5.1 Energy Payback Time (EPBT)

Energy Payback Time (EPBT) is the time required for any energy producing system or device to produce as much energy as was required in its manufacture. For solar electric panels, this is normally 6 to 36 months. (Source : http://www.solarmango.com/dictionary/energy-payback-time Links to an external site.) So, if the EPBT of a PV system is 3 years, we get energy free of cost for 27 years assuming that the life time of the system is 30 years.

$EPBT\left(year\right)=\frac{Invested\:Energy\:\left(Mj\:m^{-2}\right)}{Annual\:Energy\:Generated\:by\:the\:System\:\left(Mj\:m^{-2}\:yr^{-1}\right)}$

$Annual\:Energy\:Generated\:by\:the\:System=I\times\eta\times PR\:\slash\:\varepsilon$

ε = electrical to primary energy conversion factor;
I = total solar insolation incident on the unit-surface, per year (MJ m^-2 yr^-1 );
η = average module efficiency (%);
PR = system performance ratio (%);

(Source : http://astro1.panet.utoledo.edu/~relling2/PDF/pubs/life_cycle_assesment_ellingson_apul_(2015)_ren_and_sustain._energy_revs.pdf Links to an external site.)

EPBT of Photovoltaics 

Rooftop PV systems produce net clean electricity for approx. 95 % of their lifetime, assuming a life span of 30 years or more.

Depending on the technology and location of the PV system, the EPBT today ranges from 0.7 to 2 years. 

(Source : https://www.ise.fraunhofer.de/de/downloads/pdf-files/aktuelles/photovoltaics-report-in-englischer-sprache.pdf Links to an external site.)