(a) Such insurers shall have power to insure only real property and personal property incidental to such real property against the perils of fire, lightning and windstorm, together with perils customarily included under extended coverages.
(b) The maximum amount of insurance which the insurer shall retain on any 1 subject of insurance, after deduction of applicable reinsurance, shall not exceed 10% of its admitted assets or $5,000, whichever is the larger amount. For the purposes of this subsection a “subject of insurance” shall have the meaning ascribed in § 909 (limits of risk) of this title.