In photo voltaic Solar system, there are 2 positions of installations we have to take care about in order the solar panels can produce most of their rated power, in other word, we can get the max generation efficiency of our solar panels when we properly install them in two positions. The first position is the orientation of the solar array, we mean to point out the solar array to the sun directly, whether the array will face north, south, or whatever based on the country location, we just aim to get a best position to collection most of sun radiation over the day and the year. The second position is the installation angle of the solar panels reference to horizontal which called tilt angle or pitch angle and this position has a great effect on the solar photovoltaic system annual electrical energy production. Generally, the best pitch angle is equal to the latitude of the solar system installation location, and always there is a good margin around the latitude value, we mean it will be No efficiency loss if we increase or reduce the pitch angle by 5-10 degree, but in all cases we do Not recommend to make the pitch angle as zero degree (flat installation) to ensure the solar panels capability of self-cleaning in rainy days, so we can get rid of dusts and dirties via rainfalls.
We can make the pitch angle as zero, but in that case, we have to consider cleaning the solar panels periodically by hand, however this installation position is Not recommended and we recommend to make the pitch angle as 10 degrees at least.
Sometimes homes roofs are flat and sometimes they are slope with an angle which may be far away from the best pitch angle, so in all cases our solar panels installer will advise the best pitch angle and orientation, as the solar system installer will have most solution options, like installing tilt frame, or keeping the same slope position, or any other option.
At the end, we confirm that placing the solar panels at No angle is Not the ideal case, and having a flat roof will give us the possibility to install our solar system in the best angle and direction to get the biggest exposure to the sun.