Needs["VectorAnalysis`"]

SetCoordinates[Cartesian[x,y,z]];

Div[λ[T[x]]*Grad[T[x]]]

D[λ[T[x]]*D[T[x],x],x]

datλPlastizol={{900,0.39},{1000,0.41},{1100,0.44}};
proklad=Fit[datλPlastizol,{1,T,(T^2)},T];
pl2=ListPlot[datλPlastizol];
Plot[proklad,{T,800,1300},PlotStyle->Red,AxesLabel->{"T[°C]","λPl[W.(m^(-1)).(K^(-1))]"}];
Show[%,pl2]
λPl=proklad/.T->1300

<<PhysicalConstants`

σ=StefanConstant[[1]];

Tpec=1300;
TokoliKonvekce=25;
TokoliSalani=25;
lambda=.2;
a=0.4;
cm=0.01;
dPlastizol=2cm;
Sstr=6*0.5*((((a+dPlastizol))^2)+(((a+dPlastizol+diz))^2));
Svnejsi=6*(((a+dPlastizol+diz))^2);
α=5;
ϵ=0.9;
rceUstalene={P==((Tpec-Tplast)/(diz/(lambda*Sstr))),P==Svnejsi*(α*(Tplast-TokoliKonvekce)+ϵ*σ*((((Tplast+273))^4)-(((TokoliSalani+273))^4)))};

rce[dizz_]:=rceUstalene/.diz->dizz
res[diz_]:=FindRoot[rce[diz],{P,1000},{Tplast,100}];
vykon[diz_]:=P/.res[diz];
teplotaPlaste[diz_]:=Tplast/.res[diz];
Plot[teplotaPlaste[diz],{diz,0.1,0.5},PlotStyle->Red,AxesLabel->{"diz[m]","Tpl[°C]"},AxesOrigin->{0.1,0}];
Plot[vykon[diz],{diz,0.1,0.5},PlotStyle->{Red,Thick},AxesLabel->{"diz[m]","P[W]"},AxesOrigin->{0.1,0},GridLines->Automatic]

xmax=0.15;
ro=100;
c=500;
rceFK=ro*c*D[T[x,t],t]==lambda*D[T[x,t],{x,2}];
poc=T[x,0]==TokoliKonvekce;
okrIn=-lambda*D[T[x,t],x]==(Pzvoleny/Sstr)/.diz->xmax/.x->0;
okrOut=-lambda*D[T[x,t],x]==α*(T[xmax,t]-TokoliKonvekce)+ϵ*σ*((((T[xmax,t]+273))^4)-(((TokoliSalani+273))^4));

Pzvoleny=4000;
NDSolve[{rceFK,poc,okrIn,okrOut},T[x,t],{x,0,xmax},{t,0,5*3600}]