。
She thought he was wasting electricity.
Literal
He [subject-が] electricity [object-を] wasting is [quotative-と] thought.
~と考えた uses 考える rather than 思う for 'think' — 考える implies deliberate reasoning or judgment rather than a feeling or impression. 無駄遣いしている ('is wasting') uses the ~ている progressive to describe an ongoing behavior she's observed and formed a judgment about.