some's solar sales rep are not convinced with breaker derate load calcs , they just want to derate the breaker to 150A from 200A. NEC based load calcs doesn't support that derate, they even challenged me to visit home and run all the laods and read that instatenous Kw from meter, i was not sure we could do that at first place.
Very few (if any) utility meters care about truly instantaneous power. It is possible to measure it of course, but the typical granularity that a utility would care to measure, is demand, which is a 15-minute interval power data. That is, when applicable to the service agreement with the utility. This is the most granularity you need to care about for purposes of justifying reducing a main breaker from 200A to 150A. This would ignore the short spikes when motors start-up and draw their inrush current, but that kind of short term load exceeds its own branch breaker trip rating all the time. Breakers take several minutes to trip under moderate amounts of overload, so the standard 15 minute demand interval is realistic to consider for justifying changing your main breaker rating.
However, it is rare that this data will be available at the scale of a residential application, as most meters for services that small will only measure cumulative kilowatt-hours, rather than demand interval data. High power commercial services might have demand data available already, but for most residential applications, the meter isn't sophisticated enough to keep track of that detail of the data. If necessary, you can put your own customer-owned recording meter on the service and log the interval data. There are meters that can measure this data as granular as 5 minutes.
