On a grid-tie solar panel array all the electricity created which is not used by the home is feed back into the electrical grid.

However, on a off-grid system with batteries - where does the extra amperage created by the solar panels go if the batteries are full?

  • 1
    \$\begingroup\$ Your question contains an incorrect assumption. The charge controller for any larger system typically modulates the charge current so that the solar panel output voltage goes up and solar panel output current goes down. So there is no "extra amperage." There are some charge controllers designed for lower power systems which simply shunt the extra current to force the solar panel voltage below some level. So for those, the answer is, it goes through the shunt. \$\endgroup\$
    – user57037
    Sep 14, 2015 at 19:16
  • \$\begingroup\$ If I turn off the tap, where does the water go? \$\endgroup\$ Sep 15, 2015 at 2:31

1 Answer 1


The charging circuit won't draw much current from the PV cells when the battery is fully charged. If there is current capability left over from the demand at any given time, it's not used.

Just like a battery doesn't discharge (much) when you don't draw current from it- the voltage will increase towards the open-circuit voltage and the current will drop towards zero.

Edit: Here is the curve of the cell. Maximum power and no power points are indicated (the far left bottom point is also no power, which would correspond to shorting the cell as mkeith indicates some controllers do).

enter image description here

  • \$\begingroup\$ So the amps the solar cells create will simply remain in the panels much like a disconnected solar panel in direct sun? \$\endgroup\$
    – Xeoncross
    Sep 14, 2015 at 19:27
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    \$\begingroup\$ @Xeoncross: no, amps is a measure of moving charge. No charge moved, no amps. \$\endgroup\$
    – PlasmaHH
    Sep 14, 2015 at 19:30
  • \$\begingroup\$ See the above edit- the current will drop to zero if no current is being drawn. \$\endgroup\$ Sep 14, 2015 at 19:40

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