1) Converters and inverters are available to change DC into AC, and rectifiers change AC into DC.
2) Automatically switch themselves into the circuits if the normal electricity supply fails.
3) Small generator - alternator.
4) The time taken for the heat source to reach normality.
5) To restore small generators to service they could be started up as motors,
   with the normal energy source supplying no energy at all at first, 
   but then that energy is added slowly back to driving the generator normally.