Homework problem: A transformer on a pole near a factory steps the voltage down from 3900V to 220V. The transformer is to deliver 1180kW to the factory at 91% efficiency. Find the power delivered to the primary in kW. What I've done: Well, I thought that power in equals power out. So I worked all the math to prove that and used P=IsVs, solved for Is then used that to find Ip (Ip=VsIs/Vp). Then found P=IpVp. I got 1180kW! I then calculated in the efficiency--not correct. And then in the follow up question, I found out that my Ip isn't even correct. What's wrong here?