How can I determine how much power my solar panel outputs during winter?
it depends a lot from angle you put the panel relative to Sun's elevation angle. Someone might say or write: "solar panels next to nouthing power in winter beacouse Sun's light comes at a very steep angle", that is not particularly thue, especially if solar panel is aligned perpendicular to incoming light.  equasion should be the following: E=CTcos(Sang-PVang)Pnom(1-W), where C is a constant (I beelieve in 1/3 to 1/2 range), T - time (per day) Sun shines on solar panel, Sang - Sun's maximum angle above horizont, PVang - angle beetween vertical line and solar panel's plane, Pnom - rated solar panel power and finally W - amount of time that weather conditions cover the Sun. Equasion gives E - energy anaiable from solar panel in 1 day.