AFUE describes how much of the fuel becomes heat. It says nothing about the electricity the furnace uses, and the blower motor is a substantial and continuous load.
Two kinds of motor
PSC — permanent split capacitor. The older type. It runs at a fixed speed set by a tap selection, and it draws roughly the same power regardless of what the system needs.
ECM — electronically commutated motor. A brushless DC motor with electronic control. It varies its speed continuously and, importantly, it is far more efficient at the low speeds where it spends most of its life.
The difference in practice
A PSC blower on a residential furnace commonly draws in the region of 500 watts and can be considerably more on a large system.
An ECM at the same airflow uses substantially less, and at the reduced speeds used for continuous circulation or for low-fire heating it uses a small fraction of the PSC figure — because the power a fan consumes falls steeply as its speed drops.
Over a long heating season, and a cooling season on top of it, that is a meaningful line on the electricity bill.
Why the summer matters
On a house with central air, the furnace blower is the air handler for the cooling system.
An ECM therefore pays back in both seasons, and it enables things a PSC cannot do well: the lower airflow that improves dehumidification in a humid climate, and low-speed continuous circulation between cycles.
Half the benefit of a variable-speed furnace lands in a season the furnace itself is not running, and that is easy to overlook when comparing heating quotes.
Constant-torque and constant-airflow
Not all ECMs are the same, and the distinction is worth knowing when reading a quote.
Constant-torque ECMs are the more basic and cheaper version. They are far more efficient than a PSC and they do not compensate for duct restriction.
Constant-airflow ECMs measure and hold a target CFM regardless of static pressure. Better performance, and a caution attached: in a badly restricted duct system they will draw more power to maintain airflow rather than quietly delivering less, so the restriction shows up as an electricity bill instead of a comfort complaint.
Either way, the ductwork should be corrected rather than compensated for.
Continuous circulation
An ECM makes running the blower continuously at low speed practical, because the power draw at low speed is small.
The benefits are even temperatures between cycles, continuous filtration, and better distribution to the far rooms — all of which are the things people usually try to fix by making the furnace bigger.
On a PSC system, continuous circulation is expensive enough that it is rarely worth it.
Where this fits in a decision
The blower motor type is usually bundled with the furnace tier: single-stage units often have PSC or constant-torque ECM, and modulating units have constant-airflow ECM.
That means the electricity saving is part of the case for the higher tier, and it belongs in the comparison alongside the comfort argument rather than being left out because AFUE does not measure it.
Checking what you have
The furnace's rating plate lists the blower motor's full load amps, and the installation manual usually gives the watts at each speed tap.
For an existing furnace, that is enough to estimate the seasonal cost — multiply by the hours the blower runs, which for a system that cycles is roughly the run time already being measured for the sizing question.
The filter is part of this
An ECM will work harder against a restrictive filter, and a constant-airflow ECM will work considerably harder because it is holding the airflow target.
Filtration is worth having, and the right way to get it is filter area — a four- or five-inch media cabinet rather than a dense one-inch filter. More surface area at the same rating means much less pressure drop and much less power spent pushing air through it.
Failure and replacement
ECM motors have electronics attached, and the module is a more expensive part than a PSC motor and capacitor.
They also last well, and the seasonal energy saving over fifteen years comfortably exceeds the difference. It is worth knowing at purchase rather than discovering at the first failure, and worth asking whether the module is available separately from the motor.