Electricity is supplied to buildings in the form of 230 V a.c. mains. But solar PV panels produce d.c. so to use it, we must change d.c. into a.c. Fortunately, this is easy to do using an inverter and modern ones are very efficient.
Inverters match the frequency of the a.c. current produced to that of the mains. In the UK and Europe, it should be 50 Hz. The graph shows variation in the UK mains frequency over an hour in May 2012.
When the frequency is below 50 Hz, demand for electricity is high. When above 50 Hz, demand is low. National Grid must keep the frequency to within 0.5 Hz.
Frequency goes higher when there is less demand because generators spin faster - their energy is not taken away. Frequency drops below 50 Hz when demand is high, as more energy in needed to spin faster.
50 Hz means 50 "per second" so generators need to spin at 50 Hz too. This often described in terms of rpm - revolutions per minute. 50 Hz is produced by generators turning at 3,000 rpm.