw VAWT. [ Vertical Axis Wind Turbine ], can you suggest a good make?
I've seen older forums where some guys were saying that the Chinese Turbines aren't good and so forth. But I am getting what appears to be a god package from a Chinese company costing US\$11,000.00.
It's part of a solar wind hybrid system I'm installing.

What are your thoughts on the Chinese VAWT, and what are some of the top brands in the market?
I'm pretty new, so I'm seeking information.

BB, to answer your last question, I can't remember the link to the LED light I was asking the question of, but I got it on a B2B forum called "Global Sources." I purchase lot from China so it is a Chinese company called Beijung Solarone Energy.

This is their website: http://www.xnsolar.com/en/proinfo.aspx?nid=758
• Registered Users Posts: 9
I was trying to upload the file with the specs of the VAWT, but something's wrong with the file conversion..So I'll have to post the specs a little ;later.
• Solar Expert Posts: 5,183 ✭✭✭✭
I dont know if you have been reading on this forum, but do a search for user Keyturbocars he has an HY turbine and seems happy with it

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
VAWT. I would run away. Personally, I believe they do not reliably generate useful amounts of power (general statement).

They do not offer any advantages over HAWT systems, and as most companies install them, they simply are to close to the ground to get good wind.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 9
westbranch wrote: »
I dont know if you have been reading on this forum, but do a search for user Keyturbocars he has an HY turbine and seems happy with it

Thank you for this info.
I searched and got the thread, [ a pretty interesting thread! lol!], but I read most of the pages and experience from the guys on that thread.
I also followed a few links and got some great information on averaging output etc.
Some of the info I already knew, but a lot was new to me.
• Registered Users Posts: 9
Still on the topic of batteries, deep cycle batteries in the Caribbean are expensive and sometimes it's better to import your own stuff.
I've read somewhere about using reconditioned forklift batteries are ok to build a good battery bank. Of course, it's lifespan would be shorter.

1] So my question is, would you advise using reconditioned 12V forklift batteries for a battery bank?

2] If yes, can you please refer me to a store where I can order them?
Fork lift batteries generally need higher charging current (10% to 13% or of C20 AH capacity)... Fork Lift batteries will have higher self discharge and probably need more distilled water per month.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,739 ✭✭✭✭
Solar Wind wrote: »
I've read somewhere about using reconditioned forklift batteries are ok to build a good battery bank.

In the industries that use them, forklift batteries are considered "dead" when they have been reduced to 80% capacity. (Battery is no good if it can't support a full 8 hour shift) That 80% capacity may provide years of good service to a off-grid power system.

Also, forklift batteries (in industry) are routinely discharged down to 20% SOC (and then immediately recharged with grid power. It will help prolong their lifespan if you keep the SOC much higher than that, especially since without the grid it is unlikely that you can fully recharge them immediately after they are discharged.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Registered Users Posts: 9
Thanks guys again for your response.

vtmaps and BB, I think you are spot with your response because someone told me that they wouldn't recommend forklift batteries because they require higher maintenance.

But I was thinking that if I sized the battery bank in such a way that it's DOD is 50 % and also, since it's a solar and wind hybrid system [ 20 kW off grid ] , wouldn't the wind turbine pick up the slack when the solar array can no longer charge because of night?

Some background information here.

The location of the off grid house in on a hill on a beachfront. It is very windy.There is a constant wind and it's something like 12 m/s.[ Can't remember the average, yearly speed right now ]
The turbine we plan to use is a 5 Kilowatt. Thinking of connecting the dump load from the turbine to the water heater and the pool pumps.The solar array would be about 50 panels [ 300 watt ]. It's just the battery bank that we need to properly size to give at least three or four days power /autonomy.

BTW, thanks for the information on my initial question.
We solved the issue with the lights and we have sized and ordered everything.

Hope I can post a pic of the system here for you guys when we are finished building it!

Forklift batteries are fine for the job--They use more water and really should have enough solar/wind/etc. to charge at the higher end of the 10-13%+ range.

I am not a big fan of wind... 12 m/s is something like 26.8 Miles per hour. That is probably never going to be a typical ground speed wind except during stormy conditions... At the top of a 10 or 20+ meter tall tower--Perhaps you can get some seasonal winds that meet those numbers.

And for many turbines, around 25 MPH is their top production wind velocity. They either limit their output above that speed, or in many cases furl/shut down at wind velocities exceeding 25-30 MPH. So knowing your average and average peak wind speeds at hub elevation is going to be key to picking/designing a wind turbine that will work, reliably, for you.

If you are on a bluff, turbulence can be a huge factor too... Turbine placement, elevation, terrain, etc. are all going to critical. If the turbine is installed where there is turbulent airflow--There will be low output power and probably high wear and tear on the turbine/blades/bearings/etc...

One neat suggestion somebody posted here years ago was to go fly a kite around where you plan on installing the turbine. Where the kite starts flying in smooth/stable air is where you want to fly the turbine too.

I highly suggest you check references on both your installer (if you are using one) and the wind turbine too... I would suggest you want, at least, monthly kWH numbers for at least one continuous year of production based on that exact turbine/type of installation (support electronics, etc.)... And understand the cost of maintenance (will it be a tilt up tower, or will you need a crane/lift every time you need to service the turbine and how much will those service costs be).

http://forum.solar-electric.com/forum/solar-electric-power-wind-power-balance-of-system/general-solar-power-topics/340203-a-1-8kw-wind-turbine-and-a-1kw-solar-panel-what-could-that-run?p=340211#post340211

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset