sounds like your poco doesnt record interval or power data, just a monthly read of total energy.
realistically, the fastest way to determine this is probably to throw amp meters on main feeders (measure each phase separately, at the same time), and ask the customer to turn on EVERYTHING at once (except for loads that would never run simultaneously, like heat and cooling). Its possible but unlikely (if they only take monthly readings) that your utility meter displays instantaneous kw draw. Metering both phases will also give you a good sense of whether your panel is balanced within the generators specified tolerance (probably not a big issue, 65kw should comfortably cover most typical single-fam homes). You'll have a real, measured value in about 10 minutes.
You could also do this by summing nameplate ratings or measured draw of all equipment (this is worst-case-scenario; could apply derating factors a la NEC220 to get a number more realistic to how customers actually use energy).
also, just an opinion, but at 65kw a single family can probably get by even if it doesn't cover 100% max-full-load in the house (how often do you turn on every electric load you own simultaneously?) by either giving some customer education ("don't heat your pool to 115 while running your cooling and all the shop tools in your garage at the same time on gen") or by installing a critical load panel (or some other way to either shed or interlock loads during outage - "sorry, you can't run your hottub, electric dryer, and welder at the same time on backup")
