In many cases (mine included), SAD is not so much about how much light we get, but about when: I manage my sleep/wake cycle by getting a good dose of light in the morning (I do my morning E-mails with a nice strong blueish LED-lamp next to my monitor) and dimming/reddening my screens in the evening.

It is not even the level of darkness outside, but the direction of the level change: I need my lamp from October to January (or when I have a gig in Norway in the winter) but already at the start of February I feel the light coming back, even though it is as dark outside as at the end of November

So, in summary: it is more about the timing of the light than the amount (but YMMV, of course)