I used the App Solar Consult (for iPhone/iPad) and entered the size of a flat roof. Although the output is 1% more at an angle of 10 degrees (in an East-West setup), I've entered in both cases an angle of 15 degrees. I entered in the app that we have a flat roof of 27 feet (the sunny side) at 20 feet (the other roof side).
Traditional setup of solar panels with a direction to the sun:
First I calculated the optimal setup with the solar panels of Autarco. The result of the calculation was 2 rows of 8 solar panels (16 panels of 315 Wp per panel size 6.4 x 3.3 feet). This setup has a capacity of 5,040 Wp and on my location (in The Netherlands) the output was 91% relative to the power (Wp), so it was 4,571 kWh.
Calculation of an East-West setup of solar panels:
If you choose in the App to calculate with an East-West setup, the result is 2 rows to the East and 2 rows to the West. In each row 6 panels fit next to each other and in that case the rows lie with their backs against each other (see the picture which displays an East-West setup).
4 rows of 6 panels is a total of 24 panels with a power of 315 Wp per panel. This setup has a capacity of 7,560 Wp (50% more then a setup to the sun) and the App Solar Consult calculated that this arrangement will give an output of 82% relative to the power (Wp), so it was 6,256 kWh (37% more).
Furthermore, solar panels are usually placed in two strings, wherein the output of the worst panel determines the output of all the panels in the string. In an East-West setup, you could place the panels Eastwards in one string and the panels towards the West in the other string.
In summary:
It appears that the East-West setup the output per panel is (relative to the power) 10% less, but you can place more panels (24 instead of 16). This is only profitable if the payback period is the same or less. I helped my father to buy 13 of these solar panels and he paid $500 per panel on a flat roof (including the inverter, standards, mounting hardware, wiring, assembly, travel costs and additional group in the fuse box).
Payback time of solar panels with a direction to the sun:
The more panels you let install the cheaper per panel, but how much is now required to make it profitable. I used the App Solar ROI (only for iPhone/iPad) to calculate the payback time. With this App, you can enter some variables to calculate the payback period of solar panels. I have entered 16 panels of $500 each and an output of 4,571 kWh, according to the App Solar ROI, the payback time was 7.9 years.
Payback time of solar panels in an East-West setup:
I entered 24 solar panels of $500 each and an output of 6.256 kWh. The payback time was in an East West setup 8.7 years. If you enter 24 panels of $458 each (8% cheaper) and an output of 6,256 kWh, then the payback time is also 7.9 years (same result as in the first calculation).
Conclusion:
In case of a flat roof you can place in an East-West setup 50% more solar panels and this results is 37% more kWh. The payback time is only shorter if the price is at least 8% per panel lower. If you buy 50% more solar panels then the price per solar panel should be lower, because:
- The travel expenses are only once and this will not be different if you order 50% more panels;
- The work and time for mounting and installation of an inverter with more power, is the same;
- The peak power of the inverter does not need to be 50% higher in an East-West setup. In the morning generates the Eastern side more electricity and in the afternoon the Western side;
- Some suppliers have special standards for East-West setups and these are cheaper than traditional standards (less material).
If you have a flat roof and want to generate as much electricity as possible, ask for an estimate for the traditional setup compared to 50% extra panels in an East-West setup. Divide the price by the number of panels and see if the price is at least 8% lower per panel.
My experience is that the investment of an offer from 16 to 24 panels is only 4% cheaper per panel and therefore the payback time will be five months longer. Still, my advice is to choose the East-West setup and accept the extra five months. Accordance the App Solar ROI, you will save annually $1,052 with 16 panels and $1,439 with 24 panels in an East-West setup (this is a difference of $387 extra per year). The App Solar ROI shows a result of $7,500 more profit for an East-West setup after 25 years.