Hi Bill

There is an easy answer and a technical answer to this query?

The easy question to this is speaking to your installer/company who fitted it. They should be able to give you an estimate of performance for your house/location and system type at various times of the year.

They do this using sophisticated software. A good example of this is TSOL http://www.solardesign.co.uk/tsol.php

If you are looking for a more technical response, then more details of testing your PV panel system or installation can be located here:

http://www.electricalreview.co.uk/features/7586-119795

Hope this helps.