When solar panels are being tested at Standard Test Conditions, the efficiency number that they receive in the end corresponds to their performance at 77 degrees Fahrenheit, air mass 1.5 spectrum and irradiance of 1000 W/m2. These conditions represent performance of panels tilted at approximately 37 degrees in continental US in spring or autumn. In the USA if the angle of panels is somewhere between 30-45 degrees, they are probably doing a fine job.
What if you're determined to find out the optimal tilt for your location? There are a couple of formulas that can give you a more precise number. For example, some solar consultants propose multiplying your latitude by 0.9, then adding 29 degrees in winter, subtracting 23.5 degrees in summer or just subtracting 2.5 degrees from latitude for spring/autumn. For example, if you are living in New York, then spring/autumn angle is 40 degrees latitude - 2.5 = 37.5. In winter it should be 40 degrees * 0.9 + 29 = 65 degrees: quite steep, but in theory it should let panels receive the maximum amount of sunlight during midday hours. In summer it's 40 * 0.9 - 23.5 = 12.5 degrees.
Landau, who appeared earlier in this article, claims that for fixed solar panels an angle should be calculated as follows:
- if your latitude is less than 25 degrees, multiply it by 0.87
- if your latitude is more than 25 degrees, but less than 50, then multiply it by 0.76 then add 3.1 degrees
This method gives you the best angle for solar panels if you don't plan to adjust them during the year. If you plan to make corrections for winter and summer, then the formula changes a bit. If your latitude is somewhere between 25 and 50 degrees then you multiply your latitude by 0.93 minus 21 degrees for summer, and you multiply the latitude by 0.875 and add 19.2 in winter. Let's continue with New York's latitude as an example: fixed angle in this situation equals 40 * 0.76 + 3.1 = 33.5. This angle should give you about 71% of the maximum amount of sunlight. In situation where you adjust an angle two times a year, the winter angle is 40 * 0.875 + 19.2 = 54.2 and summer's angle is 40 * 0.93 - 21 = 16.2. In this situation your panels catch around 75% of annual sunlight — of course, approximately. Landau himself notes that he describes the most ideal situation, plus points out that the best angle can be different at high altitudes.