Complex sentence: These kettles were popular because a stove was no longer needed to heat water, thus there was no need to heat up the entire stove just for a cup of tea, and less thermal energy was required to heat up the water.
Split into simpler sentences: These kettles were popular, as a stove was no longer needed to heat up water. Also this meant that people had no need to heat up the entire stove just for a cup of tea, plus less thermal energy was required to heat up the water.