To compare the area needed for solar panels to meet the United States' energy requirements to the combined size of all Walmart parking lots, we need to estimate the total area of Walmart parking lots.

As of 2021, there were around 4,756 Walmart stores in the United States. The size of a Walmart parking lot can vary depending on the store format and location. Let's assume an average parking lot size of 4 acres per store (an acre is a common unit of area used in the US). This is a rough estimate, considering that Supercenters might have larger lots, while Neighborhood Markets and smaller format stores might have smaller lots.

1 acre = 4,047 m², so:

4 acres/store * 4,047 m²/acre ≈ 16,188 m²/store

Now we can estimate the combined area of all Walmart parking lots:

4,756 stores * 16,188 m²/store ≈ 77,000,000 m² or 77 km²

Recall that we calculated that approximately 80.3 km² of solar panels would be needed to fulfill the entire energy requirements of the USA. Comparing these values, we can see that the total area of solar panels required (80.3 km²) is slightly larger than the combined area of all Walmart parking lots in the United States (77 km²).

This comparison demonstrates that it would take a vast area of solar panels, roughly equivalent to the total area of all Walmart parking lots, to meet the energy needs of the United States. However, it's important to remember that this is a simplified analysis, and factors such as energy storage, transmission losses, and seasonal variations in solar irradiance would influence the actual required area.