𝒟Battery=ExponentialDistribution[1/300];
𝒟Solarpanels=ExponentialDistribution[1/1000];
𝒮battery=StandbyDistribution[𝒟Battery,{𝒟Battery}];
ℛsatellite=ReliabilityDistribution[solarpanel∧battery,{{solarpanel,𝒟Solarpanels},{battery,𝒮battery}}];

μ=NExpectation[t,t\[Distributed]ℛsatellite]

Plot[SurvivalFunction[ℛsatellite,t],{t,0,800},Filling->Axis,PlotStyle->Thick]

Plot[Evaluate@ImprovementImportance[ℛsatellite,t],{t,0,800},PlotLegends->{"Solar panel","Battery"},Filling->Axis]