Grid tie inverting

[Jason Wilkinson]

 I want to understand how grid tie works, more specficially battery back up. I understand in part ,panels send power to  inverter , inverter split power to loads/back feed grid. No power from panels.inverer switches to grid and draws power, so far so good.
   With battery backup , no power from the panels , does the battery have to "replace" the panels ? ie send to the inverter what the panels would have been doing ?
  I will add further questions as i get answers from this one
 Jason

[richp0169]

Your system will generate power and store them in your battery bank.  Whatever excess power you have based on how you set your charge limits, will be fed back to the grid.  If your battery bank is fully charged, again based on how you have your controls set, you can run from your battery bank to the limits you determined.  Generally speaking that is.

I know it's a vague answer, but it's a vague question without any hard info.

[joestue]

generally, you do not want to ever discharge the battery into the grid through the inverter.
the reason why is the grid won't pay you full value for the battery.

lead acid is around 25-50 cents per kilowatt hour life cycle cost.

[Jason Wilkinson]

Ok,  my question may not have been easilly understood.
 1) during sunlight  inverter get's it's power from the panels to power loads
2)  during the night inverter " switches " to the grid to power it's load (this becomes a straight   through  from grid to load)
  With battery backup, does the voltage of the battery has to be large enough (as what the panels would have been producing" so the inverter does not switch ?
 At this stage i'm not considering net metering tariffs etc   only how the inverter works
   Jason

[Flux]

With a direct grid tie the panels export power to the mains. During a mains failure you have no power. If you use power then your panel production is an energy saver with the grid supplying anything beyond what the panels are producing. if you use less than the panels make your used power is supplied free from the panels ( not directly) and any excess panel output goes to the grid at a different  ( or zero) tariff.

With a battery system your panels never power the load, they charge the battery and the inverter runs from the battery. When set to export, excess power is fed to the grid. You use grid power but up to the point where you use more than the panels produce what you are using from the grid is balanced by what you are producing so your net grid consumption is zero. If you take more than the panels produce then the grid supplies the excess. It is an energy saver, deducting what you produce from your grid consumption.

At night or during a power out it reverts to a normally battery fed inverter to supply your load as long as you have the battery power to do it.

Still not sure that is what you want to know but a battery grid tie inverter never powers the load direct from the panels, the panels charge the battery.

Flux

[Jason Wilkinson]

OK folks lets discuss this one item / person at a time
richp0169 you said     "Your system will generate power and store them in your battery bank."   got that
                 "  Whatever excess power you have  {based on how you set your charge limits,} will be fed back to the grid "
  explain the charge limit .    I thought grid tie didn't use a controller
 Flux   said " With a direct grid tie the panels export power to the mains."    { one say battery bank  the other say mains}
   " During a mains failure you have no power." understand.
    "  If you use power then your panel production is an energy saver with the grid supplying anything beyond what the panels are producing."    I understand that to mean if my needs are say 20 watts  panel only producing  8 watts  then the other 12 watts comes from the grid .    But do the two work in tandum ?
      " When set to export, excess power is fed to the grid."  Is this done automatically by the inverter ? if so it's getting clearer to understand
      " At night or during a power out it reverts to a normally battery fed inverter to supply your load"   got that . I thought ihe inverter used to "switch so fast from grid to panels " but now i understand the balancing/sharing .
    Thanks again   it's a bit clearer
   Jason

[Flux]

A direct grid tie doesn't use batteries, for those who don't need battery back up it is a much cheaper option. As you need the battery back up it will not be an option. I just mentioned it to give a complete picture of the available schemes. Sorry if it confused things.

You have grasped most of the basics and you understand the power flow bit correctly.

I think the confusion is about the changeover. The panels never supply the load directly, as a current source that wouldn't work.

When in export mode the inverter is in parallel with the grid and power flow depends on load and panel power. In the event of grid failure the load continues to be supplied by the inverter just as in stand alone mode.

However it is programmed, the whole thing happens automatically.

Those familiar with this scheme will be able to tell you what happens if the grid fails and the batteries are fully charged I don't have the experience to know if you need a dc dump controller or whether you can programme it to dump on the ac side via the inverter. I suspect you can arrange the dump in some way without another controller.

Flux

[Flux]

I think there is another option where it can be used as a static switch when your batteries are low. The grid will power the load so your batteries can be charging. If the grid fails it will transfer to inverter in a few milliseconds and the inverter will supply the load as long as the battery holds up. This is probably the thing you were thinking about.

Again I don't know if all inverters are the same, some may have more options than others but they are top end units and will be very versatile. If you have specific programming questions I am sure others will be able to answer that.

Flux

[Jason Wilkinson]

thanks again Flux