Ditto Rich's replies. Our boat came with a 26 gallon holding tank and seems adequate for several days of cruising. Yes, ours is noisy for that brief period while it macerates, brings in sea water and sends bowl contents to the tank. I tell guests to operate the flush for five seconds, which seems to work.On the Raritan Sea Era the motor is a dual shaft- it runs the input pump for seawater and the macerator at the same time. Yes it is a bit noisy, but only for a few seconds.
Power consumption is minimal. Since I cruise solo most of the time, being an old backpacker, I use a pee bottle most of the time, or just go over the side. Digging a cat hole for number two used to work in the bush, but on the water not so much.