You actually are using the wrong formula for the kinetic energy. Use Einstein's, not Newton's. Your answer is two orders of magnitude low.

0.0453592 kg at 0.50c= 6.304 times 10^14 joules, according to Wolfram alpha. 500 grams of antimatter is capable of producing 9 times 10^17 joules, therefore, 500 grams can propel over 100 such objects.

 In 2009, humanity produced about 9 times 10^19 joules of electricity.