So I did some math... if someone makes a Mr. Fusion... and you use one can of beer... the potential energy...
E=MC^2 so… we assume 371.38g of beer (I took the weight of 12 fluid ounces of water and did some guestimation since beer isn't pure water)
3.3378e+16 joules
2.4618e+16 foot-pounds
7.9775e+3 kilotons of TNT
7.9775 megatons of TNT
3.16362398 x 10^13 BTU
So
9.27166665 × 10^9 kwh or 7.97753345 × 10^12 kilocalories
Which is 9,271,666,650 kwh where the U.S. used an estimated 3.741 trillion kWh in 2009. So .002% of the U.S. energy consumption for 2009. Now... that's 1.243350979e+10 horsepower hours.
It’s also 9,271.66665 jigawatts Marty
So I was thinking about what such a device as the Mr. Fusion would mean as far as power for space travel...
SpaceX's Falcon 9 rocket carries 395,700kg of fuel (119,100 Kilograms of Rocket Propellant 1 and 276,600kg of Liquid Oxygen). Let us assume that a Mr. Fusion operated at 13% efficiency (roughly what commercial solar panels operate at currently), this would give us the equivalent energy of 1.1050e+6 megatons of TNT which is 1,284,255.55 jigawatts Marty!
So mass equivalent to the weight of fuel in a a Falcon 9 would provide 1,284,255.55 terawatt hours of energy for our spacecraft at 13% efficiency. The ENTIRE PLANET used an estimated continuous average of 12.3 terawatts in 2013 (as in 12.3 terawatts being drawn every second of every day during the year) meaning a bit shy of 12 years worth of electricity for the planet, remember that's with 13% efficiency on that Mr. Fusion.
In the above chart, not my math, a 1g constant acceleration would get a ship to Proxima Centauri in less than 10 years ROUNDTRIP (for whoever, or whatever was on the ship, relativity and all). Now that you have essentially unlimited electrical power you'd just need some sort of propulsion system to use it!