This depends entirely on your load so there is no exact answer. 400 Watts is anywhere from 400-800 rpm and 500 Watts is anywhere from 500-1000 rpm.

"This depends entirely on your load" I generally know what is meant by 'load', or at least think I do, but before I prove myself the village idiot, can you tell me what you mean by how much load?

Joe, I think my answer was not very good. It was late and I was tired so I will try to do a better job explaining. Generally people use PMA's in this power range to charge battery banks so I will use that as my example. First, we have to acknowledge the fact that the battery is a huge load and clamps the voltage of the generator at the voltage of the battery. If our PMA is spinning unloaded, (not connected to anything) it will produce approximately 60 volts at 500 rpm. Now, if it is connected to a 12 volt battery bank, our PMA will make 12 volts at 500 rpm. And if the PMA is connected to a 24 volt battery bank, the PMA will produce 24 volts at 500 rpm. So you see that the battery or battery bank clamps the voltage. Now, the other part of the equation is amps. We need to know how many amps the PMA will make at a certain rpm. Now this is not as simple as I am going to make it sound. I am going to simplify this as there are some other factors involved. But lets assume our PMA makes about 12-25 amps at 500 rpm into a battery bank(the amp range is so large because a nearly completely drained battery bank wants more amps than a nearly completely full battery bank.) So now we have to look at the equation that tells us how much power is being produced: Power = Volts x Amperage (P=VI). Using the information above, a 12 volt battery bank (which is more like 14 volts) could make anywhere from 168 to 350 Watts. And a 24 volt battery bank (which is more like 27 volts) can make anywhere from 324 to 675 Watts. This should give you an idea of what I am talking about. It depends on where you are sending the electricity to. And, if it is a battery, it depends also on the battery's state of charge. Note, there are some things I simplified here but the general idea is true. But do not hold me to these exact numbers.