Grid Tie Inverters

[skyl4rk]

A grid tie inverter is a device which takes power from a solar panel and outputs the power into a house 120V AC system.

Grid tie inverters are not yet legal in many places yet, so this post is about the future, not today.  They are not legal because most have not yet passed the Underwriters Laboratory testing program, and because utility providers pass policies prohibiting grid tie inverters without expensive meter upgrades.

A van or RV that has a solar panel setup can be connected to a grid tie inverter and plugged into a house to power the appliances, lights, furnace, etc. in the house. The grid tie inverter is a metal box with electrical input connectors and output connectors, similar to the inverters used in vans to provide 120VAC from a battery.  One way to connect a grid tie inverter to a house is to use an extension cord, and plug it in to the grid tie inverter on one side, and directly into a wall outlet in the home.

The grid tie inverters match the frequency and wavelength of the AC power coming in from the utility.  In case of a power failure, the grid tie inverter automatically shuts down power to grid, to avoid energizing wires outside of the home, which would endanger utility workers.


If a person installs solar panels on their home, the installation will increase the value of their home, and it will be taxed as part of property tax.  If a person installs a solar system on a vehicle, that is not classified as real property and does not increase property tax.


A typical electric circuit in a home is a 15 amp circuit.  At 120 VAC, the wires in a 15 amp circuit are rated for a maximum of 1800 W.  So a typical van or RV sized solar system of 200 W to 400 W is a size that will not overload a typical home circuit, and will not trip the breaker by moving more than 15 amps through the breaker.

The power generated by the solar system and transmitted through the grid tie inverter replaces power that otherwise would have been provided by the utility passing through the meter.  The grid tie inverter electricity is power that will not be paid for in the electric bill from the utility company.

If a person installs solar panels on their home, the installation will increase the value of their home, and it will be taxed as part of property tax.  If a person installs a solar system on a vehicle, that is not classified as real property and does not increase property tax.

Here are some examples of grid tie inverters:

http://www.amazon.com/s?ie=UTF8&page=1&rh=i:aps,k:"grid tie" inverter

If this technology takes off, it might be a way to justify the purchase of a larger solar system on the van or RV, and use it to reduce one's electric bill when the vehicle is parked in the driveway.

 

[MikeRuth]

I'm going to admit for the time being that I am not well versed on the GRID TIE system, yet as I understand it as stated above it should be able to disconnect the house from back feeding the grid.

My simple logic says that if you use this by connecting the output to a wall outlet and then the grid goes down you would indeed be back feeding the grid as this does nothing that I can see to disconnect the house from the pole. Disconnecting the grid power would mean to me a switching device capable of a minimum 100 amps if not more that has been wired into the service feed of the house.

Am I being to simple or missing something?

 

[MikeRuth]

let me add that you would be back feeding the transformer on the pole, and how far back down the line could that go?
Would your back feed be cranked up to several thousand volts>?

 

[skyl4rk]

The grid tie inverter device will not output any power into the the house system unless if finds an existing 120VAC waveform, to which it matches its wavelength.  If the power grid goes down, there is no waveform to match, so no power is output.

 

[skyl4rk]

Another problem area might be when the grid power is on, but there is no load in the house. In that case, there probably would be a backfeed into the transformer.

I don't know if the meter would turn backwards. I believe that the backfeed power would be used up by other houses on the system.

I am an electronics tech, not an engineer. I would not buy a grid tie inverter until they have a UL listing.

 

[MikeRuth]

OK I get it now. and with that, is it worth the trouble and expense to try and feed less than a KW back?

 

[SternWake]

My friend recently put 3KW on his home, and got it inspected and cleared by the power company.  Two weeks later he got a notice from the power company he had to  turn it off, then he got his electrical bill and it had doubled.


Turns out he needed a new meter and a new program for it, as the old meter was counting the Solars contribution, and the regular consumption as it only saw juice flowing through the meter, not which way it was flowing.

He was not able to get that money back from the power company, despite it being inspected by them before putting it into use.

They really go out of their way to screw the customer with Solar on their roofs here.  Lots of Hoops and small print to negotiate.

My friend was also dismayed to learn if the grid goes down, he can't use any of his solar to keep his fridge cold, and is considering having me set up a separate system feeding batteries which could then feed the fridge and some other necessities for the inevitable grid failures to come.

We had a power outage a few years back, and people were trying to get me to stuff my 1.8 cubic foot fridge with their perishables.  I only knew the grid went out because all of a sudden there were no WIFI signals to be found.

 

[LeeRevell]

I currently have a semi-fixed 5KW genset feeding my home for emergencies via a totally separate circuit. If power goes out I crank the genset, then hook whatever load I need to it. PC, fridge, etc. I plan to add solar to this too. The incoming AC line is shut off at the main breaker. No chance of lighting up the lineman.
My electric company texts me when power drops and when it is restored.
Being able to hook the van power to the house would be a bonus. Thanks for the idea!

 

[MikeRuth]

I think new static storage solutions are coming soon that will let us more cost effectively store and use the energy we produce.

When that happens I think we could easily see our investment pay off in just a few years.

In turn if so there will be a new all out war of how will the grid be maintained, who will pay for that etc. A subject for another thread though.