Following up on "Which water heater types do you recommend?" and "Are tankless water heaters worth it in cold climates", is there a way to calculate the point at which installing a tankless water heater makes economic sense? Given the cost of purchase and installation, it should be possible to determine the break-even point, factoring in the amount of hot water used and the temperature change required.
Is there a way to estimate hot water usage separately from cold water? If I have a gas furnace and gas hot water heater currently, can the gas usage by each be separated?