15kw? A 003a (or 803), will more than suffice when used in a single house application. The 15kw water cooled and up are prone to wet stacking if not loaded enough. My 003 runs water heater, water pump, sewer pump, dryer, washer, oil furnace, freezer, 2 refrigerators, electric oven, all lights, and then some. I rarely see the load meter over 100%. Not all listed loads are on at one time, but I have never had the 003a bog down under these loads when run as needed (just grunts a little and picks back up). The 003a is less prone to wet stacking as it is air cooled and runs hotter.
I also have a 002a as backup which will handle most loads with load management. Of course, there is another 002a and 003a in the garage (backup to the backups, and spare parts, etc).
David