*3.3. Energy Payback Time*

To assess the Energy Payback Time (EPBT) of the project with solar PV system, the embodied energy of each component and process of the proposed solar PV system was calculated using experiments conducted by [36] on monocrystalline PV modules as these experiments did not take into account the critical component of BOS i.e., inverter and the embodied energy associated with inverter, these data were extracted from the findings of [38]. This embodied energy value is often used to evaluate the energy balance of the solar PV system [48] i.e., energy metrics e.g., EPBT [36].


The annual electricity generated by solar PV system (Eagen) was estimated to be 105,604 kWh Figure 5.

The *EPBT* was calculated using Equation (10):

$$EPBT = \frac{E\_{\text{mat}} + E\_{\text{manu}f} + E\_{\text{trans}} + E\_{\text{inst}} + E\_{\text{cal}}}{\frac{E\_{\text{avgen}}}{\eta \text{G}} - E\_{\text{OekM}}} \tag{15}$$

*EPBT* =1,000,513

> ((105,604/0.483) − 0)

*EPBT* of the proposed project was 4.59 years.

### *3.4. GHG Payback Time (GBPT) and Carbon Credits*

To assess the GBPT of the project, the emissions associated with the embodied energy of the proposed solar PV system were determined [41]. Calculated solar PV system emissions ranged between 50 g to 120 g CO2 eq. kWh −1 which agreed with the findings of [4] (69 g CO2 eq kWh−1, or 0.000069 tonne CO2 eq. kWh−1. The CO2 intensity of electricity in Ireland was 0.000437 tonne CO2 eq. kWh−<sup>1</sup> [5]. The annual electricity generated by solar PV system was estimated to be 105,604 kWh.

For the project the total embodied emissions, using results calculated from Equation (5):

*CO*2 *eq*·*embodied* = (Total embodied energy for all units *Ein* in kWh)·(solar PV system emissions) = 1,002,513 kWh \* 0.000069 tonne CO2 eq. kWh −1 = = 69.2 t of CO2 eq.

GHG payback time is defined as the number of years it takes solar PV system to pay back its embodied emissions through solar PV generation. The *GPBT* [41] is described by Equation (11).

For the project, the total GHG emissions avoided in a year:


Using Equation (11) for GHG payback time:

```
GPBT = 69.2 t of CO2 eq./46.15 t of CO2 eq
             = 1.5 years.
```
The total carbon credits earned from the project was calculated [36] based on the amount of CO2 mitigated by the project with solar PV systems at its current market trading price (€21 per tonne CO2 eq.) [43].

*Net CO*2 *mitigation* = (*CO*2 *eq avoided* (*year*) ·*TLS*) – *CO*2 *eq embodied* = (46.15 t CO2 eq.)·(30 years) – 69.2 t CO2 eq. = 1315 t of CO2 eq.

Carbon credits are awarded for the reduction in GHG emissions which can be traded in international market at their current market price and each ton reduction in CO2 is a carbon credit earned [43]. Thetotalcarboncreditsearnedisdescribed byEquation(12):

$$\text{Total carbon credits earned} = \text{Net CO}\_2 \times \text{height} \times \text{price} \tag{16}$$

where, *price* = Current market trading price (€21 per tonne CO2 eq.) [41]

=(1315 t of CO2 eq.)·(€21per t CO2 eq.) =€27,615

The yearly carbon credits earned is described by Equation (13):

$$\text{Yearly carbon credit earned} = \frac{\text{Total carbon credit earned}}{T\_{LS}} \tag{17}$$

= €27,615/30 years = €920

*3.5. Financial Assessment of the Solar PV System*
