Balancing storage can be calculated using the mass curve method.

A mass diagram is the plot of accumulated inflow (i.e. supply) or outflow (i.e. demand) versus time. The mass curve of supply (i.e. supply line) is, therefore, first drawn and is superimposed by the demand curve. The procedure to construct such diagram is as follows:

From the past records, determine the hourly demand for all 24 hours for typical days (maximum, average and minimum).

Calculate and plot the cumulative demand against time, and thus plot the mass curve of demand.

Read the storage required as the sum of the two maximum ordinates between demand and supply line.

Repeat the procedure for all the typical days (maximum, average and minimum), and determine the maximum storage required for the worst day.