As a native speaker of English, I can assure you that there is no need to reflect the word „santykinės“ in the English translation. The most idiomatic way to translate „siekiant sumažinti šiluminės energijos santykines sąnaudas iki 30 procentų“ is simply: "in an effort to reduce the consumption of thermal energy to 30% of its present level". Arba, dar geriau: "in an effort to reduce the consumption of thermal energy by 70%". [Sic!]