Equivalence number method

As with the other cost allocation methods, the conservation of the cost sum applies, that is:

${\displaystyle {\text{sum of cost input}}={\text{sum of cost output}}}$

The cost of the main product, usually for the product with the highest physical or economical output, receives for example the equivalence number 1. On the basis of selected indicators (average market prices, physical properties, etc.) other equivalence numbers are formed, using suitable ratios between the different co-products. Multiplying the equivalence numbers by the production or sales figures results in the allocation keys for a specific product type. From this the cost of a co-product can be calculated, both for main and by-products.

An airline can determine the cost of the transportation service by dividing air freight and passengers by weight. The average passenger weight of booked seats is to be compared to the weight of the loaded air cargo containers.

• a_pass = m_pass / (m_pass + m_freight)
• a_freight = m_freight / (m_pass + m_freight)

In a refinery, one can assume the input as crude oil and as output gasoline, diesel and heavy fuel oil as well as (flare) losses. The equivalence number method can use the energy content of the products as an allocation key. E is the product of energy density and production quantity.

• a_gas = E_gas / (E_gas + E_diesel + E_HFO)
• a_diesel = E_diesel / (E_gas + E_diesel + E_HFO)
• a_HFO = E_HFO / (E_gas + E_diesel + E_HFO)

In the cogeneration plants, the Carnot method allocates the fuel to the products useful heat and electrical work. The weighting key is the exergy content of the output energies.

• a_el= η_el / (η_el + η_c × η_th)
• a_th= (η_c x η_th) / (η_el + η_c × η_th)

In the alternative generation method, the key is thermal and weighted electrical efficiency, where the weighting factor is the ratio of thermal to electrical reference efficiencies (γ = η_{th, ref}/η_el,ref).

• a_el= (γ η_el) / (γ η_el + η_th)
• a_th= η_th / (γ η_el + η_th)

Criticism of the equivalence number method is justified by the fact that completely arbitrary and random keys can be chosen. For example, in the case of allocating the potable water bill in a house with only one common meter, the water consumption could be divided according to the number of occupants per apartment or the apartment's net dwelling area in m².

From a one-dimensional input I, a two-dimensional output is assumed with O₁ = f₁(I) * I and O₂ = f₂(I) * I.

Note: One interpretation for f is a conversion efficiency from the input to the respective output. More than 2 co-products are also conceivable.

The costs k₁, k₂ are the variable costs of the two outputs which need to be determined. k_I represents the known variable costs of the input. K^var denotes the respective sum of the variable costs. a₁ and a₂ are the allocation factors for the respective output, i.e. they describe the proportion of the input that is assigned to a co-product.

The weighting keys are f₁ and f₂:

${\displaystyle K_{1}^{var}=a_{1}\cdot K_{I}^{var}={\frac {f_{1}}{f_{1}+f_{2}}}\cdot k_{I}\cdot I\quad {\text{respectively}}\quad K_{2}^{var}=a_{2}\cdot K_{I}^{var}={\frac {f_{2}}{f_{1}+f_{2}}}\cdot k_{I}\cdot I}$

This results in specific variable costs k₁ and k₂:

${\displaystyle k_{1}={\frac {K_{1}^{var}}{O_{1}}}={\frac {K_{1}^{var}}{f_{1}\cdot I}}\quad {\text{respectively}}\quad k_{2}={\frac {K_{2}^{var}}{O_{2}}}={\frac {K_{2}^{var}}{f_{2}\cdot I}}}$

According to the introducing relation of the cost allocation, the following applies:

${\displaystyle k_{I}\cdot I=k_{1}\cdot O_{1}+k_{2}\cdot O_{2}\quad {\text{or}}\quad K_{I}^{var}=K_{1}^{var}+K_{2}^{var}}$

• Günter Wöhe: Einführung in die allgemeine Betriebswirtschaftslehre (DE: Introduction in general business economics). 18th edition. Vahlen, München 1993, ISBN 3-8006-1728-5.

• Cross-multiplication (or rule of proportion)
• Cost allocation