In the middle of winter in Massachusetts, solar panels produce far less than the middle of summer (meaning a fraction of the energy). A couple of days can be lost to snow cover (not a huge loss due to less production), but when the sun returns, it penetrates the snow and creates a thin layer of water on the panels. Typically with a day or two of sun, all the snow slides off because the panels are smooth, the roof is pitched, and the layer of water lubricates the snow's movement.