Why does my solar output vary so much?

First check what the reading actually represents: panel power, inverter output, or power going into the battery. Compare readings taken at the same time, and note sunlight, shading, battery charge level, and whether the controller is charging or holding the battery at its target voltage. If the battery is full or nearly full, the controller may reduce panel output because there is less need to charge.

The panel rating is a peak under particular test conditions, not a promise of constant output. Sun angle, temperature, clouds, and dirt all reduce production. Still, a very low reading in strong sun with an unfilled battery is worth investigating.

Shade can matter more than it looks like it should. In a series-connected string, one shaded panel can pull down the string, and bypass diodes limit some effects but do not make shading irrelevant. Panel differences can also matter depending on how the strings are arranged.

Check the wiring diagram and panel specifications against the charge controller's allowed input voltage and current. If you can, compare each string's voltage and current under similar sunlight, without disconnecting live wiring. That helps separate a shading or string issue from a general drop in sunlight.

If PV voltage is collapsing while current is high, or a cable or connector is getting hot, treat that as a safety issue rather than just a performance mystery. Stop using the affected circuit and let it cool. Once safely isolated according to the equipment instructions, have the connectors, terminations, cable sizing, and controller limits checked. A loose or undersized connection is one possibility, but the readings alone do not identify the cause.

Also verify that the panel strings are configured within the controller's limits. If the heating or abnormal readings return, get a qualified installer or electrician to inspect it instead of repeatedly restarting the system.