A  500 W heating unit is designed to operate from a 115 Volt line. If the line voltage drops to 110  volt. The percentage drop in heat output will be

we know P=V2R ∴P1P=V12V2⇒P1P=1101152=1−51152using binomial approx⇒P1P = 1−10115=1−223⇒P1P -1= −223multiplying bothsides with 100 we  get⇒P1P−1 × 100=−20023=−8.6 %