Datasheet of solar panels

Larger format solar panels these days are typically tested and sorted for output (have a bit "better" panel, put it in the next higher output bin and charge more for it).
 
Remember that you could have a -3% Vmp and and +3% Imp and get Pmp=specification.

More or less, +/- 3% is probably about the best accuracy you are going to get with field DMMs (digital multi-meters) and leads. Not to include the variation of sun/solar angle/atmosphere/cell temperatures, etc...

From what I have seen--Just using the GT Inverter panel, more or less, you can start questioning output when it falls by 25%, and "know" something is bad when you see 50% of expected output.

Without using a reference cell/known good panel, etc... you are going to get somewhere around 2% to 5% accuracy (at best) on power accuracy.

If you look at a Power curve (part of the current vs voltage curve in the older paper specifications), you see that the Power curve is a relatively "flat" knee at max power... if you have a panel with -3% current, it can "move" the operating point to +3% on the voltage curve.

Modern MPPT (maximum power point tracking) controllers measure/estimate the Vmp/Imp for the array, at that point in time, and work to extract the most energy available (based on sun, temperature, etc. at that time).

For PWM (pulse width modulated) charge controllers, as long as Vmp>some minimum number, then the current is Imp at that level of solar energy (sun at 50%, Imp is 50% of rated output).

The specifications on the back of the panel are tested under laboratory test conditions ("flash sun test" for a few seconds that does not heat panel, STC standard test conditions). Those numbers are not what you will see. For example Vmp is very temperature sensitive... As temperature goes up, Vmp-hot falls. On a very hot day in full sun and no wind, you can lose almost 20% of Vmp-stc rating. Imp is also somewhat temperature sensitive and does rise a little bit as the panel gets hot.

In real life--For any moderate to hot environment, we use a derating of ~81% of Pmp for our calculations (and another ~5% for charge controller deratings for an overall derating of ~77% to battery / DC bus power). This also allows for a bit of dust on the panels, etc.

You can get Pmp or even more, but it pretty much has to be sub freezing weather and/or at high elevations (less atmosphere for sun to shine through),

On some panels these days, you will find a PTC rating--Which is supposed to be the actual Vmp/Imp/Pmp under a set operating conditions (warm weather, full sun, panel/cells get hot from sun). It is a closer to real life number and how (for example) California, rates their installations for solar rebates.

-Bill