The reaction of 1 kg of antimatter with 1 kg of matter would produce 1.8×10**17 J (180 petajoules) of energy (by the equation E=mc²). This is about 134 times as much energy as is obtained by nuclear fusion of the same mass of hydrogen (fusion of 1H to 4He produces about 7 MeV per nucleon, or 1.3×10**15 J for 2 kg of hydrogen). This amount of energy would be released by burning 5.6 billion liters (1.5 billion US gallons) of gasoline (the combustion of one liter of gasoline in oxygen produces 3.2×10**7 J), or by detonating 43 million tonnes of TNT (at 4.2×10**6 J/kg). Some researchers claim that with current technology, it is possible to attain antimatter for US$25 million per gram by optimizing the collision and collection parameters (given current electricity generation costs). Antimatter production costs, in mass production, are almost linearly tied in with electricity costs.
Assuming the economical mass production of antimatter was possible it would cost $25 billion to make 1 kg of antimatter which is equal to 1.5 billion gallons of gasoline. So for mundane energy usage other forms of energy storage are better. It is a net energy loser. However, a lot of relatively inexpensive antimatter would be great for space propulsion systems and for antimatter catalyzed fusion and fission.
Molecular nanotechnology could reduce electricity costs with mass production of high efficiency solar cells on earth and in space.
Antimatter harvesting could also be cheaper and more efficient using molecular nanotechnology. 20 kilograms are within the orbit of Saturn. This does not have to be an energy loser but it would require being very clever to find an efficient harvesting system. Perhaps having superconducting antimatter traps that moved around the gravitational manifolds in the solar system. (Interplanetary superhighway)
Here is a pdf that talks about a development path to get the Z Pinch system to enabling fusion power generation. Molecular nanotechnology could help make better targets for the z pinch, better lasers, better magnets and better chambers.
Shortly after (or before, but definitely after) real molecular nanotechnology arrives, then we should finally get commercial fusion power generation, over 10 times our current power generation for super-efficient solar power, and kilograms of antimatter.
A pdf from 2006 that discusses antimatter production