how can we calculate the total utility if there is money saving?

Answer 1

What do you mean, saving money gives you a certain amount of utility. Add this to your other utility and you got total utility

Answer 2

Total utility is based on the valuation (or utility a person gets from doing a certain activity) they have for their choices of consumption.

In this case total utility would be equal to the utility gained from consumption plus the utility gained from saving.

Keep in mind that utility of saving may not necessarily be equal to the amount saved. If they have high discount rates (ie they want to consume today vs. tommorow) then the utility from saving will be less than the utility from spending. The opposite is true as well, someone who wants to save (low discount rate) will gain more utility from saving than spending. A lot of this will depend on the marginal utility gained. The marginal utility from saving will equal the marginal utility of consumption at equilibrium. Just calculate the area under the line at each of these points for consumption and savings to figure total utility.