Onan 5500 marqui gen set stumbles on demand

Steve,

The microwave alone shouldn't cause a problem for the generator as even the most powerful models run about 1200 watts or about 10 amps. An air conditioner on the other hand, can draw considerably more power to run depending on BTU size; 15 amps and up is not uncommon (and even twice as much as that though only briefly when starting).

If these were the only things connected and on, the generator should be able to handle them - again depending on the BTU size of the AC unit. Generators state their capacity in Watts, but must of the devices we connect are in Amps. Calculating Watts in an AC circuit is complicated, but pretty close to the much easier Watts in a DC circuit for most residential settings. Here's how it's done:

Watts = volts x amps. Pretty simple stuff. Your generator is rated at 5500 watts. The microwave is say, 120 volts / 10 amps - which equals 1200 watts. The generator has 4300 capacity available now. Suppose your air conditioner is rated at 120 volts / 15 amps - which equals 1800 watts. 4300watts - 1800watts = 2400watts capacity is left. But, the starting current for the AC is as much as 25 amps for a few seconds (and once started drops back to 15 amps) - which means there's only 1200 watts capacity left. Add up the rest of the appliances you're running at the same time (TV set, Cable / Satellite box, stereo, toaster, lights, computer, etc.- you get the idea), and you can see how you might have exceeded the 5500 watts capacity of the generator for a few seconds. It's at these times that your hear / see the generator speed falter and sputter, lights dim, etc.. If this is happening a lot, you may need a larger or additional generator.

I hope this helps and good luck!

The generator is not capable of handling that much power. You can buy a giant Truck or Cat Battery and use an invertor and everything will be fine. You will use less gas as well.