How many watts load can you use with a 30amp trailer?

It depends on the voltage of your electrical system. The basic formula is watts = volts X amps. So, if your voltage is 120V then the absolute maximum watts available would be 3600 (30 * 120 = 3600). Note that this is the point at which the breaker would trip, so you could not count on 3600 watts for more than a few minutes. A good rule of thumb is to limit your normal load to 80% of the maximum available. In this case, 80% of 3600W is 2880W. If you put a frozen dinner in the microwave for 5 or 6 minutes and happened to push the watts up to, say, 3500, you would probably get away with it. But try the same trick with the air conditioning that runs for an hour or two and you will trip the breaker. Keep the watts below 80% and you can run forever.