It's in the name. Solar panels need sunlight to generate electricity. The more sunlight hits a panel, the more energy it produced
The sun comes up in the East, is at noon South and goes down in the West, in the Washington DC area and surroundings. Solar panels convert sunlight into electricity. To do this, a solar panels needs direct sunlight, so sunlight straight onto the panel works best. Thus, the moment the sun is the highest in the sky at noon is when the most power will be generated. This means that solar panels installed facing South will produce the most power.
East and west facing panels work perfectly fine, just a little less efficient, but still fine.
We even install solar panels on low sloped roofs facing North. It's less efficient, but it still works. Think about it like this: Your car has a thermal engine with an efficiency not greater than 40%, but does that mean you never drove with a car that has a bad efficiency? You probably have. It may not be optimal or the highest efficiency, but you still drove your car up a hill and it worked just fine, just not optimal. Same with solar panels. We install them facing all directions and some directions are just more optimal than others.
If you want to read more technical info about solar panels, continue to the 301 Solar Technical info section.