That's just the raw kinetic energy a projectile travelling at such high speeds will also 'stretch' along its direction of movement because of the effects of relativity, basically the round will already be inside the ship by the time it hits the shields. Now for the Mass Effect side of things and I love BioWare because they gave out the numbers for the mass and speed of a mass accelerator round as 25 kg moving at one third of the speed of light so the math is pretty simple.Į=25kg*1/3c^2 which gives me 2.49654216316*10^17 joules divided by 4.184*10^15 joules per megaton gives me 59.6687897504 megatons per shot from the main cannon of an Everest class ship. Star wars has never been clear about the destructive force of the weaponry used what I've heard and read has put SW weaponry between a couple hundred kilotons and ten to thirty megatons, with the exception of the Death Star (which is probably in the range of teratons) it's pretty safe to say even the thirty megaton range is pretty high given the fact that it has been seen that Imperial forces were operating within a few kilometers ( at fifty MT Tsar Bomba had a total destruction radius of thirty five KM) of an orbit to surface strike therefore the power behind the weapons at impact must not be greater than that of tactical nukes, however in space laser based weaponry may be slightly more powerful as the laser beam is moving through a vaccum and will not lose as much energy as it would moving through an atmosphere.

Star wars vs mass effect