PV system payback analysis

The financial analysis combines the impact of different system implementation costs (400€/m2 and 600€/m2) and sale prices of PV electricity to the grid, in a total of eight scenarios:

• 0.08€/kWh: the typical average cost of electricity bought to the grid

• 0.20€/kWh: an estimate of the corrected value of electricity bought to the grid, taking in account the rise of the oil barrel prices in the last years

• 0.40€/kWh: the typical value of renewable PV generated electricity sold to the grid in Portugal

If all PV produced electricity is sold to the grid, the simple payback periods of the PV systems are:
Figure 7 — Payback of the PV system if all electricity produced by the schools is sold to the grid


Alternatively part of the PV generated electricity may be used in the school, leading to different payback periods. Considering that all the electricity produced during the week days will be consumed in the school (a saving of 0.08 €/kWh on the electricity bill of the schools, 45% of the days in a year), and that for the reminder of the days, including weekends and holidays (55% days of the year) the electricity produced is sold to the grid at the three prices previous addressed, a new payback periods is obtained (see figure 8).

Figure 8- Payback of the PV system if part of the electricity produced, 45% days of the year, is used in the

school (the rest will be sold).


3. Conclusion

This study determined the following average values for the implementation of PV system in Portuguese high schools:

Table 2 — Summary table for this study

School Type

РУ Area m2

PV Capacity kW

Cost k€

Cost Є/m2 of floor area

Electrical demand reduction %









A, A+







B, A

The total PV potential of the 172 schools is approximately 144 000m2 with a capacity of generation of 25GWh/year.

The implementation of this systems is more favorable for pavilion schools due to their higher PV to floor area ratio and easier installation and maintenance of panels (flat accessible roofs).

The electrical energy autonomy values obtained, based on the energy consumption for a typical school do not consider the effect of additional optimization measures (lighting, natural ventilation etc.). It can be expected that, for the best cases, after those optimization measures, to achieve values of autonomy close to 100% (on a net yearly base).

If the electricity produced is sold to the grid at a realistic price, unsubsidized price of 0.20€/kWh, the simple payback varies between 11 and 17 years, for both scenarios considered. The payback varies notably with the cost of PV per square meter and with the PV electricity sale price to grid.


[1] Study for Parque Escolar: Avaliagao do Potencial Solar Fotovoltaico, NaturalWorks,


[2] Decreto-Lei 80/2006, de 4 de Abril, Regulamento das Caracteristicas de Comportamento Termico dos Edificios (RCCTE)

[3] Decreto-Lei79/2006, Regulamento dos Sistemas Energeticos de Climatizaqao em Edificios (RSECE)

[4] Decreto-lei n° 78/2006 de 4 de Abril aprova o Sistema Nacional de Certificaqao Energetica e da Qualidade do Ar Interior nos Edificios (SCE)

[5] European Union directive 2002/91/CE

[6] Solterm Software, INETI, Portugal

[7] Portuguese Energy Agency (ADENE): http://www. adene. pt/

[8] International Energy Agency, Portugal, Energy Balance, 2005.

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