Energy storage problem
Advanced mechanosynthesis can provide today's missing link of efficient chemomechanical conversion. This makes long term storage of huge amounts of energy possible. Additionally chemical energy is easier to transport than electrical energy - even today (oil tankers).
Unlike other performance specs like transmittable power density. The energy density of chemical energy storage is (with gasoline and explosives as good examples) pretty much at it's limits with our current (2016) crude technology.
Advanced atomically precise technology allows now forms of chemical energy storage that at the cost of slightly lower energy density are far safer and allow energy conversion efficiencies very near 100%.
For energy densities higher than chemical only nuclear is possible. How and in how far advanced atomically precise technology may enable us using some nuclear physics as a bidirectional battery is very unclear at this moment (2016). See APM and nuclear technology for related highly speculative thoughts.