Solar question

System
System Posts: 2,511 admin
in Grid Tie and Grid Interactive Systems #1
Greetings,

very new to the topic of home solar power (first post)

I'm seeing programs that allow solar, wind and other types of power generation to be connected to your house (or business) and you are still connected to your grid. The power company meters both - what they provide to you and what your private generation provides.

Your power company gives you a credit (or cash) for the difference.

Technically - is there a problem or danger in having both connected (I know the power company would have it wired correctly I just don't see how yet)?

Would or could the two sources "fight each other" or back-feed or something?

The only wiring diagram I've seen so far is both sources end up at your breaker box.

What if you are able to generate enough power on your own to meet your needs (or more than enough) - what keeps the power company's power happy and out of your house?

If my local generation (solar, wind …) provides more than enough power for my needs does what is upstream care if I am placing power on their system? Will the two fight each other?

Also - can you point me to a good schematic of how all this connects?

Thanks
Sparky_
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
    Re: Solar question

    Sparky,

    Welcome to the forum. And there are several levels to answer your "simple" question. ;)

    First. There are Grid Tied or Utility Interactive Inverters. You simply connect a solar panel array (from 1 panel to dozens to hundreds, depending on the size/specifications of the GT inverter) and then connect the GT inverter to its own (typically) two pole 120/240 VAC circuit breaker in your main electrical panel. (obviously, there are a whole bunch of code and structural issues--but the basics are that simple).

    To understand how a Grid Tied Inverter works... A bit deeper. You are very used to how a battery works--The battery tries vary hard to maintain 12.7 volts no matter the load (charging, no current, discharging). The only time the battery voltage goes outside the 10.5-15.5 volt range is if it is dead, overcharged, too much current, or disconnected.

    Believe it or not--You can think of the Utility Grid in the same way... It controls the voltage (and frequency) and, for all the world, looks like a giant AC battery. You can pull current, take no current (discharging), or push current (aka charging) and it will work just fine (obviously, there are limitations, but for 99.999% of the people, this is it). Add a bi-directional meter to measure the amount of power you both "purchase or sell"--and you are done.

    The inverter itself is actually a high impedance source. It is a term from electrical/electronics--but basically means that it it wants to output 10 amps of current, it will output it at 240 vac, 120 vac, or 0 volts vac (or vdc for a DC current source like solar PV panels).

    The inverter basically monitors the AC sinusoidal voltage and outputs an AC sinusoidal AC current that is proportional to the grid voltage.

    The reason you cannot use a standard battery based AC inverter is they are based on a "voltage source" model. You connect an AC off-grid inverter to the AC Utility Grid, they both are going to want to set the voltage and frequency. More than likely, it is your off-grid inverter that will lose in a puff of smoke and flame.

    Second level... Note that a Grid Tied Inverter cannot operate unless the AC line (for North America) 212 to 264 VAC and 60Hz +/- 1%. Both functionally and as a safety matter, the inverter shuts down if your utility power fails.

    So the Third Level, there are Hybrid Inverters that behave like a Grid Tied inverter (when the sun is up and the utility power is OK). Will switch your loads to a local off-grid inverter mode when the AC power has failed and supply power to your home from local battery power (and solar panels). Sort of like a giant UPS for your home.

    The solar panels will keep the battery bank charged and the Hybrid inverter will take the excess energy and sell it to your utility when the AC utility power is good. And when the AC mains have failed, it will behave just like an off-grid solar system and supply power to your home's loads (and can also connect to an emergency AC generator too).

    Lastly, Grid Tied power billing becomes a fairly large issue too... Each state/utility seems to have their own rules. Some of the best have you sign up for 1 year net metering. Basically, you can generate excess power in the summer and put it into a "bank". Then draw the excess from your bank for winter usage (and, you can draw a negative balance in your bank and pay it back in summer). At the end of one year, you either pay the utility (if negative balance) or they reset your account to zero if positive balance (there are lots of variations).

    Another is they pay you for your power at the wholesale rate (~1/2 the retail rate). Not very good for you.

    And many smaller utilities/co-ops do not even allow net metering/grid tied power connections.

    The the technical and the local utility billing issues (and sometimes local building department issues) are pretty daunting for a first timer who wants to do it themselves. After one pass through the entire process--it is pretty easy to understand.

    I will stop here so as not to cause confusion--More questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Solar question

    i'll try to address your questions here and it'll be in bold.
    Sparky_ wrote: »
    Greetings,

    very new to the topic of home solar power (first post)

    I'm seeing programs that allow solar, wind and other types of power generation to be connected to your house (or business) and you are still connected to your grid. The power company meters both - what they provide to you and what your private generation provides.

    Your power company gives you a credit (or cash) for the difference.

    Technically - is there a problem or danger in having both connected (I know the power company would have it wired correctly I just don't see how yet)?

    danger, no. if for some reason the inverter goes whacky and goes out of phase with the utility it would damage the inverter and there are fuses/circuit breakers to protect the wires.

    Would or could the two sources "fight each other" or back-feed or something?

    i can tell you aren't very familiar with electricity. fight? no. back-feed to the grid is what you'd want it to do.

    The only wiring diagram I've seen so far is both sources end up at your breaker box.

    essentially so and sometimes sub boxes get involved along with disconnects for safety and servicing.

    What if you are able to generate enough power on your own to meet your needs (or more than enough) - what keeps the power company's power happy and out of your house?

    if you generate more then you use the utility like a battery and use your credits or extra cash to supplement times when the system isn't supplying all of your needs and is usually in the winter. if you have so much power being generated all of the time then you really don't need the electric company at all. it is good to have the connection to them for times of maintenance and repair. as to electric company personnel being at your place they don't have the money and manpower to stay there and would only be there if they have to be for any wiring or connecting and sometimes inspecting in some cases. building inspectors are usually present in most cases rather than the electric company.

    If my local generation (solar, wind …) provides more than enough power for my needs does what is upstream care if I am placing power on their system? Will the two fight each other?

    same question as above, but if you put 2 batteries together in parallel does anything bad happen upstream from them? no, not unless they were cross wired to short them. as was said you are just adding to the power available to the grid.

    Also - can you point me to a good schematic of how all this connects?

    i'll let you search here or let someone else come up with a schematic. those are particular for each application and is not a one schematic fits all. in its simplest form it would be just a tie point at the breaker box with consideration to wire sizing, disconnects, and buss bar rating in lieu of the added power to the box.

    Thanks
    Sparky_
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