灯泡L1标有“6V 6W”,故额定电流为:I1= P1 U1 = 6W 6V =1A;电阻为:R1= U1 I1 = 6V 1A =6Ω;灯泡L2标有“12V 6W”,故额定电流为:I2= P2 U2 = 6W 12V =0.5A;电阻为:R2= U2 I2 = 12V 0.5A =24Ω;把它们串联后接入电路,电流相等,最大电流为I=0.5A,故两灯允许消耗的最大电功率为:P=I2(R1+R2)=0.52×(6+24)=7.5W故选:C.