The strength of the sun determines the energy provided, and for that the 2 main factors are aspect and inclination.
This being my first year with solar (fitted in March) I'm taking a close interest in how they perform during late autumn/winter. Using an app like 'Daff moon' can tell you the max height of the sun on any given day and time for your location. I've noticed my 4.2kwh setup was peaking at 4kwh on Sept 21st (equinox or near enough) with the sun at 38°, 2.5kwh today with the sun at 24°, and I'll be surprised if I get much more than 1kwh on Dec 21st when the sun peaks here at 14° (and that will not be for more than a few hours max by then). There's doubtless calculators out there, but the principle is the lower the sun, the far more atmosphere and pollution there is in the way, and at very oblique angles most of the light fails (except the red bit).
I'm currently waiting for my system to be doubled to 8kwh, which should easily be able to cope with my house and vehicle needs.