Although optimum electricity generation occurs with bright and sunny weather Solar Panels will still produce electricity on a cloudy day. Cloudy days provides sufficient diffuse light which is enough for Solar Panels to generate electricity.
It is estimated that under overcast conditions the modules produce half of the expected power whilst a dark overcast day may only produce five to ten percent of the expected electricity.
Renewable energy systems still remain connected to the grid therefore if electrical needs exceed the solar power output the power will be taken from the grid in the usual way. This also works in reverse as any excess electricity which is produced is exported back into the grid, see export tariff.