An excess is the amount payable by the insured and is usually expressed as the first amount falling due, up to a ceiling, in the event of a loss. An excess may or may not be applied. It may be expressed in either monetary or percentage terms. An excess is typically used to discourage moral hazard and to remove small claims, which are disproportionately expensive to handle. The term "excess" signifies the "deductible" or "retention".
A co-insurance, which typically governs non-proportional treaty reinsurance, is an excess expressed as a proportion of a claim in percentage terms and applied to the entirety of a claim. Co-insurance is a penalty imposed on the insured by the insurance carrier for under reporting/declaring/insuring the value of tangible property or business income. The penalty is based on a percentage stated within the policy and the amount under reported. As an example: a vessel actually valued at $1,000,000 has an 80% co-insurance clause but is insured for only $750,000. Since its insured value is less than 80% of its actual value, when it suffers a loss, the insurance payout will be subject to the under-reporting penalty, the insured will receive 750000/1000000th (75%) of the claim made less the deductible.