Panel output in terms of kWh?
This depends upon the time of year – in summer you would expect almost 10 times more solar energy that we get in wintertime. In the mid summer you might expect to get 14kWh of heat per day from a single 20tube panel. In December this could average only 1.15kWh per day. In winter it is still possible to preheat water to 40C or greater in the winter sunshine. If multiple panels are fitted then it is possible to heat water to 100C in January (This has been achieved in Cornwall).
