## 2 Drivers of Photovoltaic Technologies  
The price of electricity produced by a system (e.g., photovoltaic systems) is usually determined by a system LCOE analysis that allows different production methods to be compared [4; 5; 6]. The LCOE represents the total cost of a system (brought back to present value) divided by the total amount of energy produced during the system service life. Considering inflation and discount factors, LCOE equations can be complicated.  
If the system service life is \(n\) years, the price of energy can be expressed in simplified form as the sum of the total cost of acquisition and operation of the system (investment costs \(IC_{k}\), including interest and inflation in individual years and operating costs \(OC_{k}\) in the \(k\)th year of operation) and the energy \(EP_{k}\) produced in the \(k\)th year of operation.  
Since photovoltaic systems are normally commissioned and installed in much less than a year, the LCOE can be expressed in a simpler form  
\[LCOE=\frac{\sum_{1}^{n}IC_{k}\ +\sum_{1}^{n}OC_{k}}{\sum_{1}^{n}EP_{k}}=\frac{ IC+n\cdot OC_{AV}}{EP}, \tag{1}\]  
where \(IC\) is the total purchase price, \(OC_{AV}\) is the average annual operating cost, \(n\) is the service life of the system (in years), and \(EP\) is the total energy produced by the equipment over its service life.  
Different categories can be used for investment cost analyses. Usually, system costs are divided into module costs and balance of system components (BOS) costs (this division is historical from the times when module costs were the largest part of system costs). System components' market prices are relatively stable. However, in addition to these costs, there are also so-called soft costs, which represent costs associated with the project, land, administration, margins, taxes, financing, etc., which may vary depending on the type of installation and the installation location. As reported in [5], soft costs may represent from 20% to 60% of the total investment costs. When systems are installed on the roofs of buildings, these costs usually do not include the land costs (or the lease costs), so the soft costs are then limited significantly. A typical division of the costs of a 500 kW\({}_{\text{p}}\) commercial roof system within the Czech Republic is shown in Figure 2.  
For a commercial roof system, the installation cost can be written as  
\[IC=MC+BC+SC, \tag{2}\]  
where \(MC\) is the module cost, \(BC\) is the BOS cost (including cable, construction, transport, installation, and inverter costs) and \(SC\) represents the soft costs. With respect to the module parameters--the module output power and module efficiency, both at standard testing conditions (STC), module size, module weight, etc.--and the warranties for the module parameters, it is useful for further analysis to use a different non-module cost structure [6].  
Figure 1: The developments of the cumulative capacity and annual production.  
Module cost is defined as the cost of unit power (measured under STC), usually counted in USD/W\({}_{\text{P}}\). The cost of some BOS components is associated with the power of the unit (inverters, transformers, switchboards, etc.); the cost of another BOS component is associated with the area requirement per unit of power (i.e., the required area of the supporting structures, the length of cabling, etc.). Part of the BOS cost is, therefore, inversely proportional to the efficiency of the module. Accordingly, the use of less efficient (albeit cheaper) modules may not reduce investment costs [7]. From that observation, it follows that the three most important drivers of photovoltaic technology are PV module cost, PV module efficiency, and PV module service time.