what's my mistake?

7 Posts
Mar 7, 2010 04:40 pm
what's my mistake?

I built a bundled firewood vending machine. I am trying to convert to solar. The machine runs off 110 volt A/C. Here's the set up. Kyocera KC130TM 130 watt solar panel set to factory recommended angle according to latitude, Go Power GP-PWM-25 solar regulator(25 amp),Go Power GP-2500 GP inverter, battery bank totaling 600 ah(1 new 80 ah,1 new 130 ah, 2 used 190 ah approx 1 year old, all deep cycle wet cell).Site location faces south and is wide open. All wire gauges as per factory recommended

The following is power needs for 24 hrs
Bundles 24 – 10 sec’s @ 770 watts(7 amps) 4 mins
Lights 3 – 13 watt  39 watts (.35 amps) 12 hrs 720 mins Bill validator 1 amp (110 watts)1 mins
Inverter 1 amp (110 watts ) 24 hrs 1440 mins
Machine runs for approx 10 secs to push bundle out when dollar bill is inserted into bill validator

Total amps  9.35
Total watts 1030

Machine is located in Connecticut.The set up does work as long as every day is sunny. If there is 2 or more days of over cast the batteries run low. Meter indicates approx 22 volts from panel direct sun light and approx 17 volts on over cast days. I thought 600 ah of battery bank would be more than sufficient. Any thoughts???


220 Posts
Mar 8, 2010 04:34 am
Re: what's my mistake?

 hi john,

sounds like a fun contraption you have worked up there!

"Any thoughts???"

-couple things stand out...

..those lights are killing us.
..that inverter running is robbing us.
..looks like we are in the red before we even sell one stick of firewood.Sad

how it looks from here,
lights- 39 watts for 12 hours = 468 watt hours
idle inverter (however we are not really at idle here)
idle inverter- 0.8 amp draw (per spec sheet) for 24 hours = 230 watt hours. (0.8amps x 12volts x 24hours)

lets stop there for now. and add 468 and 230. so far we need 698 watt hours for a 24 hour cycle.

the perfect output for a kc130 in hartford in march is 4.5 sunhours (per data tables) x 130 watts = 585 watt hours (in reality we won't get even close to that)

the battery side of things sounds kinda ok (the mixing is not so great)..looks like about three days backup. as it is now, with a fully charged battery you might go three days without solar input (no vends)..and with a fully charged battery and perfect solar input (no vends) it might run as long as a week or so. sorry

a few ideas come to mind.

edit: re-thinking this last bit. "it might run as long as a week or so." ..it might actualy chug along for several weeks before it crashes..
« Last Edit: Mar 8, 2010 05:00 am by david ames »
578 Posts
Mar 8, 2010 11:58 am
Re: what's my mistake?

I agree with dave.

the problem you have is both units and not accounting for system inefficiencies.  your initial calculations likely doomed the project from the beginning.  your daily loads in watt-hours are more than the array is likely to harvest.

for practical design help, I would recommend this book


you can also use the calculators on the website, or consider taking classes in person if you intend to do more solar work.



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Mar 15, 2010 06:52 am
Re: what's my mistake?

Hey! Looks as though you have an anniversary of sorts coming up Mr. Laughlin. As for your "mistake", I really don't see were you made any mistakes. You are doing something, somewhere that nobody has ever done before. You are the first to have a PV powered firewood bundle vending machine in Connecticut. Like a pioneer blazing a new trial, you try a way and if it doesn't take to where you need to be, then you back up and try another way.
Posted by Ken Hall on April 24, 2009, 11:42:58 AM
Re: System requirements (Reply #2)
That machine is probably already ground fault protected. Check your owners manual or the electrical schematic for an internal GFCI.

A 20amp draw at 115V would be about 200 amps on the 12V side.  I would start thinking 24 or 48 VDC.

This is not going to be 1-2 batteries and a single panel.  You are most likely looking at somewhere between 6-12 KWH a day. Just ball parking the lower number you are looking at maybe 1400lbs worth of batteries and something above 1.5kw worth of panels.
You are probably looking at a $15K starting price and it could easily double that.

Posted by Thomas Allen Schmidt on September 26, 2009, 02:36:37 PM
Re: Firewood vending- convert to solar (Reply #2)
Oops! It appears that I made a huge blunder in one of my calculations. I happen to think to myself while I was at a snack machine a while ago waiting for my Fig Newtons to drop, "who would want to wait 15 minutes at a vending machine for their treat?" Then it hit me, Duh! You stated that the machines motor runs for 15 seconds while delivering the bundle and in my hast to simplify the math I used 15 minutes.
Sorry 'bout that.
So, instead of an 805 amphour battery bank and a 2,000 watt PV array it should have been more like a 110 amphour battery bank and a 254 watt PV array assuming all of the other circumstances used in my last post. The lights would use more power than the motor in terms of kWh's. Getting its power from a battery and inverter assuming a 15% efficiency -
The lights = .504 kWh's per 12 hours of on time during the winter.
The motor = .03 kWh's for every 15 seconds of run time, 4 times in a 24 hour period.
So .507 kWh's or 507 watt hours divided by 24 vdc nominal is 21 amphours times 5 is 106 amphours.
507 watt hours divided by 2 hours of equivalent full rated power from a PV module during the winter months is a 254 watt PV array.
That sounds a lot better huh?

Opinions are, a lot like bellybuttons. Everybody has one.
Maybe now at least you will place more importance on your own Mr. Laughlin. Opinion I mean, not your bellybutton. Not that are bellybuttons aren't important, I mean they remind us all of where we come from. I am gonna leave now.

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