Existing　research　on　phased-mission　system　reliability　has　mainly　focused　on　internal　failures.　However,　some　practical　phased-mission　systems　such　as　photovoltaic　based　electric　power　generation　systems　can　also　be　subjected　to　external　impacts.　This　paper　models　the　reliability　of　a　photovoltaic　based　electric　power　generation　system　considering　both　internal　failures　and　external　impacts　that　affect　the　failure　rate　of　photovoltaic　modules.　Two　different　cases　are　considered　when　modeling　the　system　reliability,　where　the　number　of　elements　required　during　different　phases　are　the　same　in　the　first　case　and　the　number　of　elements　required　during　different　phases　are　different　in　the　second　case.　The　configuration　of　photovoltaic　arrays　as　well　as　protection　efforts　on　each　photovoltaic　module　are　further　optimized　to　improve　the　system　reliability.　Moreover,　the　expected　system　loss　is　also　modeled　and　optimized　when　the　system　demand　is　not　satisfied.　Examples　are　presented　to　illustrate　the　applications　of　proposed　models.