Accurate, by definition, means
traceable to standards. And so the first question I have is to have you define the required accuracy and the standards organization to which it must be traced. (NIST? DIN? BSI? etc.) Separately, just the concept of taking a measurement is little easier. Any standard zero-cross SSR will trigger the load when you want it applied (half-cycle.) That's easy enough. And you can cobble up your own so that you get both the active SSR behavior and also a zero-cross signal to trigger an ADC board's sample-gathering. Should work, I think. No product recommendations.