This SAE Recommended Practice provides a test procedure and performance guideline for evaluating passenger vehicle windshield defrosting systems. It is limited to results of tests that can be conducted on uniform test equipment in commercially available laboratory facilities. The current engineering practice prescribes that for laboratory evaluation of defroster systems, a known quantity of water shall be sprayed on the windshield to form an ice coating and then melted by the defroster under specific vehicle operating conditions. The procedure provides uniform and repeatable laboratory test results, even though under actual conditions such a coating would be removed by scraping before driving the vehicle. The performance obtained, therefore, does not directly relate to actual driving conditions, but serves as a laboratory performance indicator for comparing test results within or between systems. This document is intended as a guide toward standard practice but may be subject to frequent change to keep pace with experience and technical advances and this should be kept in mind when considering its use.