Research showed impacts of over 10% for vertically (90°) mounted systems, but such effects were substantially lower for systems with lower surface tilts.
ɹᵻsˈɜːtʃ ʃˈoʊd ˈɪmpækts ʌv ˌoʊvɚ tˈɛn pɚsˈɛnt fɔːɹ vˈɜːɾɪkli nˈaɪnti dᵻɡɹˈiːz mˈaʊntᵻd sˈɪstəmz, bˌʌt sˈʌtʃ ɪfˈɛkts wɜː səbstˈænʃəli lˈoʊɚ fɔːɹ sˈɪstəmz wɪð lˈoʊɚ sˈɜːfɪs tˈɪlts.