Sample size of one, but for my solar array:
12/20/25 - 3/17/26: 1.2 MWh of power produced
12/20/24 - 3/17/25: 1.4 MWh of power produced
12/20/23 - 3/17/24: 1.4 MWh of power produced
So, call it a ~15% reduction in power produced due to cloudiness and snow cover between last winter and this winter (and the winter before that too). Feels worse than that though.