Hello,
I have a Kill-A-Watt device that can measure instantaneous Watts, Volts, Amps etc....but what Ireally need to determine is the Peak or Surge watts.

One example would be my refrigerator.

I want to determine what appliances I can safely run with my generator so I need to know not only their running watts, but more importantly, the surge requirements.

How would I build such a device or is there something already available that doesn't cost a fortune?

 

You could use a current transformer and an oscilloscope.
There are also some peak reading multimeters that may do what you want.

 

It's impossible to tell exactly given every generator has a different limit on both how much additional power its engine has and to how far above its rated maximum electrical capacity it can go. Some have very little and can actually be grossly over rated while others may have huge reserve capacity.

As for your refrigerator figure a 3 - 5x peak over its rated current draw for startup but that it only lasts for a fraction of a second to which most engine driven gensets will handle that just fine given the amount of rotating mass (flywheel effect) they have in play that can easily absorb that sort of very short duration overload.

Something solid state power inverters do not have and thusly why the need to be considerably oversized to run electric motor loads.

With any decent grade of generator the common rule of workable load capacity factoring is to add up the combined wattages of every thing you anticipate to have attached to it and running at once and have a generator that is at a best running at 3/4 it rated capacity to carry it.
Or reduce your anticipated loads to not be much over 3/4 of what the generator is rated to put out.