A grid tie novice needs help

Discussion in 'General Electronics Chat' started by qis_mo, Jan 5, 2012.

  1. qis_mo

    Thread Starter New Member

    Jan 5, 2012
    Hi all - I an an absolute beginner when it comes to grid tied inverters. I need help.
    Background - I been using a cheap HF 45 Watt kit for battery charging and driving some 12v LED lighting around the house. I recently obtained a damaged 250W PV panel that still puts out ~8 amps @ Voc 32~34v. This is a lot more than I needed for the LEDs, so the perpetual student in me said I needed to find a use for this new found source of energy. The cheaper mostly foolish side me said why not try out one of those cheap chinese made GTIs. For not much more investment I might save a buck in the long...long...run. Well the first Sun500g was pretty much DOA. Its replacement seem better but still very "inefficient".. Finally here comes my first question. How should efficiencies be measured? ..let me continue a bit more and explain what I have measured and how... Then I will badly need your advice.

    Today Jan 5, 2012 the ...
    Peak Isc measured across the inverter dc input was 7.08 amps
    Voltage measured across the inverter dc input was 28.5 volts
    The current drawn (10A meter) in series with the panel and the inverter was 4.28 a.
    The Kill-A-Watt meter between GTI and GFI measured 108 watts

    My rusty math estimates the ...
    Watts available at the panel near 7 x 28.5 = ~200 watts
    Watts consumed by the GTI near 4.3 x 28.5 = ~123 watts
    My newbe conclusion is that the ...
    PV+GTI system's efficiency = 108 watts out / 200 watts available = ~54% while the
    GTIs internal efficiency = 108 watts out / 123 watts in = ~88%

    If my logic and math is not fatally flawed.. the question becomes ...why does the GTI take so little of the "watts available" and is there a GTI spec value that I should be aware of while I learn more about GTIs and PV systems. Thanks for staying with me
    Thanks for any comment and advice.
  2. SPQR


    Nov 4, 2011
    You haven't had an answer for over 16 hours, so I'll give some references, and that will pop you back to the top.

    Wiki has a nice little review on GTIs - HERE.
    About half-way down they talk about Peak Efficiency and CEC Weighted Efficiency.

    HERE's an old thread from this forum.

    HERE and HERE are YouTubes on the calculation.

    HERE's another page.

    Googling grid tie inverter efficiency calculation will give you a ton of hits.

    Now let's wait for the experts.
  3. qis_mo

    Thread Starter New Member

    Jan 5, 2012
    Thanks for the assist and the several references.

    I must be missing something. I am still on the very steep side of this GTI inverter thing's learning curve.

    Most all of the efficiency studies I have found to date, deal with the watts going in and out of the GTI inverter. Though some reference the wattage rating on their panels. I have yet to find a study of the actual watts currently available at the inverter (basically using the amp meter across the inverter's dc input), compared to the AC watts produced as output.

    The measurements I have found seem to measure the dc current (or watts) consumed by the inverter compared to its AC watts output. This usually results in an efficiency of 85 to 95%

    When I measure the AC watts produced by my 24v Sun 500 G to the short circuited current available at the panel It results in an efficiency of 50 to 60%. A near peak "overall" system level performance. This happens when the Inverters internal efficiency (Watts-Out/Watts-In) remains at 92%

    Setting aside all the other factors used to derate the performance of a PV system, (to many to mention) is there a particular performance factor that measures the ability of the GTI Inverter to consume available PV Watts. How is it identified for and used by PV system designers

    I must be missing something obvious to everyone else?

    Thanks again for any assistance