figure out the watts you need to fly it.....then figure out the flying time you want and the weight you you can have left.

how much power ya want. for simple figure 750 watts a hp, then figure out the weight. there are lots of programs out there on google that will tell ya what ya need.

First, you need to know about how heavy your plane will be (including the motor and battery). Then you need to know the type of flying you are going to be doing, park flyer, powered glider, sport flyer/trainer, scale or 3D aerobatic. For each type of flying you will want different amounts of Watts/lb. A small park flyer will need about 70 watts/lb Sport, scale and powered gliders will need about 120 watts/lb Aerobatic planes will need at least 160 watts/lb 3D aerobatic planes will need 200+ watts/lb Once you know how many watts you need you can then choose the motor. Most motor manufacturers will recommend a range of battery voltages and capacities for each motor.

Then select the battery size for how long you want to fly. Battery capacity (watt-hrs) = Volts * ah (not mah) So if your battery is a 3S, 2200 mah, battery capacity is 11.1v times 2.2 amp-hour (2200 mah) = 24 watt-hrs. This implies that you can draw 24 watts for 1 hour from this battery. However if your motor draws 100 watts then you can fly for a maximum of 15 minutes. Flight time in hours = (battery volts * amp-hrs)/(motor watts) = (24 watts-hrs)/(100 watts)=.24 hrs = 15 minutes