Energy-efficient

adj.capable, thoughtfulprofessional

What it means

Providing a useful result while consuming relatively little energy.

Reach for it when describing performance supported by a stated energy comparison or standard

Energy-efficient windows reduced winter heat loss in the classrooms.

playful
frugal with power
when accessible informal explanation is useful
not when The personifying phrase is unsuitable for specifications or certification.
everyday
economical
when lower operating cost is the practical benefit
not when Economical may reflect cheap energy rather than lower consumption.
everyday
energy-saving
when the design reduces energy use against a familiar baseline
not when Energy-saving names a reduction but not the useful output used to judge efficiency.
professional
fuel-efficient
when less fuel is used for travel or power
not when Fuel-efficient is limited to fuel and may still produce high total emissions.
professional
high-efficiency
when a recognized performance ratio or class applies
not when High-efficiency needs a named metric and does not automatically mean low total use.
professional
low-energy
when absolute energy demand is small
not when Low-energy does not prove efficient output and can describe reduced performance.
formal
low-consumption
when measured use should be emphasized neutrally
not when Low-consumption is comparative and awkward without naming what is consumed.

energy-efficient appliance
The energy-efficient appliance carries a verified performance rating.
energy-efficient building
An energy-efficient building pairs insulation with controlled ventilation.
more energy-efficient
The new pump is more energy-efficient at the same flow rate.

Energy efficiency is relative to useful output and does not establish low total consumption or overall sustainability.

The definition and the example on this page are written here. The dictionaries above ground which meaning is being described, not the wording.