A motor efficiency determination system comprising: a motor configured to receive power from a power supply; a plurality of sensors (20,22) configured to acquire voltage and current data, wherein the plurality of sensors are positioned external to a motor; and a processor programmed to: determine a plurality of stator input currents of a motor in operation; determine electrical input data of the motor in operation, wherein the electrical input data comprises at least one of an input power P input and a plurality of stator input voltages; determine a rotor speed ω r of the motor in operation based on one of the input power P input and the plurality of stator input currents; determine a value of stator resistance of the motor in operation based on at least one of a horsepower rating and a combination of the plurality of stator input currents with the electrical input data; and determine an efficiency η of the motor in operation based on the determined rotor speed, ω r the determined value of stator resistance, the determined plurality of stator input currents, and the determined electrical input data;. wherein the efficiency η of the motor is determined according to the equation n = Poutput Pinput, wherein the output power Poutput is determined from no-load losses and an air-gap torque determined by: T ag = 3 ⁢ p 6 i a - i b ⋅ ∫ v ca + R s ⁢ 2 ⁢ i a + i b ⁢ dt + 2 ⁢ i a + i b ⋅ ∫ v ab - R s ⁢ i a - i b ⁢ dt wherein i and ν are current and voltage values, p is the number of poles in the motor, and R s represents the determined stator resistance. a motor configured to receive power from a power supply; a plurality of sensors (20,22) configured to acquire voltage and current data, wherein the plurality of sensors are positioned external to a motor; and a processor programmed to: determine a plurality of stator input currents of a motor in operation; determine electrical input data of the motor in operation, wherein the electrical input data comprises at least one of an input power P input and a plurality of stator input voltages; determine a rotor speed ω r of the motor in operation based on one of the input power P input and the plurality of stator input currents; determine a value of stator resistance of the motor in operation based on at least one of a horsepower rating and a combination of the plurality of stator input currents with the electrical input data; and determine an efficiency η of the motor in operation based on the determined rotor speed, ω r the determined value of stator resistance, the determined plurality of stator input currents, and the determined electrical input data;. determine a plurality of stator input currents of a motor in operation; determine electrical input data of the motor in operation, wherein the electrical input data comprises at least one of an input power P input and a plurality of stator input voltages; determine a rotor speed ω r of the motor in operation based on one of the input power P input and the plurality of stator input currents; determine a value of stator resistance of the motor in operation based on at least one of a horsepower rating and a combination of the plurality of stator input currents with the electrical input data; and determine an efficiency η of the motor in operation based on the determined rotor speed, ω r the determined value of stator resistance, the determined plurality of stator input currents, and the determined electrical input data;. wherein the efficiency η of the motor is determined according to the equation n = Poutput Pinput, wherein the output power Poutput is determined from no-load losses and an air-gap torque determined by: T ag = 3 ⁢ p 6 i a - i b ⋅ ∫ v ca + R s ⁢ 2 ⁢ i a + i b ⁢ dt + 2 ⁢ i a + i b ⋅ ∫ v ab - R s ⁢ i a - i b ⁢ dt wherein i and ν are current and voltage values, p is the number of poles in the motor, and R s represents the determined stator resistance.