*4.3. Energy Payback Time*

The EPBT for the solar installation for the project was 4.59 years i.e., it will take 4.59 years of operation of solar plant to generate the energy used to produce the system itself [4]. The EPBT of the

solar PV system decreases as module e fficiency increases, with an example of a rooftop mono-crystalline silicon PV system in Southern Europe (solar irradiation of 1700 kWh m<sup>−</sup><sup>2</sup> year<sup>−</sup>1) which has and EPBT of 2.5 to 3 years [39].

### *4.4. GHG Payback Time and Carbon Credits*

The GPBT for the solar installation for the project is 1.5 years. The life cycle GHG emissions from solar electricity production in Ireland are significantly lower than from electricity production on national grid. The net CO2 mitigated due to incorporation of solar PV electricity of the project over the lifespan of 30 years is 1315 t of CO2 eq. Carbon credits earned from the proposed project amounts to €27,615 which is approximately €920 per year.

The GHG emissions generally for wind and hydro power amounts to 6.2 to 46 g CO2 eq kWh−<sup>1</sup> and 2.2 to 74.8 g CO2 eq kWh−<sup>1</sup> respectively, wind power has the lower energy consumption and GHG emissions compared to solar PV system even though solar PV power has larger impact values due to module manufacturing process which has an general emission range of 2.89 to 671 g CO2 eq kWh−<sup>1</sup> (quantum dot to mono-si solar cells), but compares favourably to hard coal plant which has a general emission range of 750 to 1050 g CO2 eq kWh−<sup>1</sup> [39].

### *4.5. Financial Assessment of the Solar PV System*

The total cost of installing the solar PV system in an individual property was €4672 and for the project totaled €317,696. The NPV of the investment per property was calculated at −€1918.52, the negative NPV value means that the present value of the costs exceeds the present value of the returns at the current discount rate. In this scenario the investment cost would be repaid using the savings in electricity costs and payments for the excess electricity sent to the grid after 12 years.
