The circuit breaker is probably about 15 amps. A 15-amp circuit breaker will often go off at around 12 amps. To calculate your amps, divide the watts by 120, ie your voltage.

Your lights are probably about 4 amps total, and fans are about 1 amp each for large box fans. Smaller fans would use less. This still leaves you with about half your total amperage to run your garage door opener on. If you know the power of the motor on your garage door opener (probably about 1/2 horsepower) you can calculate the watts by multiplying this by 750. So your garage door opener is probably about 3 amps, which is still a safe margin. And even if you do trip the circuit breaker, you can always reset it.

I have never heard of federal agents busting such a small grow. Usually they go after the more commercial operations.