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The simple, and obvious, answer is ‘by pointing it at the sun’! However, this can be more involved than it first appears. The sun tracks across the sky from East to West and at its zenith (highest point, around midday)) will generally be in the direction of the equator – South in the northern hemisphere, and North in the southern hemisphere. Near the equator at midsummer, it can overtop to the opposite pole. The vertical angle of the sun (azimuth) varies through the seasons, highest at midsummer and lowest at midwinter. The midday azimuth at the equinox (midpoint between midsummer and midwinter) is the same as the angle of latitude for the location.

A photovoltaic (PV) module (‘solar panel’) will receive the most energy when it is pointing directly at the sun. Ideally, the module would track the sun east to west during the day, and tilt higher or lower during the seasons. Many plants do this, sunflowers being a great example. Tracking systems are available but these are generally expensive and inherently less reliable than a fixed mounting. It is quite simple to arrange a frame that can be raised or lowered every few months to match the season, but this adds the potential for human failure (forgetting to change it from winter to summer angle) and reduces one of PV’s best points that it is virtually ‘fit and forget’. The cost of PV is now so low that it is usually most cost effective to fit a slightly larger panel and orientate it for the best average performance for the year. The simplest approach is to mount the panel pointing towards the equator tilted at an angle from the horizontal equal to the latitude of the site. So, in Zimbabwe, this would be facing North at an angle of about 20 degrees, and in Pakistan it would be South at about 30 degrees.

If you are mounting the module on an existing structure such as a roof, it may not be pointing the right way. It is possible to set up a frame to adjust for this, but this can be quite fiddly. In general, if the main direction is within 45 degrees of the equator direction the loss of output is only a few percent. There are a couple of other considerations for the tilt. Output from the module will reduce significantly if it gets dirty. Where rain is very seasonal, it is important to clean the panel during the dry season (when there is also typically more dust). However, it is best practice to use at least 10 degrees of tilt to help self-cleaning during rain, even at the equator (with latitude zero).

The second consideration for tilt angle is how the load varies with season. In high latitudes, there is a significantly shorter day length in the winter, when a system intended primarily for lighting will have a much higher load, whilst at the same time receive a lot less solar energy as the sun is not in the sky for long. In this case, the module (or array – a collection of modules) should be mounted at an angle to horizontal 10 to 20 degrees steeper than the location’s latitude to maximise the solar energy when the sun is low in the sky. In the UK, you will often see this with solar powered streetlights with modules at angles of 60 degrees or more. On the other hand, a load that increases in the summer, such as refrigeration, would justify using a flatter tilt angle than latitude to get more energy from the higher summer sun.

So far, so good – point it at the sun and keep it clean.

However, there is a potentially more significant cause of lost energy than dust or slightly wonky orientation – shading. Because of the way silicon wafers are connected together to make a PV module, it is possible for a shadow covering 10 – 20% of the module to reduce its output by 50 - 80%. It’s not always this bad and depends a lot on the configuration of the module and the type of load e.g. battery or direct inverter. But it is always worth trying to site the module away from potential shadows. Typical culprits are buildings and trees, but even quite slender objects like poles, wire fences and aerials can cause significant losses.

The quick and dirty way to do this is to look towards the equator (a compass helps), then imagine the suns path at the equinox by looking up at the angle of latitude (don’t look directly at the sun, though). Make a rough trace to the east and west to estimate the sun’s path. Waving your arm about can help! Tilt up and down about 15 degrees for winter and summer paths. Most solar energy is received between 9am and 3pm. At the equinox, the sun is in the sky for 12 hours so if it is highest at midday, at 9am it will be halfway up and 3pm halfway down. It is hard to avoid all shading early and late in the day, but if you can get a clear sky between these hours you will harvest the bulk of the available energy.

Try to move the mounting away from shading (but not too far from the load as longer cables lose more energy in transmission). Getting higher is often the best policy, but remember you need to be able to clean the panel somehow. Maybe you can remove the shading object. Also remember that trees and bushes grow upwards and may cause shading in a year or two.

Tracing an estimated sun path by waving your arm is very rough and ready. There are physical tools to measure the path using clear plastic screens traced with sun paths for different seasons and latitudes, but these are not that easy to find or use. A great alternative is to use a solar tracking app for a smartphone. It is possible to use a star chart app like SkyMap to trace the sun, but there are several excellent apps designed specifically for solar installers that display the sun’s path at different seasons on the camera screen as you point your phone at the sky. There are several free versions available, but the ‘professional’ ones are often only a few dollars. Sun Surveyor for Android and Sun Seeker for IOS are good examples but there are others available. There are also more simple apps that turn your phone into a clinometer and tell you when you when it is in the best orientation for the PV module e.g. SolarTilt. Another useful (Android) app is Scan the Sun where you trace the horizon and the app calculates the potential output for different months of the year. If you search the app stores for Android or IOS there are quite a selection. Many are geared towards grid-connected systems but the shading and orientation tools work whatever the application.

In summary, for general applications

• Point the module towards the equator
• Tilt it at the angle from horizontal equal to the site’s latitude, but not less than 10 degrees
• Position the panel to avoid shading between 9am and 3pm, thoughout the year
• Remember that trees and plants will keep growing!
• Smartphone apps can be simple tool for assessing shade and getting orientation right.