The choice of power source usually depends on the total energy or the peak power required for the application, but may also be influenced by environmental considerations such as operating temperature and storage requirements. Power levels may range from a few tens of milliwatts for a simple electronic missile fuse, up to tens of watts for a more sophisticated missile fuse that has powerful radar or other sensors. Electrically-actuated missile control surfaces (e.g. as in Exocet, Harpoon, Martel) may require, intermittently, hundreds of watts. Electric propulsion of torpedoes may demand many tens of kilowatts.
Battery size is usually determined by the total energy to be stored, i.e. power x time, but there may be other more important considerations. For example, a modern undersea mine may need to operate for several years, and still have enough energy left at the end for detonation or sterilization. In such a case, the most crucial factor may well be the length of time for which the power supply has to sustain its energy.
The most critical stage in the duty of a power source often occurs towards the end of its life. For example, a guided missile uses most of its energy as it homes in on the target, making increasingly tight and frequent maneuvers. As with the undersea mine, the power source must still be able to meet the demands placed upon it right up to the bitter end and it is vital to allow a sufficient safety margin.