This depends on where you live and the orientation of your roof. There is standard data table for the UK that provides one single factor for Cumbria. Installers have to use this factor, which will result in an estimate that a 4.44kWp system on a south-facing roof in the north-west will produce 3,890kWh per year.  

However, there is a variance across Cumbria since some places are sunnier or cloudier than average. So be mindful that the quote you receive might be an over- or under-estimate of what your system will produce. You can explore the differences across Cumbria on this map.