The losses of a heat storage depend primarily on the so-called A/V ratio, i.e. the ratio of storage surface (A) to storage volume (V). The storage surface determines how large the losses are (double the surface leads to double the heat loss). The volume determines the energy content of the storage tank. For very large storage tanks, the A/V ratio is very small and the heat losses relative to the energy content are also small. In addition, heat losses depend strongly on the thickness of thermal insulation. This cannot be estimated across the board and depends on the manufacturer of the storage tank. In summary, the storage losses depend on:

Ratio of storage surface to storage volume (A/V)
Temperature of the medium in the storage
Thickness of thermal insulation
Ambient temperature
Temperature stratification in the storage

How large are the storage losses in heat storages?

In the nPro tool, the losses of a thermal storage can be defined by specifying the relative loss per time, e.g. "1 % per day". This means that the storage tank content decreases by 1 % over one day. Thus, if the storage is fully charged, the loss is higher than if the storage is almost completely discharged. This makes sense because a storage tank at a high temperature has higher (absolute) storage losses than a storage tank close to ambient temperature. In Table 1 rough estimates are given, which can serve as a first orientation, if no further information about the storage is available. The data are based on own calculations and literature data. However, since numerous assumptions had to be made, e.g. on the storage tank temperature or insulation thickness, the data is only a rough guideline. For small storage tanks with up to about 1 m³, the estimate "10 % per day" can be assumed. For seasonal storages, the figure "35 % per 365 days" can be found in the literature.

Table 1: Heat losses depending on storage size (orientation values)