I have built a plumbing test rig for my project. I have a couple of electrical devices on this rig and need to find out the efficiency of them/the rig. I have looked online to find equations and have found all the relevant material except for the useful output energy. in all the examples they seem to just give out this information and not tell you how to obtain it. HOW DO YOU WORK OUT THE useful energy output? I have a 3 port valve with a power consumption of 6.5W and switching current of 2.2A There is almost a cylinder thermostat with a switching current of 3A I am also using a 20amp junction box (not sure if this effects anything) and a 3amp plug. using P= V x I i worked out the pwer for the wholse system to be 1.196 watts % energy efficiency = (Useful energy produced × 100) / Total energy used THANKS!