The reason why the maximum supply power is different between summer and winter is that a periodic test for {TAIL ~ the power stations} is carried out in winter with less power demand in preparation for summer with {HEAD ~ more power demand} 
The reason why the maximum supply power is different between summer and winter is that a periodic test for { TAIL ~ the power stations } is carried out in winter with less power demand in preparation for summer with { HEAD ~ more power demand }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD