Northwest Energy Efficiency Alliance (NEEA), July 30, 2015, by Nicholas Kvaltine and Ben Larson (Ecotope, Inc.)
http://neea.org/docs/default-source/reports/laboratory-assessment-of-stiebel-eltron-accelera-220-e-heat-pump-water-heater.pdf?sfvrsn=4

"The Northwest Energy Efficiency Alliance (NEEA) contracted with Ecotope, Inc. and Cascade Engineering Services, Inc. to conduct a laboratory assessment of the Stiebel-Eltron Accelera 220 E heat pump water heater (HPWH) for northern climate installations. Cascade Engineering evaluated the Accelera 220 E using a testing plan developed by Ecotope to assess HPWH performance. Overall, the results suggest that the Stiebel-Eltron Accelera 220 E is a highly-efficient water heater."