DIAGNOSIS Yeah, this one's common on initial setup. It's almost always the governor not being properly adjusted or the carburetor needing a break-in period. The engine RPM (and thus frequency) is hunting, which causes the voltage to surge.
FIX First, make sure it's on a level surface and you're running it at least 10 feet from any structures for proper airflow. Start it and let it run at no load for 5-10 minutes to warm up. Then, with the eco-throttle switch OFF, plug in a steady, substantial load (like a space heater). The governor should settle after a minute. If it doesn't, there's a manual governor adjustment screw on the carburetor linkage, but turning it requires a special tool and is best left for a second step.
IF THAT DIDN'T WORK If it still surges after warm-up under load, the issue might be stale fuel from factory testing or a slightly misadjusted carburetor jet from the factory. Running a full tank of fresh, ethanol-free gas with a carb cleaner additive (like Seafoam) through it often clears this up after a few hours.
HEADS UP Don't try to adjust the governor or carburetor yourself if you're not comfortable—you can make it worse. has the procedure, but it's easy to mess up the 3600 RPM setting.