What roof orientation actually costs you
Why azimuth usually matters more than roof pitch
A roof facing east or west loses about 16% of annual output compared to true south at a typical 6/12 pitch, a figure in line with modeling from NLR's PVWatts calculator and with the NREL weather-data runs behind this tool. A pitch 15 degrees from the optimum costs only about 2.6%. Degree for degree a tilt miss costs more, but real roofs rarely miss the best pitch by much, while their direction can be anything. If your roof forces a tradeoff between a good pitch and a good compass direction, keep the orientation close to south first. The roof pitch output adjustment calculator covers the pitch factor in detail.
Why the loss curve is gentle near true south
At a 6/12 pitch, a roof 30 degrees off south loses only about 2.1% of annual output, and 45 degrees off loses about 4.6%. The loss then accelerates: about 16% at due east or west, about 29% at northeast or northwest, and about 35% at due north. Pitch changes the picture too. At due east or west the loss is about 7% on a low-slope roof, 16% at 6/12, and 22% on a steep 12/12 roof. That is why southeast and southwest roofs are perfectly viable for solar, while east, west, and especially north faces need a real look at the numbers, which the south-facing vs east-west array calculator sets out side by side.
Getting an accurate azimuth reading
A magnetic compass reads magnetic north, not true north, and the gap between the two, called magnetic declination, can reach 15 degrees or more in parts of the US, a variation the USGS explains varies by both location and time. Google Maps satellite view avoids that correction step entirely, since you're reading the roof's actual geographic orientation directly off the image rather than working from a compass in your hand.
When adding panels beats a tracking mount
For most homeowners with a fixed east or west facing roof, adding a modest number of extra panels is far cheaper than a solar tracker that follows the sun through the day. Trackers add real cost and maintenance for a gain that a few more panels usually match at a lower price, Whether those extra panels are worth it depends on how fast they pay for themselves, which the payback period calculator works out for your system. The solar panel calculator can size the full system once you know your orientation adjustment.
What these percentages can't tell you about your location
The loss figures come from typical-year weather data for a mid-latitude US site, and they shift with latitude and cloud cover. At near-optimal tilt, an east or west roof loses roughly 17% at a mid-latitude site but up to about 27% at a high-latitude, cloudier one. Local sun hours also set how much a given loss costs in kWh, and the peak sun hours calculator gives the location-specific piece of the puzzle.