Best angle for solar panels and how to get it right

The angle of solar panels corresponds to their vertical tilt towards the equator. Pretty simple: if panels lie flat, their angle is at 0, if they stay perpendicular to ground, the angle is 90 degrees. In our article on positioning and angle we've already discussed the best side for solar panels: in the northern hemisphere panels turned to the south side give you the maximum amount of energy (in the southern hemisphere it's therefore the north side of the roof), but an angle of panels can vary.

The angle at which solar panels are installed is not always an adjustable thing — sometimes an owner sets it once and for all, sometimes the roof is steep by itself. In theory solar panels, whose angle can be adjusted, are well suited for flat roofs — it's easier to access them and they don't have an initial tilt. When a roof is not well suited for solar panels, they also can be installed on the ground. Keep in mind that panels at low angle tend to accumulate dirt and debris faster because all the litter doesn't slide off panels with rain quite as well.

Some models of solar panels are equipped with solar trackers that make the system follow the sun at all times. These can be on average 25-30% more efficient (they squeeze out about 10% more energy in winter and 40% more energy in summer), but at the same time cost significantly more. Obviously, they don't need to be adjusted at different seasons as they adjust themselves.

If we simplify things, then the angle of your panels can be equal to your latitude. For example, New York's latitude is 40 degrees, so panels can be installed at 40 degrees angle and their performance will be just fine. This number works well for spring and autumn, but in summer sun is higher, therefore the angle should be diminished by 15 degrees. In winter the sun is lower and so you have to add 15 degrees to the angle of your panels. For example, in New York your panels' angle would be at 25 degrees for summer and 55 degrees in winter. However, this method can be optimized and improved.

First of all, let's discuss whether it's even worth it to adjust the angle of solar panels. According to data of Charles Landau, senior software engineer at Arxan Technologies, solar panels with fixed angle, which remains the same for the whole year, receive about 71.1% of optimum amount of solar energy (given that solar panels with solar trackers give you 100% of energy). When you adjust an angle of your panels 2 times a year — for summer and winter — it gives you 75.2%. 4-times a year adjustment — for each season — can raise this number only to 75.7% so it's hardly worth the time and effort. The reason why this percentage can't get too high is because solar panels lose some amount of sunlight during the day anyways — there is no "catch-all" angle.

Landau even figured out the date when the switch of angle is optimal. In the northern hemisphere summer switch should occur on March 30th and winter switch is ideal on September 10th. In southern hemispheres it's reversed: 29th of September works best for summer switch and March 12th is when it's time to adjust for winter.

Keep in mind that it's you who knows your priorities and when solar energy would be the most useful for your house. If you don't plan to adjust your solar panel system during the year, but at the same time you know that in winter solar energy is going to be the most important, you can set a winter angle once and for all. This way you're going to lose some amount of energy in summer, but in winter your panels will work at maximum capacity.

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.

While these numbers are impressive and all, in practise the changes in performance of solar panels caused by an angle are relatively minor when compared to such factors as positioning on a roof, weather and especially properties of your model. For example, the south side of a roof raises the efficiency of solar panels by 15% compared to those that are installed on a eastern or western side, while trying to get a precise angle gives your panels just around 5% more sunlight than just setting the angle to your latitude. For example, if a homeowner in New York would set his solar panels at 5 degrees for the whole year, he would lose only about 10% of annual production compared to one, whose panels were at 30 degrees — and we're talking about a 25 degrees difference!

For residential owners it's rarely ever worth it to pursue the perfect angle — it makes more sense to put more thought into positioning and choosing the right model. General homeowners too often are constrained by wrong roofs, shadows of trees and near buildings, budget. Trying to get the best possible angle is more reasonable for those who use solar panels on a much bigger, commercial scale, when it's possible to simulate perfect conditions for the work of solar panels and little details actually lead up to a big difference in the end. So if you are worried that it might not be possible to install your panels at the best angle, don't get too concerned about it. If you're currently thinking about purchasing solar panels for home or commercial use, check out our article on how to choose a solar panel system.