Concept:
Efficiency is given by:
\[
\eta=\frac{\text{Output power}}{\text{Input power}}\times 100
\]
Step 1: Given output power:
\[
P_{out}=1000\,W
\]
Step 2: Efficiency:
\[
\eta=80%=0.8
\]
Step 3: Use:
\[
\eta=\frac{P_{out}}{P_{in}}
\]
Step 4: Therefore:
\[
P_{in}=\frac{P_{out}}{\eta}
\]
\[
P_{in}=\frac{1000}{0.8}
\]
\[
P_{in}=1250\,W
\]
\[
\boxed{1250\,W}
\]