As note the primary issue with interstellar travel, for the passengers, is not speed or power, it's fuel.
You really don't want to accelerate much over 1G, but even at 1 G acceleration it would take approximately 1 year to accelerate to light speed... of course, relativity jumps in as you get near light speed, which actually works to the advantage of the passengers, so their perceived speed becomes many times the speed of light (as time slows for them) but of course the increase in mass hurts acceleration, I think, relatively, not sure (?).
Then of course, you have to spend exactly half of the trip decelerating, so you have to consider that.
As noted, fuel is the primary issue. You would need constant acceleration and deceleration for the entire trip. As noted above, something like nuclear is the only current option. Even still, I am pretty sure you only need the fuel based on the crews time perspective (?).
Here is my query to Googles AI, for half the trip, so double these numbers, so about 6 years for those on earth and 1.5 years for those in the ship, not really that long! It should be noted that longer distances will not take relatively (!) longer times, they will get progressively more efficient as trips get longer because of relativity (more % of time spent near light speed), earth perspective wise though... say goodby. I broke the AI asking about 12.5 light years (half the distance to Vega) because it assumes passing light speed, which of course is not possible.
Traveling 2.1 light-years at a constant acceleration of 1 g is a challenging task due to the limitations of relativity. As an object approaches the speed of light, its mass increases and its acceleration decreases. This makes it impossible to reach the speed of light with a finite acceleration.
However, we can approximate the time it would take to travel 2.1 light-years at 1 g acceleration using a simplified model that assumes the spacecraft can accelerate at a constant rate for the entire journey. In this model, the spacecraft would reach a speed of approximately 0.999c (99.9% of the speed of light) after about 1.14 years. At this speed, it would take an additional 2.07 years to travel 2.1 light-years, for a total travel time of approximately 3.21 years from the perspective of an observer on Earth.
From the perspective of the astronauts on the spacecraft, however, the journey would seem much shorter due to the effects of time dilation. Time dilation is a consequence of special relativity, which states that the passage of time is relative to the observer's motion. As an object approaches the speed of light, time appears to pass more slowly from the perspective of an observer at rest.
In this case, the astronauts would experience about 4.4 times less time dilation than an observer on Earth. This means that their journey would seem to last only about 0.73 years.
So, while it would take an observer on Earth about 3.21 years to travel 2.1 light-years at 1 g acceleration, the astronauts on the spacecraft would only experience about 0.73 years of time passing. This is a remarkable consequence of relativity, and it highlights the challenges of interstellar travel.