If $p$ is the actual number of panels, determine how to write the graph's $x$-value in terms of $p$. Do the same for $y$ and $E$.

Replace $x$ and $y$ in the displayed line-of-best-fit equation with expressions involving $p$ and $E$, then clear the denominator.

Algebra is faster here because the key task is converting the units on both axes. Use Desmos to verify the conversion with a point on the displayed line.

Enter $y=1.8x+4.5$. At $x=5$, the line has $y=13.5$. From the axis labels, this represents $p=50$ panels and $E=1350$ kilowatt-hours.

Enter each answer choice using $p$ as the horizontal variable. At $p=50$, only one choice gives $E=1350$, matching the converted point from the graph.

The graph uses $x$ to represent tens of panels, so $x = \frac{p}{10}$. It uses $y$ to represent hundreds of kilowatt-hours, so $y = \frac{E}{100}$.

The labeled line of best fit is $y = 1.8x + 4.5$. Substitute the scaled-variable expressions:

Multiply both sides by $100$:

This is choice C.