Lower body lifting is indeed important if one of your goals is to increase power. You need at least 6 weeks of lifting at a minimum to have gains and improvement. But your exercises or sets/reps ratio needs to be varied or else your body will become accustomed to what you're doing and plateau.
I do a 12 week strength training program over the winter. During that time I'm also doing some tempo or sub threshold work on the bike to work on my aerobic base, leaving all the "hard stuff" for the gym 2 or 3 days a week. Once that 12 week strength training is over for me in the late winter, my bike workouts change to start incorporating more threshold work and adding some neuromuscular workouts (e.g. 10 second sprints every few minutes for the duration of a ride). And then once the spring and summer role around (for me, it's racing season) then my workouts decrease in volume but increase the intensity through lots of sprint and short effort workouts (i.e. high power/high force).
Having it set up in this way helps your body to translate the strength you've acquired from lifting to putting force through the pedals and applying it directly to propelling yourself faster and harder on the bike.
This year I'm also going to try something new throughout the season by replacing one bike workout per week with explosive plyometrics coupled with power lifts. I want to see if it helps me with creating force better than just solely training on the bike.
Another thing to remember, though, is not to focus on how terrible and slow and sluggish your legs feel right now in the off season/during the winter. That's not the point of what you're doing right now. This time - in the big picture - is to rip and strain your muscles so that they repair and become stronger. Not to always ride well and fast all-year-round. If you're a competitive cyclist or if you have certain goals then you need to always remind yourself of the big picture and not so much the moment. It's okay if your legs are aweful right now. They're supposed to be! The only way your body adapts to stresses (i.e. you get better) is through stressing it!!