Adhesion requirements for photovoltaic modules to ensure reliability are often discussed but not well defined, neither in terms of tests nor actual requirements. This paper presents a new approach for realistic assessment of the adhesion strength, which shows the conventional peel test may not ensure reliability. The test presented reproduces the actual adhesion requirements for fielded modules much more closely than the commonly used peel testing. The test is conducted in situ during standard damp-heat test at a temperature of 85 °C and 85% relative humidity, with the modules installed at an angle to give an appropriate force vector perpendicular to the backsheet. This is achieved by attaching weights to the back of the tested samples which are mounted with a 45° angle on a testing rack in the environmental cabinet. With an appropriate weight holder, this could be done as part of the standard damp-heat cycle during certification and would not involve additional testing time nor significant changes in the commonly used racking. This approach will identify the weakest interface of the multilayer encapsulation system. A number of test-to-fail bespoke samples are tested to set realistic fail criteria. It is shown that the test allows discrimination between different samples and can identify unsuitable production processes.