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Solar panel installation on flat roof- west-east or south-north? We’ve got numbers!

More and more  manufacturers offer dual-tilt mounting systems known comercially as East-West construction. Is it just a new trend or new, better way of designing? 

The PV design has evolved during the last few years. In the past, it was obvious that the inclination of photovoltaic modules should be taken from the latitude of the project location and the optimal direction was the South one. This way, the energy output was optimized for the power installed (kWh/kW).

Thich approach was justified when photovoltaic modules were the most expensive part of photovoltaic system. However, when prices fell down and the industry got interested in different ways of mounting photovoltaic modules on flat or low slope roof. The new dual-tilt mounting applied more modules mounted per square meter (kW/m2). The modules were installed under low inclination of 10-15 degrees. Although we cannot still talk about era of super-cheap solar modules yet,  this design is already justified in some cases. Let’s compare South and dual-tilt mounting on flat roofs.

Making good use of rooftop space

In this category wins dual-tilt construction.

South-oriented mounting for flat or low-slope roofs requires providing large inter-row gaps that will protect rows of modules from self-shading. In dual-tilt constructions, modules are installed at lower angle that eliminates self-shading. This mean roof area is used much better than in case of South-oriented modules.

The ground coverge ratio in case of dual-tilt modules is 0,9 (not counting space left for maintenance or obstacles on the roof) while in case of South-oriented panels- 0,5-0,8. It means that system efficiency can rise up by 15 to even 35% thanks to dual-tilt mounting.

Energy production

The winner of this category is the South-oriented construction although… it depends on season of the year.

Although dual-tilt mounting is less sensitive on azimuth changes, it’s not a big surprise that modules oriented this way will produce less than those oriented into the South. Nevertheless, let’s remember that the amount of modules installed per square meter also counts. Also, in summer months, East-West-oriented modules will produce much more energy in afternoon hours that South-oriented modules.

PV system losses

Reflected radiation and low radiation 

These losses are higher in case of dual-tilt modules since they are the consequence of incident light with angle different that 90 degrees. Low-irradiation losses together with reflection light losses together account for 1% in case of dual-tilt mounting.

Inverter losses

Although inverters work better with higher load ratio, there are also sensitive to sudden load changes. In case of East-West mounting, inverter losses as calculated as slightly lower than in South-oriented modules. Cheaper, smaller inverters can be adapted to same installation. It’s important to choose the inverter with multiple MPP trackers.


Dual-tilt constructions have more aerodynamic shape that makes them more resistant to wind loads and distributes forces affecting the roof. On the other hand, what matters is the number of modules per square meter. The disadvantage is self-cleaning of the module.

Material and installation costs

This time dual-tilt mounting wins again.

And it happens for 3 reasons:

– some elements of construction are shared between modules

– thanks to the module configuration, modules act like wind protector so this component is not necessary

– thanks to smaller inclination angle, load is distributed better than in case of classic mounting ballast material can be reduced

Less material means lower costs of mounting as well as installation. Still, it depends on number of modules mounted per square meter.


The choice between Sounth-oriented or dual-tilt mounting has a lot to do with energy rates (both utility one as the energy fed into the grid). If the rate for energy sold is higher in afternoons and higher than auto-consumed energy, you can choose South-oriented modules with no hesitation.

The dual-tilt mounting on the flat roof is recommended when:

– the roof area is limited and has almost no obstacles

– the rooftop orientation is deviated from South orientation

– the consumption profile is the highest in the mornings and afternoons and it’s better to self-consme than to sell

Check the energy production of modules with different azimuth and in different configurations to choose the ideal PV system. Try EasySolar app!