A solar cell will output a constant amount of voltage if it is exposed to light. This voltage is about 0.5VDC. Different voltages are achieved by wiring cells in series. If you measure 2.5VDC on a cloudy day (assuming all electrical connections are good in the panel) you will get 2.5VDC on a sunny day. The amount of light and the area of the silicon effect the amperage.
In short, your panel is not working properly and should probably be replaced.
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