What is your supply water temperature year-round? Does it change much in the winter? Compare your incoming water temp with the specs to see what temperature rise you can get out of it, it may be okay in the summer if the water temps are higher, but not in the winter. And, it's not the duration of the draw, but the maximum draw you might want. You can't prevent someone from turning on the washing machine, dishwasher, or a faucet to wash their hands while you are in the shower. If you have a boiler, an indirect may be much more economical, and the tanks are smaller (typically) than a stand-alone tank.