It's a W-M WTGO tankless boiler so domestic hot water is included. There's only my wife & I and some weekend days the 2 grandkids are over and get a bath. Hot water usage is primarily kitchen, bathroom sinks and occasional full baths & showers (not every day). Not a lot of hot water used.
Our oil costs (~2712/yr) are consistant since we have been under their fuel cap for last and this season.
There is one problem with the hot water delivery plumbing. The mixing valve gets corroded and the built up residues affects it's behavior. If newly adjusted so water out of kitchen faucet is steaming after a couple of months it's still plenty hot but not steaming anymore. This problem is due to the "crap" in our city water supply. Also the burner relief valve shows lots of corrosion and discoloration after a while.