Graphical way of sizing a storage tank

[TSN79]

The attachment shows an example from a book on how to size a hot water storage tank (for tap water). During the 24h period a total of 55 kWh is used. The red curve shows the variation in usage along the period. The slope of the green lines is determined by the heat source's power, in this case 3,5 kW, which manages to heat up 55 kWh in 10 hours. Tangents are then placed along the curve as shown in the topmost example to determine how much water must be stored.

My question is this; how do I know where exactly to place the tangents? The book shows the topmost placing in this case but I don't see how this is obvious. The bottom example (by me) results in more storage but how do I know that's not the correct choice...?

 

[anorlunda]

The tangents are separated by a distance proportional to energy, in kWh. Note that the second plot, shows two lines separated by 2 kWh.

How to place the tangent? You must know your hot water needs. How many kWh per day of hot water heating do you need? Suppose that is 29 kwh, then one 29 kWh cycle per day serves that need. However, if the temperature drops too low between cycles, it might be better to split it into 10, 10 and 9 kWh smaller cycles.