How much performance do you want? A trainer needs less power than a 3D aerobatic flyer.
You can start with the standard "input watts per pound" guidelines that have been around for quite a while (see the "Watts per pound/ounce").

When you have some idea of how many watts per pound (or per ounce) you need, the next step is to check your model's weight with batteries. This can just be an educated guess; it doesn't have to be an exact weight. Now start looking at brushless motors to check the range of watts output.

Use these standards to determine the type of performance you can expect from new power system. For light park flyers, use the watts/oz. column.
Watts/lb.           Watts/oz.           Type of aircraft
50 - 70 . . . . ... . . . .4.35 - 5.65 . . . . . . . . .Trainer & slow-flying scale models
50 - 70 ..................3.125 - 4.35 . . . . . . . .Minimum power level for decent performance; good for lightly loaded slow and park flyers
70 - 90 . . . . . . . . . .4.35 - 5.65 ..................Trainer & slow flying scale models
90 - 110 . . . . . . . . .5.65 - 6.87 . . . . . . . . .Spor1 aerobatic & fast scale flyers
110 - 130 . . . . . . . .6.87 - 8.15 . . . . . . . . .Advanced aerobatic & high-speed models
130 - 150 . . . . . . . .8.15 - 9.35 . . . . . . . . .Lightly loaded 3D models & ducted fans
150-200+ . . . . . . . .9.35 - 12.5+ . . . . . . . .Unlimited performance 3D models

Use these standards to determine the type of performance you can expect from new power system. For light park flyers, use the watts/oz. column.

Watts/lb.  ....... Watts/oz.                      Type of aircraft
50 - 70..........4.35 - 5.65..........Trainer & slow-flying scale models
50 - 70 .........3.125 - 4.35........Minimum power level for decent performance;                           good for lightly loaded slow and park flyers
70 - 90..........4.35 - 5.65..........Trainer & slow flying scale models
90 - 110........5.65 - 6.87..........Spor1 aerobatic & fast scale flyers
110 - 130......6.87 - 8.15..........Advanced aerobatic & high-speed models
130 - 150......8.15 - 9.35..........Lightly loaded 3D models & ducted fans
150-200+......9.35 - 12.5+......Unlimited performance 3D models

The instructions with many motors don't show how much power in watts you can expect from them, so you have to do a little math. Most motor stat sheets (they're often online) give the motor's battery-cell count-voltage. It's also important to know the continuous and maximum current, in amps, a motor can safely draw. Multiplying the amps by the voltage gives you the motor's wattage.
Example: let's say that your motor will run on a 2- or 3-cell LiPo-a voltage of 7.4 or 11.1 volts, respectively. The motor specifications suggest a continuous current of 10 amps. Multiply the volts by the amps and you get 74 and 111 watts, depending on the voltage used. If you power a 10-ounce model with this motor and a 2-cell battery, it would have enough power to do advanced aerobatics. With a 3-cell pack, that same motor would be powerful enough for unlimited 3D performance. That's all there is to selecting the best motor for your aircraft; now you have to match the ESC and battery to the motor's current draw.

Now that you've chosen a motor, select- ing the battery and ESC is a snap. First, consider the battery. From my example, we know that the motor needs to pull at least 10 amps continuously from the bat- tery and that it can run on a 2- or 3-cell pack. The performance you want will determine how many cells you should use. But you have to pick a pack that can supply the amps, or current, you want. You do this by checking the pack's "C" rating. Multiply this rating by the pack's capacity in mAh to determine how many amps can safely be pulled from the pack.

An 800mAh pack with a 10C rating can provide only 8 amps continually
(10xO.800 = 8 amps). If your motor draws 10 amps, the pack will get hot and most likely become bloated and unusable. But if you use an 800mAh battery that has a 20C rating (20xO.800 = 16 amps), it will safely provide 16 amps continually and is safe to use. You can also use a pack with a larger capacity of, for example, 1500mAh with
the same 10C rating. It will safely provide 15 amps of continuous power and will also work well with a 10A motor setup.
The advantage of a pack with a higher C rating is that it's lighter and smaller, but there's a downside-shorter flight times.

Once you know the motor's requirements, deciding which ESC to use is easy. For the ESC to work with your
motor, it must be rated for voltage of your LiPo pack, and it must be able to handle the motor's amp draw. Using my example, the correct ESC for your motor would have to be able to handle at least 10 amps continuously and work, with 7.4 to 11.1 volts (2- or 3-cell LiPos). It is always better to have an ESC that is rated slightly higher than the amps you really need. But don't exceed three times the required amperage, and remember that larger ESCs weigh more.
Keep in mind that these are guidelines, and when you have your motor system in place, you'll be able to measure the current going through it. Variables such as prop size can push a motor's amp '" draw above the recommend limit, and ~ that subjects the entire power system to !! unnecessary stress and wear. The only'::) precise way to test your system is with a watt meter (see the sidebar "Use a watt-meter").
If you follow these guidelines, you'll be able to select a safe and suitable power system for your park flyer. You won't
have to rely on the electrics gurus at the field for help. You'll do it yourself.