Overpaneling has two related meanings; both refer to having more solar panel output than a system might otherwise require.
Note: overpaneling is not overvolting, which would be exceeding the maximum input voltage of the charge controller. Overvolting often results in damage to the controller; it can be caused by linking too many panels in series, or running too close to the voltage max in conditions that raise the panels' Voc.1)
Overpaneling, or having “too much” panel for the rating of your MPPT charge controller, can be desirable in some situations. It is a trade-off that biases the setup to get more power out of normal and marginal conditions at the expense of the peak output during optimal conditions.2)
In car terms, the overpaneled system trades away some of the panels' top-end output for more low- and mid-range grunt. – secessus
The overpaneled system's power output bell curve is shifted upwards and cut off by the controller when it reaches its limits. This is known as clipping, trimming, or clamping due to the “flat top” appearance of the overpaneled output curve.
With appropriate overpaneling the clipping is relatively minor. Victron notes that:
Generally total energy harvested from a 130% panel oversizing results in less than 1% annual energy loss. – source
Example: Consider the image to the right.3) The smaller/interior bell curve is the normally paneled configuration.
The larger/exterior curve is the overpaneled configuration.
The striped area is the peak power “clipped” by the controller when the controller is overpaneled.
The horizontal line is the peak output of the normal and overpaneled configurations; they are both capped at the controller's rated output.
The green area is the additional power produced during suboptimal conditions. These areas are sometimes called shoulders due to their shape.
Overpaneling might be a good fit for you if your MPPT controller is designed to clamp down on excessive power without damage and
Overpaneling can increase heat in PPT charge controllers
This occurs because the PPT controller moves Vpanel toward Voc to reduce current when less is needed but battery voltage is high, as in late Absorption. For this reason adding some load to the system in Absorption can reduce PV voltage (and therefore CC temps) to lower levels.4)
[This section is based on a forum post - secessus]
Perhaps counterintuiitively, overpaneled systems in normal5) conditions don't need to constrain power that often. A couple reasons for this:
[note: this section was based on a forum post and is therefore more personal in style – secessus]
The ability of MPPT controllers to clip power is a function of their ability to select arbitary powerpoints, thereby forcing the panel to produce the amount of power associated with a given powerpoint.
If the controller was rated at 10A it would run the panel wide open at Vmp (18.9v). That would mean ~7.7A going into the battery.6)
But that can't happen because the 5A controller is undersized. 5A output at our present 13v charging voltage would be 65w, quite a bit down from the rated 100W. The controller protects itself by forcing the panel to produce less power.7) This is called clamping, clipping, current limiting, overcurrent protection and a few other marketing terms.
The dotted green line is Pmax (max power). The solid green line is the “power curve”. The heavy horizontal black line is at 65w.8)
The intersection of Watts and Volts on the curve is a power point.9)
In this case the controller wants to limit panel output to 65w So it runs the panel at 21v, which is a far cry from Vmp. Then it downconverts the excess voltage to amps: it charges the battery12) 5A @ 13v. This same mechanism is used to reduce power in Absorption and Float stages.
If you have the curve and Vpanel you can derive the power output.
If you have the curve and you know Wpanel you can derive Vpanel.
If you have a lot of time and patience, you can plot Vpanel and Wpanel to draw the curve yourself.
An MPPT controller is always Tracking and using powerpoints, not just when the Maximum Power Point is required.
In this configuration there is abundant solar power and relatively little bank capacity. This is sometimes called underbanking.
Overpaneling the bank gets lead-acid batteries charged faster14) and keeps them charged longer than other approaches. This minimizes battery murdering partial state of charge conditions found in overbanking.
Overpaneling the bank would work best when:
Since the underbanked system has less capacity will reduce your actual reserve days of power to some extent. But during daytime there is some solar power coming in and an overpaneled configuration will harvest relatively more power in sub-optimal conditions. The smaller bank will get through Bulk and well into Absorption faster, too. This will help extend the effective reserve days of power.