Tin=1250;
cm=0.01;
a=50cm;
b=50cm;
c=70cm;
Rt1=(d1/(λ1*S1));
S1=2*((a+d1)*(b+d1)+(a+d1)*(c+d1)+(c+d1)*(b+d1));
d1=4cm;
dataPlastizol={{1000,0.36},{1100,0.4}};
λ1=Fit[dataPlastizol,{1,T},T]/.T->1250;
Print["λ1= ",λ1," W.(m^(-1)).(K^(-1))"];

Rt2=(d2/(λ2*S2));
S2=2*((a+2d1+d2)*(b+2d1+d2)+(a+2d1+d2)*(c+2d1+d2)+(c+2d1+d2)*(b+2d1+d2));
dataSibral={{600,0.147},{800,0.207}};
λ2=Fit[dataSibral,{1,T},T]/.T->650;
Print["λ2= ",λ2," W.(m^(-1)).(K^(-1))"];

S3=2*((a+2d1+2d2)*(b+2d1+2d2)+(a+2d1+2d2)*(c+2d1+2d2)+(c+2d1+2d2)*(b+2d1+2d2));
Tpovrch=50;
To=20;
α=6;
Qk=α*S3*(Tpovrch-To);
σ=5.67*(10^(-8));
ϵ=0.9;
Qr=ϵ*σ*S3*((((Tpovrch+273))^4)-(((To+273))^4));
rce=Qk+Qr==((Tin-Tpovrch)/(Rt1+Rt2));
dosd2=Solve[rce]//Last;
Pel=Qk+Qr/.dosd2;
Print["Pel= ",(Pel/1000)," kW"]

Plot[{Sin[t],0.8*Sin[t-0.86]},{t,0,2Pi},PlotStyle->Thick]