Power Jack 300W Grid Tie Inverters normally get around 78% efficiency at full output (220W to 250W).

They are much more efficient at lower power levels, typically under 100W. And that's where they rate them for marketing purposes, of course.

Today I was measuring stuff and measured a small panel I have hooked up to it's own GTI. I made several measurements and came up with about the same results every time.

16.3V at 3.5A = 57.05W input to the GTI.

GTI output (from the Kill-A-Watt) 52.0W.

52 / 57.05 = 0.9114 = 91% efficiency!

Nice, and the power company will never know what hit them!