When you hear the words electric RC aeroplane, the first thing that comes to mind is a park flyer. These planes are light weight, small, and can be flown anywhere. However, there are electric motors on the market for RC planes of all sizes, so you are no longer limited to park flyers. In fact, most gasoline-powered planes can be converted over to electric, making it easier to get your aircraft off of the ground.
Look at the plans or manual that came with your plane. Usually they specify what size electric engine to use. The size will be listed under kV, which is the voltage constant. This will tell you how many RPMs the engine turns per volt. It could also have a specified torque constant, or Kt. One Kt equals 1355 kV.
Weigh the plane. If you are new to flying or want a plane that flies simply, the measurement to go with is 100 Watts per pound of plane. If you want a plane that does tricks and flies at higher speeds, use a 150 to 200 Watts per pound ratio.
Measure your propeller. For a propeller with a large diameter and pitch, you will need an engine capable of producing more RPMs. Otherwise, the propeller will damage the motor by drawing too much power. Once you have the amount of wattage needed, multiply it by the battery voltage and you know how much power you need.