Flat roofs: south-facing or east/west-facing? Advantages, disadvantages and how to choose

On a flat roof the structure decides much more than the inclination of the modules: it decides how many modules fit , How much energy do they produce and at what times , How much does the system weigh on the roof? and how the electrical part should be designed. The two most common configurations are South and East/West Neither is better overall: let's see how they work, where they perform best, and how to choose the right one for your system. South Configuration The modules are arranged in parallel rows, all inclined towards the South. On flat roofs the typical inclination is 10–15° , a compromise between production, space and wind load. Advantages Maximum production per kWp installed : each module works close to the optimal orientation Peak production at midday , when the irradiation is maximum Simple electrical design : all modules see the same sun, the strings are homogeneous Disadvantages They are needed distances between rows to prevent one row from shading the next, especially in winter when the sun is low: for the same surface area, two rows are installed fewer modules The production is concentrated in the central hours ; in the early morning and late afternoon it is lower The back of the form is open and it gets more wind : usually more ballast per module is needed, or rear deflectors East/West Configuration The modules are mounted in pairs, back to back: one slope faces the other. East , the other one West , with low inclinations (typically 10th ). The rows are close together, with almost no spaces. Advantages Multiple modules on the same surface : without anti-shade distances the roof fills almost completely. It is generally installed from 30% to over 50% more modules compared to the South, depending on latitude and inclination (see calculation below) Production distributed throughout the day : the eastern aquifer produces more in the morning, the western one in the afternoon More aerodynamic structure : the double closed flap reduces the sail effect and therefore the ballast per module Smaller inverter for the same kWp : the peak is lower, so the photovoltaic field can be oversized compared to the AC power of the inverter Disadvantages Lower production per kWp : each module is less favored than one facing south. With low inclinations the annual difference is generally a few percentage points (approximately 5–10%): it must be verified on a case-by-case basis with a simulation, for example with PVGIS With side-by-side rows you have to foresee the corridors for maintenance and distances required for fire prevention Production curves compared With the same installed power, the South has a higher peak at midday. The East/West has a lower peak (in the graph approximately 70% ), but produces more in the early morning and late afternoon. Its curve is wider and flatter. South or East/West: the comparison It's not just the performance of a single panel that counts The most common mistake is to compare production of a module instead of that one of the plant South wins on a single panel, but East/West often wins on the entire roof. Because with East/West more modules enter To the south, each row of modules casts a shadow on the one behind it. To avoid this, you need to leave one free space between the rows, and it is calculated on the worst case: December 21st at noon , when the sun is at its lowest point during the year. In the East/West orientation, however, the two roofs are back to back and the rows are placed side by side: the space lost is almost zero. In the example, in Central Italy, each 15° South row occupies 1.74 m of roof: 1.10 m of more module 0.64 m of free space. In East/West each module takes up only 1.17 m . On 7 meters of roof they enter like this 4 rows to the South against 6 modules in East/West : The 50% more . How much does it change from North to South? The space to be left between the South rows depends on two things: the latitude : the further north you go, the lower the winter sun is and the longer the shadows are; the inclination of the modules to the South: the more they are inclined, the higher they are and the more shade they cast. Therefore the East/West advantage ranges from approximately +27% in Sicily with the South at 10° up to over +60% in the North with the South at 15°: These values ​​are calculated using the minimum criteria. To avoid shade even in the morning and afternoon in winter, the south rows should be spaced even further apart, increasing the east/west advantage. Corridors, edges, and obstacles reduce the usable area in both cases. From modules to energy More modules doesn't mean that each one produces more: each kWp in the East/West yields a little less than one in the South. But the total counts. An indicative example: if they enter on the same roof 100 kWp in the South or 140 kWp in East/West , even with 7% less yield per kWp the East/West produces 140 × 0.93 ≈ 130: approximately 30% more energy per year from the same roof. The right comparison then is on overall production and on the cost per kWh produced , also taking into account the consumption profile. What to take care of in design Strings and MPPT East and West modules receive sunlight at different times, thus producing different currents. Two East and West strings can be in parallel on the same MPPT only if they have the same number and type of modules , because the tension is similar. When possible, it is advisable to assign Dedicated MPPTs at each exposure. Never mix East and West modules in the same string . Inverter sizing With East/West, the actual peak remains below the nominal field power, so you can operate with a higher DC/AC ratio without significant clipping losses. This should always be verified with a simulation and the input limits of the selected inverter. Wind, ballast and roof load The ballast calculation depends on the wind zone, the height of the building, its position on the roof (edges and corners are the most critical areas), and the type of structure. First of all, the scope of coverage If load is a constraint, there are lightweight, aerodynamic structures that significantly reduce ballast. Tilt and clean Very shallow inclinations reduce shadows and wind loads, but the modules are less self-cleaning in the rain. Below 10°, more frequent washing is recommended, especially in dusty or agricultural areas. Which One to Choose? > > > Choose SOUTH if - the available surface is abundant compared to the power you need - do you want it? maximum yield per kWp (and for purchased module) - consumption is concentrated in central hours of the day > > > Choose EAST/WEST if - the surface is the real limit and you want to install as much power as possible - consumption is distributed from morning to evening (offices, commercial activities, users with heat pumps) - you want to increase the self-consumption and reduce the energy fed into the grid at midday - the wind exposure is high and you want a more aerodynamic structure In short, South and East/West are not in competition: the right configuration is the one that best combines available surface area, installable power, production and consumption profile. Flat roof structures in the catalogue You can find both configurations in the structures we distribute: South Configuration GB Solar Ballast System – concrete ballast (download the catalog in PDF) S:FLEX LEICHTmount RAIL 2.1 S – lightweight aluminum structure SL Rack Fast Flat Sud East/West Configuration S:FLEX LEICHTmount RAIL 2.1 EW – lightweight aluminum structure SL Rack Fast Flat East-West Do you need support? The Solarplace Team is available to compare the two configurations on your roof, including layout, producibility, and ballast calculations. ☎️ Or call us to the +39 070 796 8010