Spacecraft vibration qualification is traditionally performed using sequential single-axis sine tests, which may not accurately replicate the multi-degree-of-freedom dynamic environment encountered during launch. Especially, under-testing and/or over-testing might occur. A different approach would be to reproduce the transient launch events on a multi-axis shaker platform using a time waveform controller. The severity of transient events is commonly captured by the shock response spectrum, which is described by a single-input single-output transformation and does not account for the cross-correlation among different axis. This paper proposes an energy-based metric to evaluate the severity of multi-degree-of-freedom transient environments, in particular with respect to sequential single-axis tests. The severity is assessed through the relative input energy, which inherently accounts for cross-axis coupling and dynamic interactions. A simplified 6-degree-of-freedom model demonstrates the method’s capability to compare qualification and operational load cases. This approach quantifies the degree of under-/over-testing with respect to a reference environment.