Ventricular volumetric ejection fraction (VV EF) is often normal in patients with single ventricle circulations despite them experiencing symptoms related to circulatory failure. We sought to determine if kinetic energy (KE) could be a better marker of ventricular performance. KE was prospectively quantified using 4D flow MRI in 41 patients with a single ventricle circulation (aged 0.5 - 28 years) and compared to 43 healthy volunteers (aged 1.5 - 62 years) and 14 patients with left ventricular (LV) dysfunction (aged 28 - 79 years). Intraventricular end-diastolic blood was tracked through systole and divided into ejected and residual blood components. Two ejection fraction (EF) metrics were devised based on the KE of the ejected component over the total of both the ejected and residual components using: (1) instantaneous peak KE to assess KE EF or; (2) summating individual peak particle energy (PE) to assess PE EF. KE EF and PE EF had a smaller range than VV EF in healthy subjects (97.9 {plus minus} 0.8% vs. 97.3 {plus minus} 0.8% vs. 60.1 {plus minus} 5.2%). LV dysfunction caused a fall in KE EF (p =0.01) and PE EF (p =0.0001). VV EF in healthy LVs and single ventricle hearts was equivalent however KE EF and PE EF were lower (p <0.001) with a wider range indicating a spectrum of severity. Those reporting the greatest symptomatic impairment (NYHA II) had lower PE EF than asymptomatic subjects (p =0.0067). KE metrics are markers of healthy cardiac function. PE EF may be useful in grading dysfunction.