Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

rss2qrss2q Solar Expert Posts: 75 ✭✭✭✭
I have 3x 12v (3.1A) 55watt panels that are hooked up to a MorningStar Prostar 30 charge controller, which is hooked up to a 12v deep cycle battery. The panels are producing a little over 10A while in direct sunlight.

Currently, I do not have any use for the solar system execpt as a backup power source if the power shall ever go out.

My question is, am I even producing enough power to look into turning it into a grid tie system? And if so, what extra equipment, other than a grid tie inverter, would I need for my system to pump energy back into the grid?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

    Say you get an average of 4.5 hours of sun over the year and pay $0.12 per kWH for power:
    • 3x.055 kWatt panels * 4.5 hours of sun * 365 days per year * 0.77 GT efficiency * $0.12 per kWH = $25 per year worth of electricity
    Your building department may charge you between $45 and $900+ for the building permits and inspection. And the utility may charge you $277 for a new meter, or charge you $10-$25 per month for billing, etc... And your utility may only pay you $0.05 per kWH (they may also pay you more--you have to check).

    And you will have to purchase a micro inverter (and possibly a power line interface to monitor the output power from the 1x micro inverter).

    My own two cents--I would suggest that a 3kW array is probably the smallest "cost effective" array for many people based on the above issues... Of course, some areas may cost less for an install/utility charges.

    Your choice.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SevenSeven Solar Expert Posts: 292 ✭✭
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

    Not really with your current setup.
  • rss2qrss2q Solar Expert Posts: 75 ✭✭✭✭
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

    Thanks bill, that's basically what I was looking for. Good info to know.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

    Actually, that is Bill's way of saying here are some starting assumptions... Plug in your own (if different) and see what it looks like. Bill has no magical way of ascertaining other people's requirements. Bill is still trying to figure out his own wife's requirements after 25+ years.:-)

    Then make your own decisions. :p

    -Bill ":roll:" B. ;)
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rss2qrss2q Solar Expert Posts: 75 ✭✭✭✭
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?
    BB. wrote: »
    Actually, that is Bill's way of saying here are some starting assumptions... Plug in your own (if different) and see what it looks like. Bill has no magical way of ascertaining other people's requirements. Bill is still trying to figure out his own wife's requirements after 25+ years.:-)

    Then make your own decisions. :p

    -Bill ":roll:" B. ;)

    Haha, well I figured there was more to it. What I don't understand is, I know some grid tie inverters I've seen allow you to just plug it directly into one of your wall outlets in your home. I'm curious if you do, will your electric company fine you or something for not letting them know?

    Like I see all these other policies you posted about, but I'm wondering what happens if you don't even bother with them? I'm not going to do it :p, but I am curious.
  • SevenSeven Solar Expert Posts: 292 ✭✭
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

    From what I understand those are usually not legal, and dangerous.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

    To keep it simple--The NEC is not a magic safety document--but if various people follow the basic rules (different electricians over the decades in the life of a home/building). Then the overall installation should be/remain relatively safe.

    The NEC is pretty much setup to have "one power source"--the GRID--and multiple distributed loads. In some cases, such as with backup generators, it is a simple transfer switch that moves the loads from one power source to another.

    And with just those assumptions--it can be amazing how complex/confusing a large/complex/"special" installation can become (ground loops, back feeds, etc.).

    So--when you add multiple power sources (like Grid and a Grid Tied Inverter) to a power system--we now have multiple sources of current/energy into the building wiring.

    In the simplest example--You have a 15 amp branch circuit that is designed to carry 80% of rated current, or 12 amp max continuous current.

    Now, you add 165 watts of solar panels--which could add:
    • 165 watts / 120 VAC = 1.4 amps
    Adding 1.4 amps to a 15 amp branch circuit does not really hurt much (14 AWG wiring is actually OK to 20 amps)...

    But what happens when somebody adds 1,650 watts of solar panels to a 15 amp branch circuit:
    • 1,650 watts /120 VAC = 14 amps
    Now, if you had ~10 of those micro-inverter setups, you have 29 amps available in that circuit. And since heating of wiring (resistance heating/voltage drop) is:
    • power = I^2 * R
    • 29 amps^ / 15 amps^2 = 3.7x more heating available in wiring
    So--if somebody just kept plugging in additional solar (or wind, etc.) GT micro inverters--until they popped a breaker, then backed of one... They have seriously way too much current available in the upstairs bedroom circuit. And if somebody plugs in an alarm clock or vacuum cleaner, or the wiring up to the roof, with an electrical short--the chances of starting a fire are much higher.

    The only "safe" way to connect a Grid Tied Inverter is to hardwire a dedicated branch circuit to the GT inverter... And engineer the Main Panel (starts at 20% of main breaker bus bar rating) and inspected by the building department to insure that the wiring system still meets requirements.

    Add the fact that (by code) you probably need to bring the solar power in through the roof/wall with a metal conduit, some locations may have hurricane tie down requirements, others safety grounding the frames against shorts circuits and lightning... etc...

    It becomes near impossible to make a generic "plug in" GT inverter that will be safely installed by the "average" person. (there are other issues too--but these are enough to kill "homeowner" plug-in installs to existing wiring).

    Remember, UL was originally created by Insurance Companies to have some standard way of insuring that various "stuff" used in building met some minimum level of safety requirements. Started back in 1894 testing a non-combustible insulation material.

    I could see some day that, perhaps, there will be a series of "plugs on the roof" pre-wired into new homes/businesses for solar arrays that could reduce the cost and complexity of installation. But I doubt that many people would pay to "pre-wire" solar and never install it on the assumption that someday, somebody else may.

    Anyway, my two cents worth.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rss2qrss2q Solar Expert Posts: 75 ✭✭✭✭
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?
    BB. wrote: »
    Anyway, my two cents worth.

    -Bill

    More like $25.02 worth :p
    I'll have to read through this article at work today. :D
  • rss2qrss2q Solar Expert Posts: 75 ✭✭✭✭
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

    BB or someone, can you please check out this calculator and see if it's a good tool to base rather or not you should look into grid tie systems or not, and check to see if my values I used are in the right neighborhood:
    http://www.ecowho.com/tools/solar_savings_calculator.php

    I attached two screenshots which shows the values I used, and the results from entering those values.

    For the Annual Output, I got that value from this equation:
    • 3x.055 kWatt panels * 4.5 hours of sun * 365 days per year * 0.77 GT efficiency = 209 annual output (kWh)

    The other info I just put an average low percentage such as how much my electric company would pay per kWh and how much they charge per kWh.

    From the looks of it, if it's somewhat accurate, I should pay for my panels in 15 years?
  • solar_davesolar_dave Solar Expert Posts: 2,382 ✭✭✭✭
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?

    I think you have missed a couple costs.
    1) permits to add the system to the grid
    2) cost of electrician to do the wiring (or at least approve) to NEC code.
  • rss2qrss2q Solar Expert Posts: 75 ✭✭✭✭
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?
    solar_dave wrote: »
    I think you have missed a couple costs.
    1) permits to add the system to the grid
    2) cost of electrician to do the wiring (or at least approve) to NEC code.

    I'm not focused on those costs at the moment, but focused on how accurate the calculator used is for the other information.
  • fixerfixer Registered Users Posts: 12
    Re: Based On The Power I'm Producing Is It Worth Looking Into A Grid Tie System?
    BB. wrote: »
    In the simplest example--You have a 15 amp branch circuit that is designed to carry 80% of rated current, or 12 amp max continuous current.

    Now, you add 165 watts of solar panels--which could add:
    • 165 watts / 120 VAC = 1.4 amps
    Adding 1.4 amps to a 15 amp branch circuit does not really hurt much (14 AWG wiring is actually OK to 20 amps)...

    But what happens when somebody adds 1,650 watts of solar panels to a 15 amp branch circuit:
    • 1,650 watts /120 VAC = 14 amps
    Now, if you had ~10 of those micro-inverter setups, you have 29 amps available in that circuit. And since heating of wiring (resistance heating/voltage drop) is:
    • power = I^2 * R
    • 29 amps^ / 15 amps^2 = 3.7x more heating available in wiring
    So--if somebody just kept plugging in additional solar (or wind, etc.) GT micro inverters--until they popped a breaker, then backed of one... They have seriously way too much current available in the upstairs bedroom circuit. And if somebody plugs in an alarm clock or vacuum cleaner, or the wiring up to the roof, with an electrical short--the chances of starting a fire are much higher.

    The only "safe" way to connect a Grid Tied Inverter is to hardwire a dedicated branch circuit to the GT inverter... And engineer the Main Panel (starts at 20% of main breaker bus bar rating) and inspected by the building department to insure that the wiring system still meets requirements.
    It's understandable that this gets confusing to people.

    You simply need to understand that a standard wall outlet gets you 15A at 120V, or 1800W (most USA places).

    It goes both ways, in or out. If you are going to exceed that then you will simply pop the breaker, not burn down the house, because the wires can handle more than that. When they are installed there is already a margin for safety built in, meaning it could go to 20A or 25A before heating up to a point where there *may be* concern. It's overbuilt for safety.

    You can put a power strip on the GTI if you are really worried about it, those have a 15A breaker.

    Here's where I see the confusion coming in, if someone plugs in a 100W load to the same circuit as the GTI producing 100W, then the load is being completely powered from the GTI !

    The 15A "house" breaker sees ZERO amps flowing. The circuit is as cool as can be.

    It doesn't add up to more amperage on the wire, it's actually less.

    Most of the small GTIs are fused anyway, and for the power the OP was talking about, a 300W ebay under $90 GTI would work perfectly.

    It's in a metal enclosure, unlike a lot of household items I plug in daily that are totally plastic, but no one blinks an eye about that. I have a space heater that's all plastic with lots of warning labels all over the place, talk about dangerous, and those are not "illegal". The power company doesn't tell you to not use them. In fact they love it because it makes them $$.

    The 300W GTIs off ebay are fused at 5A on the AC side, or 600W max and it's a fast blow fuse, unlike the house breaker that would allow for a surge before going off. The GTI has a 20A fuse on the DC side, (400W at 20V for example) so if you want to look at it, it's doubly protected.

    So technically and electrically, I don't see any "danger" in using a simple plug in GTI, any more than any other plug in appliance. Save some money and go green, easy and cheap.

    No inspection needed, it's a plug in appliance. In the OPs case it would most likely cover a few of his minor loads which are 24/7 anyway, so the power would never go beyond his house.

    Even if he had a rip off "smart meter" that won't count in reverse, he could still benefit from a small cheap GTI. Most houses have 24/7 loads of 40W+ lurking about.

    And of course the GTIs shut off within a few milliseconds of losing AC. "Anti Islanding".

    If the OP was producing way more than he can ever use, then I can see how he might want all the inspections and a net meter and sign a big long contract with the power company, get the lawyers involved and all that.

    With that much power of course anyone with a brain would seriously think about having a dedicated circuit for a *large* single GTI, but that is the case with anything that would be over the 15A limit anyway.

    So basically the wiring is already inspected, conforms to NEC and rated for this use!

    Did I mention that the GTIs also have overheat protection? Yea, there's a temperature sensor in there and it will shut down and run the fan till it cools off.
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