I Have My 3 Kits Connected to a GTI and Putting 135 Watts Back on the Grid

OK, I have my 3 sets of 45w panels (135w Total) connected to my 300w GTI (Grid Tie Inverter) I bought off eBay. The Sun is Out today and I am putting Power back on the grid. Granted it is probably like throwing pocket change at the National Debt but I am at least trying. I have 2 more kits to go collect, this will give me a total of 225W's. I think I am going to look into getting some more powerful Panels but I want to keep experimenting with the 45w kits for now. If any thing changes or I have anything to report I will.

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You are jamming now cueball! Would you mind sharing a link from eBay on which GTI you bought? Also, have all of your control boxes from HF worth without a any problems?

Also would love to see some pictures or even videos.
OK, I realize this is a Very Quick Update but It WORKS!! It Really Really WORKS!!!. I just have on the basic items in the house right now. Hot water tank, Fridge and computer. With the panels and GTI connected the wheel at the pole slows down to a crawl!!.. I think with the two other sets panels I could actually stop the wheel or make it go in reverse.
Here is the eBay Item Number (120520449658) of the kind I bought from the vendor whom I bought mine from. I offered them $90.00 instead of $99.99 and they excepted my offer. I would buy from them again as their shipping from China was fast, like 5-6 days and there was a weekend in there as well. I did have an issue with one controller not working as far as the light outlets did not work on one of my controllers. I have not had a chance yet to return to HFT to see if they will replace the faulty unit. How do I share pictures? I would love to show off my set up.
Here are some pictures I hope??
Thank you for sharing your experience. I need your one last clarification before I can confidently copy what you have done.

From IMG_2302.JPG above, I see you are using a 14-28 volt, 300 Watt GTI --- which is connected to nine 15-Watt solar panels (each 12 volt, connected in parallel) bought from Harbor Freight. Despite the panels being 12 volts each, but the GTI being 14-28 volts, the system works fine. Exciting!

But according to the helpful video by Dan Rojas,
the 12 volt panels are too low, and will not work for the 14-28 volt GTI. To overcome this Rojas hooks up two 12-volt batteries in series (at segment 1:52-2:50 of the video) to make it a 24 volt battery. This battery is used to store solar power first, then after charged, feeds to the grid through GTI. However, he provides a warning (at segment 6:10-6:40 of the video) that we should be very cautious if we try to connect the panels in series to create a higher voltage to meet the 14-28 volt range because the high voltage will damage the panels. After watching the video and understanding his warning, I am confused now. Should I disregard the discrepancy between the solar panels (12 volts) and GTI (14-28 volt)?

Thank you again for your sharing and guidance.
ultraghostrider (and others),

Thank you for the additional inputs. To clear my confusion, I appreciate if anyone can advise me with the following questions.

(1) Should I disregard the discrepancy between GTI (14-28 volt) and parallel connected solar panels (12 volt), just connect as Cueball shows? Dan Rojas in his video says this won't work. But Cueball seems to have success.

(2) ultraghostrider says it's better to set panels in series (to gain higher voltage/efficiency, and save wiring costs). But the first ebay link provided by him (above) says we'd better connect panels in parallel so if one panel malfunction, the system still works. Dan Rojas also says connecting panels in series might damage the solar panel due to higher voltage through the panels. I am really confused with these conflicting comments.

Please help me. I really wish to duplicate what Cueball has done. But I am not 100% confident.

the HF panels are part of a 12v kit but individually they each have a rating. most panels have labels on the back that tell you what the volts and amps.


the HF kits have the following specs on the back. I had a 10.5-30v GTI and my 2 HF kits worked well wired in parallel. My meter would show about 14-17v and 2-4 amps coming from the panels. I chose the 10.5 since i wanted the GTI to kick in as soon as possible. Having a higher Voltage GTI like 28-55 would mean that the GTI will not start producing until your panels hit the cut in voltage (probably lower than 28 someone with one would have to validate).

15w HF panel specs

Power (W) 15
Open Circuit Voltage (V) 23.57
Short Circuit Current (A) 1.15
Maximum Power Voltage (V) 17.5
Maximum Power Current (A)


I'm not sure I understand the diff. between short circuit and max power.


When I use the formula...

Current equals watts divided by volts...

I = w/v

so watts being 15 per panel divided by volts 12 the answer is 1.25 amps or I panel = 1 and 1/4 amps

That to me means 3 panels under very good conditions can produce 3.75 amps.  Most of the time we will get something like 1 amp per panel maybe a little over.


However if you use max volts number (17.5) and use the same formula Watts divided by Volts the answer is .85 amps per panel and for set of 3 panels 2.57amps.


My guess is that with amps being as they are your true amps per set of 3 panels, a harbor freight set of panels, will produce between 2.57 and 3.75 amps. if all is connected properly, good connections, bright sunny day, low humidity, not too hot, etc. etc.


That tells me my 12 panels, 4 sets, will produce between 10.28 and 15 amps per hour into battery bank or GTI or an inverter.


Stay with me here...for example if I had a battery bank that is rated at 200 amp hours and I used 50 amps last night (roughly 25% of battery bank) and my panels put out 12 amps per hour back into the batteries they should be recharged and full in about 4 to 5 hours and I'm ready to do it again and again.


Now to back up just a bit I have 6 batteries at 33 amp hours each wired plus to plus and negative to negative which gives me 33 X 6 = 198 amp hours total.  They are rated at 1.75 amps per hour ea.  That tells me I can take (since I have 6 of them) 6 x 1.75 amps per hour which equals 10.5 amps).


Now you can take more or less amps out of the battery but the more you take and the faster you take it will not be very good for your batteries.  Best to stay within the 1.75 amps per hour.  Suppose you will be pulling more amps per hour then get batteries that have a higher amp per hour rate...you will be glad you did.


BTW these are AGM batteries, AGM batteries are the most trouble free and safest out there, for one thing normal freezing will not harm unless a hard freeze may hurt them a bit....not so with acid type or others.


Can any of you tell me if this is correct or am I out in left field?

Dave T

Hi Dave,

Your math is correct but won't exactly fit the HF solar panel since the panel output (Watts) is not linear.  To be linear, the curved line in the following graph would be a straight line from the maximum voltage to the maximum current.  W/Volts = Amps is a linear equation.

This graph is approximately how the HF panel outputs its wattage.


HF rates its panel at 15 Watts, using an Operating Point of 17.5 volts and 0.857 amps (0.86 amps). 

The panel Short Circuit current would be 1.15 amps per specifications posted by Raymond Diaz above. 

As you can see from the graph, maximum Short Circuit current comes at close to zero volts (the only voltage available is what the internal resistance of an ammeter develops).  The red box would collapse to a straight line to the right.  Not many people can use the maximum 1.15 amps at close to zero volts.

Now, starting at the Operating Point of 17.5 volts, raise your voltage to the Open Circuit Voltage of 23.57 volts.  As the voltage goes up the red box gets more narrow to the left until it becomes a straight line up and down.  The only current available is whatever the voltmeter needs to produce a reading.  Again, not many people can use the 23.57 volts with essentially no current available.

15 Watts/12 volts = 1.25 Amps is correct math, but that HF panel won't produce 1.25 Amps at 12 volts.  It will only produce 1.15 amps, and that's with the panel leads shorted. 


The only way to tell exactly how many amps your panel will produce at 12 volts is to measure it with an ammeter in series with the panel positive output lead and the 12 volt battery.


Hillbilly Gene

Okay my math does not work but is correct.  I think you are telling me how amps work under a load and not under a load.

When calculating battery drain and recharge time are my numbers correct or what?


Dave T

Hi David,


My reply was intended as more of an explanation of the limitations of using Ohm's Law on the HF panel rather than of how amps work under load or no load, but you seem to get the concept.


I see no fault with your reasoning or math with either of your two battery examples.


The AGM 33 amp-hour/1.75 amps gives an 18.86 hour rating.  A little odd, but if that's how that battery manufacturer rates its batteries, then by all means stay within their ratings.  As you say, if you need more amps per hour than that, get higher amperage rated batteries. Your battery bank will last longer.


Hillbilly Gene

I have 6ea  33ah batteries based on 20hr rate of discharge at 1.75 amps per hour per battery is...1.75 x 6 =  I can draw 10.5 amps per hour total being nice to the batteries.

I think you goofed with the 18.86 hour rating or I'm loosing it.


Help me here Gene?


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