Roof direction, tilt and shade

Across the Virginia towns on this site, the sunniest spot makes 11% more per kilowatt than the least sunny (Danville: 1,430, Covington: 1,289 kWh per kW per year). That is the entire geographic effect. Your own roof can swing the number more than moving across the state would. Here is what matters, biggest first.

1. Shade

A tree that shades part of the array for part of the day costs more than any other factor on this page, and it is the one a model cannot see. Installers measure it with a shade tool or a drone and report an annual "solar access" percentage. The calculator offers three coarse buckets: none (100%), some (90%) and heavy (75%). If you have heavy shade, the honest advice is usually to trim, pick a different roof plane, or wait.

Microinverters or optimizers reduce how much one shaded panel drags down the rest, and most residential systems installed today use one or the other. They do not make shaded panels produce; they stop them from penalizing unshaded ones.

2. Direction (azimuth)

In the northern hemisphere, south is best. The calculator uses these factors relative to south, applied to PVWatts' south-facing output:

• South-facing: 100%
• Southeast / southwest: 95%
• East / west: 85%
• Ground mount (ideal tilt): 103%

These are planning approximations, stated on the methodology page. An east-west split array is common and perfectly viable: it produces less in total but spreads production across the day. Under Virginia's one-for-one net metering, total production is what counts, so south still wins on economics. North-facing roofs are generally not worth it.

3. Tilt

Virginia sits between roughly 36.5° and 39.5° north. For maximum annual production a fixed array wants a tilt somewhere near latitude minus a bit; for roof mounts, the roof decides. A common 6-in-12 pitch is about 27°, close to the 25° used in this site's PVWatts runs. Steeper roofs favor winter, flatter roofs favor summer, and the annual difference between 20° and 35° is a few percent either way. Not worth re-framing a roof over.

4. Location

Last, and smallest. Coastal and Southside towns are a little sunnier than the mountains; the city pages have each town's PVWatts number and monthly profile. Median across the state: 1,389 kWh per kW per year.

Things that don't matter as much as people think

• Heat. Panels lose a little output when hot. PVWatts includes it. Virginia summers cost a few percent, not tens.
• Snow. A few lost days a year in most of Virginia. PVWatts does not model snow cover (its default snow loss is zero), so treat it as a small extra haircut in the mountains.
• Brand. Within reputable tier-one panels, a few percent of efficiency changes how many panels fit, not how much a kilowatt produces.