140 is a common number...anything over 120 (and in some places it doesn't matter, you) need a tempering valve to limit it to 120-degrees. Having higher storage temperature gives you a greater quantity of hot water since you'll almost always be mixing it with cold, either via the tempering valve or at the point of use. This means you don't need as much hot volume for things like a shower (makes no difference on the quantity of hot used to say fill a tub, though). Mine is set to 140. A higher temp also means things can't grow in the warm water, which is a good thing. A typical indirect WH has great insulation, so there is not a lot of standby loss verses say a gas fired WH.